diff mbox series

[v8,3/4] fs: move S_ISGID stripping into the vfs

Message ID 1650971490-4532-3-git-send-email-xuyang2018.jy@fujitsu.com (mailing list archive)
State New, archived
Headers show
Series [v8,1/4] fs: add mode_strip_sgid() helper | expand

Commit Message

Yang Xu April 26, 2022, 11:11 a.m. UTC
Creating files that have both the S_IXGRP and S_ISGID bit raised in
directories that themselves have the S_ISGID bit set requires additional
privileges to avoid security issues.

When a filesystem creates a new inode it needs to take care that the
caller is either in the group of the newly created inode or they have
CAP_FSETID in their current user namespace and are privileged over the
parent directory of the new inode. If any of these two conditions is
true then the S_ISGID bit can be raised for an S_IXGRP file and if not
it needs to be stripped.

However, there are several key issues with the current state of things:

* The S_ISGID stripping logic is entangled with umask stripping.

  If a filesystem doesn't support or enable POSIX ACLs then umask
  stripping is done directly in the vfs before calling into the
  filesystem.
  If the filesystem does support POSIX ACLs then unmask stripping may be
  done in the filesystem itself when calling posix_acl_create().

* Filesystems that don't rely on inode_init_owner() don't get S_ISGID
  stripping logic.

  While that may be intentional (e.g. network filesystems might just
  defer setgid stripping to a server) it is often just a security issue.

* The first two points taken together mean that there's a
  non-standardized ordering between setgid stripping in
  inode_init_owner() and posix_acl_create() both on the vfs level and
  the filesystem level. The latter part is especially problematic since
  each filesystem is technically free to order inode_init_owner() and
  posix_acl_create() however it sees fit meaning that S_ISGID
  inheritance might or might not be applied.

* We do still have bugs in this areas years after the initial round of
  setgid bugfixes.

So the current state is quite messy and while we won't be able to make
it completely clean as posix_acl_create() is still a filesystem specific
call we can improve the S_SIGD stripping situation quite a bit by
hoisting it out of inode_init_owner() and into the vfs creation
operations. This means we alleviate the burden for filesystems to handle
S_ISGID stripping correctly and can standardize the ordering between
S_ISGID and umask stripping in the vfs.

The S_ISGID bit is stripped before any umask is applied. This has the
advantage that the ordering is unaffected by whether umask stripping is
done by the vfs itself (if no POSIX ACLs are supported or enabled) or in
the filesystem in posix_acl_create() (if POSIX ACLs are supported).

To this end a new helper vfs_prepare_mode() is added which calls the
previously added mode_strip_setgid() helper and strips the umask
afterwards.

All inode operations that create new filesystem objects have been
updated to call vfs_prepare_mode() before passing the mode into the
relevant inode operation of the filesystems. Care has been taken to
ensure that the mode passed to the security hooks is the mode that is
seen by the filesystem.

Following is an overview of the filesystem specific and inode operations
specific implications:

arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
fs/9p/vfs_inode.c:      inode_init_owner(&init_user_ns, inode, NULL, mode);
fs/bfs/dir.c:   inode_init_owner(&init_user_ns, inode, dir, mode);
fs/btrfs/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
fs/btrfs/tests/btrfs-tests.c:   inode_init_owner(&init_user_ns, inode, NULL, S_IFREG);
fs/ext2/ialloc.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
fs/ext4/ialloc.c:               inode_init_owner(mnt_userns, inode, dir, mode);
fs/f2fs/namei.c:        inode_init_owner(mnt_userns, inode, dir, mode);
fs/hfsplus/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
fs/hugetlbfs/inode.c:           inode_init_owner(&init_user_ns, inode, dir, mode);
fs/jfs/jfs_inode.c:     inode_init_owner(&init_user_ns, inode, parent, mode);
fs/minix/bitmap.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
fs/nilfs2/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
fs/ntfs3/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
fs/ocfs2/dlmfs/dlmfs.c:         inode_init_owner(&init_user_ns, inode, NULL, mode);
fs/ocfs2/dlmfs/dlmfs.c: inode_init_owner(&init_user_ns, inode, parent, mode);
fs/ocfs2/namei.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
fs/omfs/inode.c:        inode_init_owner(&init_user_ns, inode, NULL, mode);
fs/overlayfs/dir.c:     inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
fs/ramfs/inode.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
fs/reiserfs/namei.c:    inode_init_owner(&init_user_ns, inode, dir, mode);
fs/sysv/ialloc.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
fs/ubifs/dir.c: inode_init_owner(&init_user_ns, inode, dir, mode);
fs/udf/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
fs/ufs/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
fs/xfs/xfs_inode.c:             inode_init_owner(mnt_userns, inode, dir, mode);
fs/zonefs/super.c:      inode_init_owner(&init_user_ns, inode, parent, S_IFDIR | 0555);
kernel/bpf/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
mm/shmem.c:             inode_init_owner(&init_user_ns, inode, dir, mode);

All of the above filesystems end up calling inode_init_owner() when new
filesystem objects are created through the following ->mkdir(),
->symlink(), ->mknod(), ->create(), ->tmpfile(), ->rename() inode
operations.

Since directories always inherit the S_ISGID bit with the exception of
xfs when irix_sgid_inherit mode is turned on S_ISGID stripping doesn't
apply. The ->symlink() inode operation trivially inherit the mode from
the target and the ->rename() inode operation inherits the mode from the
source inode.

All other inode operations will have the S_ISGID bit stripped once in
vfs_prepare_mode() before.

In addition to this there are filesystems which allow the creation of
filesystem objects through ioctl()s or - in the case of spufs -
circumventing the vfs in other ways. If filesystem objects are created
through ioctl()s the vfs doesn't know about it and can't apply regular
permission checking including S_ISGID logic. Therfore, a filesystem
relying on S_ISGID stripping in inode_init_owner() in their ioctl()
callpath will be affected by moving this logic into the vfs.

So we did our best to audit all filesystems in this regard:

* btrfs allows the creation of filesystem objects through various
  ioctls(). Snapshot creation literally takes a snapshot and so the mode
  is fully preserved and S_ISGID stripping doesn't apply.

  Creating a new subvolum relies on inode_init_owner() in
  btrfs_new_inode() but only creates directories and doesn't raise
  S_ISGID.

* ocfs2 has a peculiar implementation of reflinks. In contrast to e.g.
  xfs and btrfs FICLONE/FICLONERANGE ioctl() that is only concerned with
  the actual extents ocfs2 uses a separate ioctl() that also creates the
  target file.

  Iow, ocfs2 circumvents the vfs entirely here and did indeed rely on
  inode_init_owner() to strip the S_ISGID bit. This is the only place
  where a filesystem needs to call mode_strip_sgid() directly but this
  is self-inflicted pain tbh.

* spufs doesn't go through the vfs at all and doesn't use ioctl()s
  either. Instead it has a dedicated system call spufs_create() which
  allows the creation of filesystem objects. But spufs only creates
  directories and doesn't allo S_SIGID bits, i.e. it specifically only
  allows 0777 bits.

* bpf uses vfs_mkobj() but also doesn't allow S_ISGID bits to be created.

While we did our best to audit everything there's a risk of regressions
in here. However, for the sake of maintenance and given that we've seen
a range of bugs years after S_ISGID inheritance issues were fixed (see
[1]-[3]) the risk seems worth taking. In the worst case we will have to
revert.

Associated with this change is a new set of fstests to enforce the
semantics for all new filesystems.

Link: e014f37db1a2 ("xfs: use setattr_copy to set vfs inode attributes") [1]
Link: 01ea173e103e ("xfs: fix up non-directory creation in SGID directories") [2]
Link: fd84bfdddd16 ("ceph: fix up non-directory creation in SGID directories") [3]
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Suggested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
---
 fs/inode.c         |  2 --
 fs/namei.c         | 22 +++++++++-------------
 fs/ocfs2/namei.c   |  1 +
 include/linux/fs.h | 11 +++++++++++
 4 files changed, 21 insertions(+), 15 deletions(-)

Comments

Christian Brauner April 26, 2022, 10:38 a.m. UTC | #1
On Tue, Apr 26, 2022 at 07:11:29PM +0800, Yang Xu wrote:
> Creating files that have both the S_IXGRP and S_ISGID bit raised in
> directories that themselves have the S_ISGID bit set requires additional
> privileges to avoid security issues.
> 
> When a filesystem creates a new inode it needs to take care that the
> caller is either in the group of the newly created inode or they have
> CAP_FSETID in their current user namespace and are privileged over the
> parent directory of the new inode. If any of these two conditions is
> true then the S_ISGID bit can be raised for an S_IXGRP file and if not
> it needs to be stripped.
> 
> However, there are several key issues with the current state of things:
> 
> * The S_ISGID stripping logic is entangled with umask stripping.
> 
>   If a filesystem doesn't support or enable POSIX ACLs then umask
>   stripping is done directly in the vfs before calling into the
>   filesystem.
>   If the filesystem does support POSIX ACLs then unmask stripping may be
>   done in the filesystem itself when calling posix_acl_create().
> 
> * Filesystems that don't rely on inode_init_owner() don't get S_ISGID
>   stripping logic.
> 
>   While that may be intentional (e.g. network filesystems might just
>   defer setgid stripping to a server) it is often just a security issue.
> 
> * The first two points taken together mean that there's a
>   non-standardized ordering between setgid stripping in
>   inode_init_owner() and posix_acl_create() both on the vfs level and
>   the filesystem level. The latter part is especially problematic since
>   each filesystem is technically free to order inode_init_owner() and
>   posix_acl_create() however it sees fit meaning that S_ISGID
>   inheritance might or might not be applied.
> 
> * We do still have bugs in this areas years after the initial round of
>   setgid bugfixes.
> 
> So the current state is quite messy and while we won't be able to make
> it completely clean as posix_acl_create() is still a filesystem specific
> call we can improve the S_SIGD stripping situation quite a bit by
> hoisting it out of inode_init_owner() and into the vfs creation
> operations. This means we alleviate the burden for filesystems to handle
> S_ISGID stripping correctly and can standardize the ordering between
> S_ISGID and umask stripping in the vfs.
> 
> The S_ISGID bit is stripped before any umask is applied. This has the
> advantage that the ordering is unaffected by whether umask stripping is
> done by the vfs itself (if no POSIX ACLs are supported or enabled) or in
> the filesystem in posix_acl_create() (if POSIX ACLs are supported).
> 
> To this end a new helper vfs_prepare_mode() is added which calls the
> previously added mode_strip_setgid() helper and strips the umask
> afterwards.
> 
> All inode operations that create new filesystem objects have been
> updated to call vfs_prepare_mode() before passing the mode into the
> relevant inode operation of the filesystems. Care has been taken to
> ensure that the mode passed to the security hooks is the mode that is
> seen by the filesystem.
> 
> Following is an overview of the filesystem specific and inode operations
> specific implications:
> 
> arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
> arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
> fs/9p/vfs_inode.c:      inode_init_owner(&init_user_ns, inode, NULL, mode);
> fs/bfs/dir.c:   inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/btrfs/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
> fs/btrfs/tests/btrfs-tests.c:   inode_init_owner(&init_user_ns, inode, NULL, S_IFREG);
> fs/ext2/ialloc.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ext4/ialloc.c:               inode_init_owner(mnt_userns, inode, dir, mode);
> fs/f2fs/namei.c:        inode_init_owner(mnt_userns, inode, dir, mode);
> fs/hfsplus/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/hugetlbfs/inode.c:           inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/jfs/jfs_inode.c:     inode_init_owner(&init_user_ns, inode, parent, mode);
> fs/minix/bitmap.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/nilfs2/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ntfs3/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
> fs/ocfs2/dlmfs/dlmfs.c:         inode_init_owner(&init_user_ns, inode, NULL, mode);
> fs/ocfs2/dlmfs/dlmfs.c: inode_init_owner(&init_user_ns, inode, parent, mode);
> fs/ocfs2/namei.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/omfs/inode.c:        inode_init_owner(&init_user_ns, inode, NULL, mode);
> fs/overlayfs/dir.c:     inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
> fs/ramfs/inode.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/reiserfs/namei.c:    inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/sysv/ialloc.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ubifs/dir.c: inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/udf/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ufs/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/xfs/xfs_inode.c:             inode_init_owner(mnt_userns, inode, dir, mode);
> fs/zonefs/super.c:      inode_init_owner(&init_user_ns, inode, parent, S_IFDIR | 0555);
> kernel/bpf/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
> mm/shmem.c:             inode_init_owner(&init_user_ns, inode, dir, mode);
> 
> All of the above filesystems end up calling inode_init_owner() when new
> filesystem objects are created through the following ->mkdir(),
> ->symlink(), ->mknod(), ->create(), ->tmpfile(), ->rename() inode
> operations.
> 
> Since directories always inherit the S_ISGID bit with the exception of
> xfs when irix_sgid_inherit mode is turned on S_ISGID stripping doesn't
> apply. The ->symlink() inode operation trivially inherit the mode from
> the target and the ->rename() inode operation inherits the mode from the
> source inode.
> 
> All other inode operations will have the S_ISGID bit stripped once in
> vfs_prepare_mode() before.
> 
> In addition to this there are filesystems which allow the creation of
> filesystem objects through ioctl()s or - in the case of spufs -
> circumventing the vfs in other ways. If filesystem objects are created
> through ioctl()s the vfs doesn't know about it and can't apply regular
> permission checking including S_ISGID logic. Therfore, a filesystem
> relying on S_ISGID stripping in inode_init_owner() in their ioctl()
> callpath will be affected by moving this logic into the vfs.
> 
> So we did our best to audit all filesystems in this regard:
> 
> * btrfs allows the creation of filesystem objects through various
>   ioctls(). Snapshot creation literally takes a snapshot and so the mode
>   is fully preserved and S_ISGID stripping doesn't apply.
> 
>   Creating a new subvolum relies on inode_init_owner() in
>   btrfs_new_inode() but only creates directories and doesn't raise
>   S_ISGID.
> 
> * ocfs2 has a peculiar implementation of reflinks. In contrast to e.g.
>   xfs and btrfs FICLONE/FICLONERANGE ioctl() that is only concerned with
>   the actual extents ocfs2 uses a separate ioctl() that also creates the
>   target file.
> 
>   Iow, ocfs2 circumvents the vfs entirely here and did indeed rely on
>   inode_init_owner() to strip the S_ISGID bit. This is the only place
>   where a filesystem needs to call mode_strip_sgid() directly but this
>   is self-inflicted pain tbh.
> 
> * spufs doesn't go through the vfs at all and doesn't use ioctl()s
>   either. Instead it has a dedicated system call spufs_create() which
>   allows the creation of filesystem objects. But spufs only creates
>   directories and doesn't allo S_SIGID bits, i.e. it specifically only
>   allows 0777 bits.
> 
> * bpf uses vfs_mkobj() but also doesn't allow S_ISGID bits to be created.
> 
> While we did our best to audit everything there's a risk of regressions
> in here. However, for the sake of maintenance and given that we've seen
> a range of bugs years after S_ISGID inheritance issues were fixed (see
> [1]-[3]) the risk seems worth taking. In the worst case we will have to
> revert.
> 
> Associated with this change is a new set of fstests to enforce the
> semantics for all new filesystems.
> 
> Link: e014f37db1a2 ("xfs: use setattr_copy to set vfs inode attributes") [1]
> Link: 01ea173e103e ("xfs: fix up non-directory creation in SGID directories") [2]
> Link: fd84bfdddd16 ("ceph: fix up non-directory creation in SGID directories") [3]
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> Suggested-by: Dave Chinner <david@fromorbit.com>
> Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
> ---

Thanks for using my commit message!

One thing that I just remembered and which I think I haven't mentioned
so far is that moving S_ISGID stripping from filesystem callpaths into
the vfs callpaths means that we're hoisting this logic out of vfs_*()
helpers implicitly.

So filesystems that call vfs_*() helpers directly can't rely on S_ISGID
stripping being done in vfs_*() helpers anymore unless they pass the
mode on from a prior run through the vfs.

This mostly affects overlayfs which calls vfs_*() functions directly. So
a typical overlayfs callstack would be (roughly - I'm omw to lunch):

sys_mknod()
-> do_mknodat(mode) // calls vfs_prepare_mode()
   -> .mknod = ovl_mknod(mode)
      -> ovl_create(mode)
         -> vfs_mknod(mode)

I think we are safe as overlayfs passes on the mode on from its own run
through the vfs and then via vfs_*() to the underlying filesystem but it
is worth point that out.

Ccing Amir just for confirmation.

>  fs/inode.c         |  2 --
>  fs/namei.c         | 22 +++++++++-------------
>  fs/ocfs2/namei.c   |  1 +
>  include/linux/fs.h | 11 +++++++++++
>  4 files changed, 21 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index e9a5f2ec2f89..dd357f4b556d 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2246,8 +2246,6 @@ void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
>  		/* Directories are special, and always inherit S_ISGID */
>  		if (S_ISDIR(mode))
>  			mode |= S_ISGID;
> -		else
> -			mode = mode_strip_sgid(mnt_userns, dir, mode);
>  	} else
>  		inode_fsgid_set(inode, mnt_userns);
>  	inode->i_mode = mode;
> diff --git a/fs/namei.c b/fs/namei.c
> index 73646e28fae0..5dbf00704ae8 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3287,8 +3287,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
>  	if (open_flag & O_CREAT) {
>  		if (open_flag & O_EXCL)
>  			open_flag &= ~O_TRUNC;
> -		if (!IS_POSIXACL(dir->d_inode))
> -			mode &= ~current_umask();
> +		mode = vfs_prepare_mode(mnt_userns, dir->d_inode, mode);
>  		if (likely(got_write))
>  			create_error = may_o_create(mnt_userns, &nd->path,
>  						    dentry, mode);
> @@ -3521,8 +3520,7 @@ struct dentry *vfs_tmpfile(struct user_namespace *mnt_userns,
>  	child = d_alloc(dentry, &slash_name);
>  	if (unlikely(!child))
>  		goto out_err;
> -	if (!IS_POSIXACL(dir))
> -		mode &= ~current_umask();
> +	mode = vfs_prepare_mode(mnt_userns, dir, mode);
>  	error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
>  	if (error)
>  		goto out_err;
> @@ -3850,13 +3848,12 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
>  	if (IS_ERR(dentry))
>  		goto out1;
>  
> -	if (!IS_POSIXACL(path.dentry->d_inode))
> -		mode &= ~current_umask();
> +	mnt_userns = mnt_user_ns(path.mnt);
> +	mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
>  	error = security_path_mknod(&path, dentry, mode, dev);
>  	if (error)
>  		goto out2;
>  
> -	mnt_userns = mnt_user_ns(path.mnt);
>  	switch (mode & S_IFMT) {
>  		case 0: case S_IFREG:
>  			error = vfs_create(mnt_userns, path.dentry->d_inode,
> @@ -3943,6 +3940,7 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
>  	struct path path;
>  	int error;
>  	unsigned int lookup_flags = LOOKUP_DIRECTORY;
> +	struct user_namespace *mnt_userns;
>  
>  retry:
>  	dentry = filename_create(dfd, name, &path, lookup_flags);
> @@ -3950,15 +3948,13 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
>  	if (IS_ERR(dentry))
>  		goto out_putname;
>  
> -	if (!IS_POSIXACL(path.dentry->d_inode))
> -		mode &= ~current_umask();
> +	mnt_userns = mnt_user_ns(path.mnt);
> +	mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
>  	error = security_path_mkdir(&path, dentry, mode);
> -	if (!error) {
> -		struct user_namespace *mnt_userns;
> -		mnt_userns = mnt_user_ns(path.mnt);
> +	if (!error)
>  		error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry,
>  				  mode);
> -	}
> +
>  	done_path_create(&path, dentry);
>  	if (retry_estale(error, lookup_flags)) {
>  		lookup_flags |= LOOKUP_REVAL;
> diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
> index c75fd54b9185..961d1cf54388 100644
> --- a/fs/ocfs2/namei.c
> +++ b/fs/ocfs2/namei.c
> @@ -197,6 +197,7 @@ static struct inode *ocfs2_get_init_inode(struct inode *dir, umode_t mode)
>  	 * callers. */
>  	if (S_ISDIR(mode))
>  		set_nlink(inode, 2);
> +	mode = mode_strip_sgid(&init_user_ns, dir, mode);
>  	inode_init_owner(&init_user_ns, inode, dir, mode);
>  	status = dquot_initialize(inode);
>  	if (status)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 98b44a2732f5..914c8f28bb02 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3459,6 +3459,17 @@ static inline bool dir_relax_shared(struct inode *inode)
>  	return !IS_DEADDIR(inode);
>  }
>  
> +static inline umode_t vfs_prepare_mode(struct user_namespace *mnt_userns,
> +				   const struct inode *dir, umode_t mode)
> +{
> +	mode = mode_strip_sgid(mnt_userns, dir, mode);
> +
> +	if (!IS_POSIXACL(dir))
> +		mode &= ~current_umask();
> +
> +	return mode;
> +}
> +
>  extern bool path_noexec(const struct path *path);
>  extern void inode_nohighmem(struct inode *inode);
>  
> -- 
> 2.27.0
>
Amir Goldstein April 26, 2022, 11:20 a.m. UTC | #2
On Tue, Apr 26, 2022 at 1:38 PM Christian Brauner <brauner@kernel.org> wrote:
>
> On Tue, Apr 26, 2022 at 07:11:29PM +0800, Yang Xu wrote:
> > Creating files that have both the S_IXGRP and S_ISGID bit raised in
> > directories that themselves have the S_ISGID bit set requires additional
> > privileges to avoid security issues.
> >
> > When a filesystem creates a new inode it needs to take care that the
> > caller is either in the group of the newly created inode or they have
> > CAP_FSETID in their current user namespace and are privileged over the
> > parent directory of the new inode. If any of these two conditions is
> > true then the S_ISGID bit can be raised for an S_IXGRP file and if not
> > it needs to be stripped.
> >
> > However, there are several key issues with the current state of things:
> >
> > * The S_ISGID stripping logic is entangled with umask stripping.
> >
> >   If a filesystem doesn't support or enable POSIX ACLs then umask
> >   stripping is done directly in the vfs before calling into the
> >   filesystem.
> >   If the filesystem does support POSIX ACLs then unmask stripping may be
> >   done in the filesystem itself when calling posix_acl_create().
> >
> > * Filesystems that don't rely on inode_init_owner() don't get S_ISGID
> >   stripping logic.
> >
> >   While that may be intentional (e.g. network filesystems might just
> >   defer setgid stripping to a server) it is often just a security issue.
> >
> > * The first two points taken together mean that there's a
> >   non-standardized ordering between setgid stripping in
> >   inode_init_owner() and posix_acl_create() both on the vfs level and
> >   the filesystem level. The latter part is especially problematic since
> >   each filesystem is technically free to order inode_init_owner() and
> >   posix_acl_create() however it sees fit meaning that S_ISGID
> >   inheritance might or might not be applied.
> >
> > * We do still have bugs in this areas years after the initial round of
> >   setgid bugfixes.
> >
> > So the current state is quite messy and while we won't be able to make
> > it completely clean as posix_acl_create() is still a filesystem specific
> > call we can improve the S_SIGD stripping situation quite a bit by
> > hoisting it out of inode_init_owner() and into the vfs creation
> > operations. This means we alleviate the burden for filesystems to handle
> > S_ISGID stripping correctly and can standardize the ordering between
> > S_ISGID and umask stripping in the vfs.
> >
> > The S_ISGID bit is stripped before any umask is applied. This has the
> > advantage that the ordering is unaffected by whether umask stripping is
> > done by the vfs itself (if no POSIX ACLs are supported or enabled) or in
> > the filesystem in posix_acl_create() (if POSIX ACLs are supported).
> >
> > To this end a new helper vfs_prepare_mode() is added which calls the
> > previously added mode_strip_setgid() helper and strips the umask
> > afterwards.
> >
> > All inode operations that create new filesystem objects have been
> > updated to call vfs_prepare_mode() before passing the mode into the
> > relevant inode operation of the filesystems. Care has been taken to
> > ensure that the mode passed to the security hooks is the mode that is
> > seen by the filesystem.
> >
> > Following is an overview of the filesystem specific and inode operations
> > specific implications:
> >
> > arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
> > arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
> > fs/9p/vfs_inode.c:      inode_init_owner(&init_user_ns, inode, NULL, mode);
> > fs/bfs/dir.c:   inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/btrfs/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
> > fs/btrfs/tests/btrfs-tests.c:   inode_init_owner(&init_user_ns, inode, NULL, S_IFREG);
> > fs/ext2/ialloc.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/ext4/ialloc.c:               inode_init_owner(mnt_userns, inode, dir, mode);
> > fs/f2fs/namei.c:        inode_init_owner(mnt_userns, inode, dir, mode);
> > fs/hfsplus/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/hugetlbfs/inode.c:           inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/jfs/jfs_inode.c:     inode_init_owner(&init_user_ns, inode, parent, mode);
> > fs/minix/bitmap.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/nilfs2/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/ntfs3/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
> > fs/ocfs2/dlmfs/dlmfs.c:         inode_init_owner(&init_user_ns, inode, NULL, mode);
> > fs/ocfs2/dlmfs/dlmfs.c: inode_init_owner(&init_user_ns, inode, parent, mode);
> > fs/ocfs2/namei.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/omfs/inode.c:        inode_init_owner(&init_user_ns, inode, NULL, mode);
> > fs/overlayfs/dir.c:     inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
> > fs/ramfs/inode.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/reiserfs/namei.c:    inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/sysv/ialloc.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/ubifs/dir.c: inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/udf/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/ufs/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
> > fs/xfs/xfs_inode.c:             inode_init_owner(mnt_userns, inode, dir, mode);
> > fs/zonefs/super.c:      inode_init_owner(&init_user_ns, inode, parent, S_IFDIR | 0555);
> > kernel/bpf/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
> > mm/shmem.c:             inode_init_owner(&init_user_ns, inode, dir, mode);
> >
> > All of the above filesystems end up calling inode_init_owner() when new
> > filesystem objects are created through the following ->mkdir(),
> > ->symlink(), ->mknod(), ->create(), ->tmpfile(), ->rename() inode
> > operations.
> >
> > Since directories always inherit the S_ISGID bit with the exception of
> > xfs when irix_sgid_inherit mode is turned on S_ISGID stripping doesn't
> > apply. The ->symlink() inode operation trivially inherit the mode from
> > the target and the ->rename() inode operation inherits the mode from the
> > source inode.
> >
> > All other inode operations will have the S_ISGID bit stripped once in
> > vfs_prepare_mode() before.
> >
> > In addition to this there are filesystems which allow the creation of
> > filesystem objects through ioctl()s or - in the case of spufs -
> > circumventing the vfs in other ways. If filesystem objects are created
> > through ioctl()s the vfs doesn't know about it and can't apply regular
> > permission checking including S_ISGID logic. Therfore, a filesystem
> > relying on S_ISGID stripping in inode_init_owner() in their ioctl()
> > callpath will be affected by moving this logic into the vfs.
> >
> > So we did our best to audit all filesystems in this regard:
> >
> > * btrfs allows the creation of filesystem objects through various
> >   ioctls(). Snapshot creation literally takes a snapshot and so the mode
> >   is fully preserved and S_ISGID stripping doesn't apply.
> >
> >   Creating a new subvolum relies on inode_init_owner() in
> >   btrfs_new_inode() but only creates directories and doesn't raise
> >   S_ISGID.
> >
> > * ocfs2 has a peculiar implementation of reflinks. In contrast to e.g.
> >   xfs and btrfs FICLONE/FICLONERANGE ioctl() that is only concerned with
> >   the actual extents ocfs2 uses a separate ioctl() that also creates the
> >   target file.
> >
> >   Iow, ocfs2 circumvents the vfs entirely here and did indeed rely on
> >   inode_init_owner() to strip the S_ISGID bit. This is the only place
> >   where a filesystem needs to call mode_strip_sgid() directly but this
> >   is self-inflicted pain tbh.
> >
> > * spufs doesn't go through the vfs at all and doesn't use ioctl()s
> >   either. Instead it has a dedicated system call spufs_create() which
> >   allows the creation of filesystem objects. But spufs only creates
> >   directories and doesn't allo S_SIGID bits, i.e. it specifically only
> >   allows 0777 bits.
> >
> > * bpf uses vfs_mkobj() but also doesn't allow S_ISGID bits to be created.
> >
> > While we did our best to audit everything there's a risk of regressions
> > in here. However, for the sake of maintenance and given that we've seen
> > a range of bugs years after S_ISGID inheritance issues were fixed (see
> > [1]-[3]) the risk seems worth taking. In the worst case we will have to
> > revert.
> >
> > Associated with this change is a new set of fstests to enforce the
> > semantics for all new filesystems.
> >
> > Link: e014f37db1a2 ("xfs: use setattr_copy to set vfs inode attributes") [1]
> > Link: 01ea173e103e ("xfs: fix up non-directory creation in SGID directories") [2]
> > Link: fd84bfdddd16 ("ceph: fix up non-directory creation in SGID directories") [3]
> > Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> > Suggested-by: Dave Chinner <david@fromorbit.com>
> > Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
> > ---
>
> Thanks for using my commit message!
>
> One thing that I just remembered and which I think I haven't mentioned
> so far is that moving S_ISGID stripping from filesystem callpaths into
> the vfs callpaths means that we're hoisting this logic out of vfs_*()
> helpers implicitly.
>
> So filesystems that call vfs_*() helpers directly can't rely on S_ISGID
> stripping being done in vfs_*() helpers anymore unless they pass the
> mode on from a prior run through the vfs.
>
> This mostly affects overlayfs which calls vfs_*() functions directly. So
> a typical overlayfs callstack would be (roughly - I'm omw to lunch):
>
> sys_mknod()
> -> do_mknodat(mode) // calls vfs_prepare_mode()
>    -> .mknod = ovl_mknod(mode)
>       -> ovl_create(mode)
>          -> vfs_mknod(mode)
>
> I think we are safe as overlayfs passes on the mode on from its own run
> through the vfs and then via vfs_*() to the underlying filesystem but it
> is worth point that out.
>
> Ccing Amir just for confirmation.

Looks fine to me, but CC Miklos ...

Thanks,
Amir.

>
> >  fs/inode.c         |  2 --
> >  fs/namei.c         | 22 +++++++++-------------
> >  fs/ocfs2/namei.c   |  1 +
> >  include/linux/fs.h | 11 +++++++++++
> >  4 files changed, 21 insertions(+), 15 deletions(-)
> >
> > diff --git a/fs/inode.c b/fs/inode.c
> > index e9a5f2ec2f89..dd357f4b556d 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -2246,8 +2246,6 @@ void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
> >               /* Directories are special, and always inherit S_ISGID */
> >               if (S_ISDIR(mode))
> >                       mode |= S_ISGID;
> > -             else
> > -                     mode = mode_strip_sgid(mnt_userns, dir, mode);
> >       } else
> >               inode_fsgid_set(inode, mnt_userns);
> >       inode->i_mode = mode;
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 73646e28fae0..5dbf00704ae8 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -3287,8 +3287,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
> >       if (open_flag & O_CREAT) {
> >               if (open_flag & O_EXCL)
> >                       open_flag &= ~O_TRUNC;
> > -             if (!IS_POSIXACL(dir->d_inode))
> > -                     mode &= ~current_umask();
> > +             mode = vfs_prepare_mode(mnt_userns, dir->d_inode, mode);
> >               if (likely(got_write))
> >                       create_error = may_o_create(mnt_userns, &nd->path,
> >                                                   dentry, mode);
> > @@ -3521,8 +3520,7 @@ struct dentry *vfs_tmpfile(struct user_namespace *mnt_userns,
> >       child = d_alloc(dentry, &slash_name);
> >       if (unlikely(!child))
> >               goto out_err;
> > -     if (!IS_POSIXACL(dir))
> > -             mode &= ~current_umask();
> > +     mode = vfs_prepare_mode(mnt_userns, dir, mode);
> >       error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
> >       if (error)
> >               goto out_err;
> > @@ -3850,13 +3848,12 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
> >       if (IS_ERR(dentry))
> >               goto out1;
> >
> > -     if (!IS_POSIXACL(path.dentry->d_inode))
> > -             mode &= ~current_umask();
> > +     mnt_userns = mnt_user_ns(path.mnt);
> > +     mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
> >       error = security_path_mknod(&path, dentry, mode, dev);
> >       if (error)
> >               goto out2;
> >
> > -     mnt_userns = mnt_user_ns(path.mnt);
> >       switch (mode & S_IFMT) {
> >               case 0: case S_IFREG:
> >                       error = vfs_create(mnt_userns, path.dentry->d_inode,
> > @@ -3943,6 +3940,7 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
> >       struct path path;
> >       int error;
> >       unsigned int lookup_flags = LOOKUP_DIRECTORY;
> > +     struct user_namespace *mnt_userns;
> >
> >  retry:
> >       dentry = filename_create(dfd, name, &path, lookup_flags);
> > @@ -3950,15 +3948,13 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
> >       if (IS_ERR(dentry))
> >               goto out_putname;
> >
> > -     if (!IS_POSIXACL(path.dentry->d_inode))
> > -             mode &= ~current_umask();
> > +     mnt_userns = mnt_user_ns(path.mnt);
> > +     mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
> >       error = security_path_mkdir(&path, dentry, mode);
> > -     if (!error) {
> > -             struct user_namespace *mnt_userns;
> > -             mnt_userns = mnt_user_ns(path.mnt);
> > +     if (!error)
> >               error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry,
> >                                 mode);
> > -     }
> > +
> >       done_path_create(&path, dentry);
> >       if (retry_estale(error, lookup_flags)) {
> >               lookup_flags |= LOOKUP_REVAL;
> > diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
> > index c75fd54b9185..961d1cf54388 100644
> > --- a/fs/ocfs2/namei.c
> > +++ b/fs/ocfs2/namei.c
> > @@ -197,6 +197,7 @@ static struct inode *ocfs2_get_init_inode(struct inode *dir, umode_t mode)
> >        * callers. */
> >       if (S_ISDIR(mode))
> >               set_nlink(inode, 2);
> > +     mode = mode_strip_sgid(&init_user_ns, dir, mode);
> >       inode_init_owner(&init_user_ns, inode, dir, mode);
> >       status = dquot_initialize(inode);
> >       if (status)
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 98b44a2732f5..914c8f28bb02 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -3459,6 +3459,17 @@ static inline bool dir_relax_shared(struct inode *inode)
> >       return !IS_DEADDIR(inode);
> >  }
> >
> > +static inline umode_t vfs_prepare_mode(struct user_namespace *mnt_userns,
> > +                                const struct inode *dir, umode_t mode)
> > +{
> > +     mode = mode_strip_sgid(mnt_userns, dir, mode);
> > +
> > +     if (!IS_POSIXACL(dir))
> > +             mode &= ~current_umask();
> > +
> > +     return mode;
> > +}
> > +
> >  extern bool path_noexec(const struct path *path);
> >  extern void inode_nohighmem(struct inode *inode);
> >
> > --
> > 2.27.0
> >
Miklos Szeredi April 26, 2022, 11:52 a.m. UTC | #3
On Tue, 26 Apr 2022 at 13:21, Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Tue, Apr 26, 2022 at 1:38 PM Christian Brauner <brauner@kernel.org> wrote:

> > One thing that I just remembered and which I think I haven't mentioned
> > so far is that moving S_ISGID stripping from filesystem callpaths into
> > the vfs callpaths means that we're hoisting this logic out of vfs_*()
> > helpers implicitly.
> >
> > So filesystems that call vfs_*() helpers directly can't rely on S_ISGID
> > stripping being done in vfs_*() helpers anymore unless they pass the
> > mode on from a prior run through the vfs.
> >
> > This mostly affects overlayfs which calls vfs_*() functions directly. So
> > a typical overlayfs callstack would be (roughly - I'm omw to lunch):
> >
> > sys_mknod()
> > -> do_mknodat(mode) // calls vfs_prepare_mode()
> >    -> .mknod = ovl_mknod(mode)
> >       -> ovl_create(mode)
> >          -> vfs_mknod(mode)
> >
> > I think we are safe as overlayfs passes on the mode on from its own run
> > through the vfs and then via vfs_*() to the underlying filesystem but it
> > is worth point that out.
> >
> > Ccing Amir just for confirmation.
>
> Looks fine to me, but CC Miklos ...

Looks fine to me as well.  Overlayfs should share the mode (including
the suid and sgid bits), owner, group and ACL's with the underlying
filesystem, so clearing sgid based on overlay parent directory should
result in the same mode as if it was done based on the parent
directory on the underlying layer.

AFAIU this logic is not affected by userns or mnt_userns, but
Christian would be best to confirm that.

Thanks,
Miklos
Christian Brauner April 26, 2022, 2:53 p.m. UTC | #4
On Tue, Apr 26, 2022 at 01:52:11PM +0200, Miklos Szeredi wrote:
> On Tue, 26 Apr 2022 at 13:21, Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > On Tue, Apr 26, 2022 at 1:38 PM Christian Brauner <brauner@kernel.org> wrote:
> 
> > > One thing that I just remembered and which I think I haven't mentioned
> > > so far is that moving S_ISGID stripping from filesystem callpaths into
> > > the vfs callpaths means that we're hoisting this logic out of vfs_*()
> > > helpers implicitly.
> > >
> > > So filesystems that call vfs_*() helpers directly can't rely on S_ISGID
> > > stripping being done in vfs_*() helpers anymore unless they pass the
> > > mode on from a prior run through the vfs.
> > >
> > > This mostly affects overlayfs which calls vfs_*() functions directly. So
> > > a typical overlayfs callstack would be (roughly - I'm omw to lunch):
> > >
> > > sys_mknod()
> > > -> do_mknodat(mode) // calls vfs_prepare_mode()
> > >    -> .mknod = ovl_mknod(mode)
> > >       -> ovl_create(mode)
> > >          -> vfs_mknod(mode)
> > >
> > > I think we are safe as overlayfs passes on the mode on from its own run
> > > through the vfs and then via vfs_*() to the underlying filesystem but it
> > > is worth point that out.
> > >
> > > Ccing Amir just for confirmation.
> >
> > Looks fine to me, but CC Miklos ...
> 
> Looks fine to me as well.  Overlayfs should share the mode (including
> the suid and sgid bits), owner, group and ACL's with the underlying
> filesystem, so clearing sgid based on overlay parent directory should
> result in the same mode as if it was done based on the parent
> directory on the underlying layer.

Ah yes, good point.

> 
> AFAIU this logic is not affected by userns or mnt_userns, but
> Christian would be best to confirm that.

It does depend on it as S_ISGID stripping requires knowledge about
whether the caller has CAP_FSETID and is capable over the parent
directory or if they are in the group the file is owned by.

I think ultimately it might just come down to moving vfs_prepare_mode()
into vfs_*() helpers and not into the do_*at() helpers.

That would be cleaner anyway as right now we have this weird disconnect
between vfs_tmpfile() and vfs_{create,mknod,mkdir}(). IOW, vfs_tmpfile()
doesn't even have an associated do_*() wrapper where we could call
vfs_prepare_mode() from.

So ultimately it might be nicer if we do it in vfs_*() helpers anyway.

The less pretty thing about it will be that the security_path_*() hooks
also want a mode.

Right now these hooks receive the mode as it's passed in from userspace
minus umask but before S_ISGID stripping happens.

Whereas I think they should really see what the filesystem sees and
currently it's a bug that they see something else.

I need to think about this a bit.
Christian Brauner April 26, 2022, 3:52 p.m. UTC | #5
On Tue, Apr 26, 2022 at 07:11:29PM +0800, Yang Xu wrote:
> Creating files that have both the S_IXGRP and S_ISGID bit raised in
> directories that themselves have the S_ISGID bit set requires additional
> privileges to avoid security issues.
> 
> When a filesystem creates a new inode it needs to take care that the
> caller is either in the group of the newly created inode or they have
> CAP_FSETID in their current user namespace and are privileged over the
> parent directory of the new inode. If any of these two conditions is
> true then the S_ISGID bit can be raised for an S_IXGRP file and if not
> it needs to be stripped.
> 
> However, there are several key issues with the current state of things:
> 
> * The S_ISGID stripping logic is entangled with umask stripping.
> 
>   If a filesystem doesn't support or enable POSIX ACLs then umask
>   stripping is done directly in the vfs before calling into the
>   filesystem.
>   If the filesystem does support POSIX ACLs then unmask stripping may be
>   done in the filesystem itself when calling posix_acl_create().
> 
> * Filesystems that don't rely on inode_init_owner() don't get S_ISGID
>   stripping logic.
> 
>   While that may be intentional (e.g. network filesystems might just
>   defer setgid stripping to a server) it is often just a security issue.
> 
> * The first two points taken together mean that there's a
>   non-standardized ordering between setgid stripping in
>   inode_init_owner() and posix_acl_create() both on the vfs level and
>   the filesystem level. The latter part is especially problematic since
>   each filesystem is technically free to order inode_init_owner() and
>   posix_acl_create() however it sees fit meaning that S_ISGID
>   inheritance might or might not be applied.
> 
> * We do still have bugs in this areas years after the initial round of
>   setgid bugfixes.
> 
> So the current state is quite messy and while we won't be able to make
> it completely clean as posix_acl_create() is still a filesystem specific
> call we can improve the S_SIGD stripping situation quite a bit by
> hoisting it out of inode_init_owner() and into the vfs creation
> operations. This means we alleviate the burden for filesystems to handle
> S_ISGID stripping correctly and can standardize the ordering between
> S_ISGID and umask stripping in the vfs.
> 
> The S_ISGID bit is stripped before any umask is applied. This has the
> advantage that the ordering is unaffected by whether umask stripping is
> done by the vfs itself (if no POSIX ACLs are supported or enabled) or in
> the filesystem in posix_acl_create() (if POSIX ACLs are supported).
> 
> To this end a new helper vfs_prepare_mode() is added which calls the
> previously added mode_strip_setgid() helper and strips the umask
> afterwards.
> 
> All inode operations that create new filesystem objects have been
> updated to call vfs_prepare_mode() before passing the mode into the
> relevant inode operation of the filesystems. Care has been taken to
> ensure that the mode passed to the security hooks is the mode that is
> seen by the filesystem.
> 
> Following is an overview of the filesystem specific and inode operations
> specific implications:
> 
> arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
> arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
> fs/9p/vfs_inode.c:      inode_init_owner(&init_user_ns, inode, NULL, mode);
> fs/bfs/dir.c:   inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/btrfs/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
> fs/btrfs/tests/btrfs-tests.c:   inode_init_owner(&init_user_ns, inode, NULL, S_IFREG);
> fs/ext2/ialloc.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ext4/ialloc.c:               inode_init_owner(mnt_userns, inode, dir, mode);
> fs/f2fs/namei.c:        inode_init_owner(mnt_userns, inode, dir, mode);
> fs/hfsplus/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/hugetlbfs/inode.c:           inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/jfs/jfs_inode.c:     inode_init_owner(&init_user_ns, inode, parent, mode);
> fs/minix/bitmap.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/nilfs2/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ntfs3/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
> fs/ocfs2/dlmfs/dlmfs.c:         inode_init_owner(&init_user_ns, inode, NULL, mode);
> fs/ocfs2/dlmfs/dlmfs.c: inode_init_owner(&init_user_ns, inode, parent, mode);
> fs/ocfs2/namei.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/omfs/inode.c:        inode_init_owner(&init_user_ns, inode, NULL, mode);
> fs/overlayfs/dir.c:     inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
> fs/ramfs/inode.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/reiserfs/namei.c:    inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/sysv/ialloc.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ubifs/dir.c: inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/udf/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/ufs/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
> fs/xfs/xfs_inode.c:             inode_init_owner(mnt_userns, inode, dir, mode);
> fs/zonefs/super.c:      inode_init_owner(&init_user_ns, inode, parent, S_IFDIR | 0555);
> kernel/bpf/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
> mm/shmem.c:             inode_init_owner(&init_user_ns, inode, dir, mode);
> 
> All of the above filesystems end up calling inode_init_owner() when new
> filesystem objects are created through the following ->mkdir(),
> ->symlink(), ->mknod(), ->create(), ->tmpfile(), ->rename() inode
> operations.
> 
> Since directories always inherit the S_ISGID bit with the exception of
> xfs when irix_sgid_inherit mode is turned on S_ISGID stripping doesn't
> apply. The ->symlink() inode operation trivially inherit the mode from
> the target and the ->rename() inode operation inherits the mode from the
> source inode.
> 
> All other inode operations will have the S_ISGID bit stripped once in
> vfs_prepare_mode() before.
> 
> In addition to this there are filesystems which allow the creation of
> filesystem objects through ioctl()s or - in the case of spufs -
> circumventing the vfs in other ways. If filesystem objects are created
> through ioctl()s the vfs doesn't know about it and can't apply regular
> permission checking including S_ISGID logic. Therfore, a filesystem
> relying on S_ISGID stripping in inode_init_owner() in their ioctl()
> callpath will be affected by moving this logic into the vfs.
> 
> So we did our best to audit all filesystems in this regard:
> 
> * btrfs allows the creation of filesystem objects through various
>   ioctls(). Snapshot creation literally takes a snapshot and so the mode
>   is fully preserved and S_ISGID stripping doesn't apply.
> 
>   Creating a new subvolum relies on inode_init_owner() in
>   btrfs_new_inode() but only creates directories and doesn't raise
>   S_ISGID.
> 
> * ocfs2 has a peculiar implementation of reflinks. In contrast to e.g.
>   xfs and btrfs FICLONE/FICLONERANGE ioctl() that is only concerned with
>   the actual extents ocfs2 uses a separate ioctl() that also creates the
>   target file.
> 
>   Iow, ocfs2 circumvents the vfs entirely here and did indeed rely on
>   inode_init_owner() to strip the S_ISGID bit. This is the only place
>   where a filesystem needs to call mode_strip_sgid() directly but this
>   is self-inflicted pain tbh.
> 
> * spufs doesn't go through the vfs at all and doesn't use ioctl()s
>   either. Instead it has a dedicated system call spufs_create() which
>   allows the creation of filesystem objects. But spufs only creates
>   directories and doesn't allo S_SIGID bits, i.e. it specifically only
>   allows 0777 bits.
> 
> * bpf uses vfs_mkobj() but also doesn't allow S_ISGID bits to be created.
> 
> While we did our best to audit everything there's a risk of regressions
> in here. However, for the sake of maintenance and given that we've seen
> a range of bugs years after S_ISGID inheritance issues were fixed (see
> [1]-[3]) the risk seems worth taking. In the worst case we will have to
> revert.
> 
> Associated with this change is a new set of fstests to enforce the
> semantics for all new filesystems.
> 
> Link: e014f37db1a2 ("xfs: use setattr_copy to set vfs inode attributes") [1]
> Link: 01ea173e103e ("xfs: fix up non-directory creation in SGID directories") [2]
> Link: fd84bfdddd16 ("ceph: fix up non-directory creation in SGID directories") [3]
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> Suggested-by: Dave Chinner <david@fromorbit.com>
> Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
> ---
>  fs/inode.c         |  2 --
>  fs/namei.c         | 22 +++++++++-------------
>  fs/ocfs2/namei.c   |  1 +
>  include/linux/fs.h | 11 +++++++++++
>  4 files changed, 21 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index e9a5f2ec2f89..dd357f4b556d 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2246,8 +2246,6 @@ void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
>  		/* Directories are special, and always inherit S_ISGID */
>  		if (S_ISDIR(mode))
>  			mode |= S_ISGID;
> -		else
> -			mode = mode_strip_sgid(mnt_userns, dir, mode);
>  	} else
>  		inode_fsgid_set(inode, mnt_userns);
>  	inode->i_mode = mode;
> diff --git a/fs/namei.c b/fs/namei.c
> index 73646e28fae0..5dbf00704ae8 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3287,8 +3287,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
>  	if (open_flag & O_CREAT) {
>  		if (open_flag & O_EXCL)
>  			open_flag &= ~O_TRUNC;
> -		if (!IS_POSIXACL(dir->d_inode))
> -			mode &= ~current_umask();
> +		mode = vfs_prepare_mode(mnt_userns, dir->d_inode, mode);
>  		if (likely(got_write))
>  			create_error = may_o_create(mnt_userns, &nd->path,
>  						    dentry, mode);
> @@ -3521,8 +3520,7 @@ struct dentry *vfs_tmpfile(struct user_namespace *mnt_userns,
>  	child = d_alloc(dentry, &slash_name);
>  	if (unlikely(!child))
>  		goto out_err;
> -	if (!IS_POSIXACL(dir))
> -		mode &= ~current_umask();
> +	mode = vfs_prepare_mode(mnt_userns, dir, mode);
>  	error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
>  	if (error)
>  		goto out_err;
> @@ -3850,13 +3848,12 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
>  	if (IS_ERR(dentry))
>  		goto out1;
>  
> -	if (!IS_POSIXACL(path.dentry->d_inode))
> -		mode &= ~current_umask();
> +	mnt_userns = mnt_user_ns(path.mnt);
> +	mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
>  	error = security_path_mknod(&path, dentry, mode, dev);
>  	if (error)
>  		goto out2;
>  
> -	mnt_userns = mnt_user_ns(path.mnt);
>  	switch (mode & S_IFMT) {
>  		case 0: case S_IFREG:
>  			error = vfs_create(mnt_userns, path.dentry->d_inode,
> @@ -3943,6 +3940,7 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
>  	struct path path;
>  	int error;
>  	unsigned int lookup_flags = LOOKUP_DIRECTORY;
> +	struct user_namespace *mnt_userns;
>  
>  retry:
>  	dentry = filename_create(dfd, name, &path, lookup_flags);
> @@ -3950,15 +3948,13 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
>  	if (IS_ERR(dentry))
>  		goto out_putname;
>  
> -	if (!IS_POSIXACL(path.dentry->d_inode))
> -		mode &= ~current_umask();
> +	mnt_userns = mnt_user_ns(path.mnt);
> +	mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
>  	error = security_path_mkdir(&path, dentry, mode);
> -	if (!error) {
> -		struct user_namespace *mnt_userns;
> -		mnt_userns = mnt_user_ns(path.mnt);
> +	if (!error)
>  		error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry,
>  				  mode);
> -	}
> +
>  	done_path_create(&path, dentry);
>  	if (retry_estale(error, lookup_flags)) {
>  		lookup_flags |= LOOKUP_REVAL;
> diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
> index c75fd54b9185..961d1cf54388 100644
> --- a/fs/ocfs2/namei.c
> +++ b/fs/ocfs2/namei.c
> @@ -197,6 +197,7 @@ static struct inode *ocfs2_get_init_inode(struct inode *dir, umode_t mode)
>  	 * callers. */
>  	if (S_ISDIR(mode))
>  		set_nlink(inode, 2);
> +	mode = mode_strip_sgid(&init_user_ns, dir, mode);
>  	inode_init_owner(&init_user_ns, inode, dir, mode);
>  	status = dquot_initialize(inode);
>  	if (status)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 98b44a2732f5..914c8f28bb02 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -3459,6 +3459,17 @@ static inline bool dir_relax_shared(struct inode *inode)
>  	return !IS_DEADDIR(inode);
>  }
>  
> +static inline umode_t vfs_prepare_mode(struct user_namespace *mnt_userns,
> +				   const struct inode *dir, umode_t mode)

Sorry, since you're only calling the helper in fs/namei.c you don't need
to expose it in fs.h; just keep it local to fs/namei.c.
Yang Xu April 27, 2022, 1:21 a.m. UTC | #6
on 2022/4/26 23:52, Christian Brauner wrote:
> On Tue, Apr 26, 2022 at 07:11:29PM +0800, Yang Xu wrote:
>> Creating files that have both the S_IXGRP and S_ISGID bit raised in
>> directories that themselves have the S_ISGID bit set requires additional
>> privileges to avoid security issues.
>>
>> When a filesystem creates a new inode it needs to take care that the
>> caller is either in the group of the newly created inode or they have
>> CAP_FSETID in their current user namespace and are privileged over the
>> parent directory of the new inode. If any of these two conditions is
>> true then the S_ISGID bit can be raised for an S_IXGRP file and if not
>> it needs to be stripped.
>>
>> However, there are several key issues with the current state of things:
>>
>> * The S_ISGID stripping logic is entangled with umask stripping.
>>
>>    If a filesystem doesn't support or enable POSIX ACLs then umask
>>    stripping is done directly in the vfs before calling into the
>>    filesystem.
>>    If the filesystem does support POSIX ACLs then unmask stripping may be
>>    done in the filesystem itself when calling posix_acl_create().
>>
>> * Filesystems that don't rely on inode_init_owner() don't get S_ISGID
>>    stripping logic.
>>
>>    While that may be intentional (e.g. network filesystems might just
>>    defer setgid stripping to a server) it is often just a security issue.
>>
>> * The first two points taken together mean that there's a
>>    non-standardized ordering between setgid stripping in
>>    inode_init_owner() and posix_acl_create() both on the vfs level and
>>    the filesystem level. The latter part is especially problematic since
>>    each filesystem is technically free to order inode_init_owner() and
>>    posix_acl_create() however it sees fit meaning that S_ISGID
>>    inheritance might or might not be applied.
>>
>> * We do still have bugs in this areas years after the initial round of
>>    setgid bugfixes.
>>
>> So the current state is quite messy and while we won't be able to make
>> it completely clean as posix_acl_create() is still a filesystem specific
>> call we can improve the S_SIGD stripping situation quite a bit by
>> hoisting it out of inode_init_owner() and into the vfs creation
>> operations. This means we alleviate the burden for filesystems to handle
>> S_ISGID stripping correctly and can standardize the ordering between
>> S_ISGID and umask stripping in the vfs.
>>
>> The S_ISGID bit is stripped before any umask is applied. This has the
>> advantage that the ordering is unaffected by whether umask stripping is
>> done by the vfs itself (if no POSIX ACLs are supported or enabled) or in
>> the filesystem in posix_acl_create() (if POSIX ACLs are supported).
>>
>> To this end a new helper vfs_prepare_mode() is added which calls the
>> previously added mode_strip_setgid() helper and strips the umask
>> afterwards.
>>
>> All inode operations that create new filesystem objects have been
>> updated to call vfs_prepare_mode() before passing the mode into the
>> relevant inode operation of the filesystems. Care has been taken to
>> ensure that the mode passed to the security hooks is the mode that is
>> seen by the filesystem.
>>
>> Following is an overview of the filesystem specific and inode operations
>> specific implications:
>>
>> arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
>> arch/powerpc/platforms/cell/spufs/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode | S_IFDIR);
>> fs/9p/vfs_inode.c:      inode_init_owner(&init_user_ns, inode, NULL, mode);
>> fs/bfs/dir.c:   inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/btrfs/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
>> fs/btrfs/tests/btrfs-tests.c:   inode_init_owner(&init_user_ns, inode, NULL, S_IFREG);
>> fs/ext2/ialloc.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/ext4/ialloc.c:               inode_init_owner(mnt_userns, inode, dir, mode);
>> fs/f2fs/namei.c:        inode_init_owner(mnt_userns, inode, dir, mode);
>> fs/hfsplus/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/hugetlbfs/inode.c:           inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/jfs/jfs_inode.c:     inode_init_owner(&init_user_ns, inode, parent, mode);
>> fs/minix/bitmap.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/nilfs2/inode.c:      inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/ntfs3/inode.c:       inode_init_owner(mnt_userns, inode, dir, mode);
>> fs/ocfs2/dlmfs/dlmfs.c:         inode_init_owner(&init_user_ns, inode, NULL, mode);
>> fs/ocfs2/dlmfs/dlmfs.c: inode_init_owner(&init_user_ns, inode, parent, mode);
>> fs/ocfs2/namei.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/omfs/inode.c:        inode_init_owner(&init_user_ns, inode, NULL, mode);
>> fs/overlayfs/dir.c:     inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
>> fs/ramfs/inode.c:               inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/reiserfs/namei.c:    inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/sysv/ialloc.c:       inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/ubifs/dir.c: inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/udf/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/ufs/ialloc.c:        inode_init_owner(&init_user_ns, inode, dir, mode);
>> fs/xfs/xfs_inode.c:             inode_init_owner(mnt_userns, inode, dir, mode);
>> fs/zonefs/super.c:      inode_init_owner(&init_user_ns, inode, parent, S_IFDIR | 0555);
>> kernel/bpf/inode.c:     inode_init_owner(&init_user_ns, inode, dir, mode);
>> mm/shmem.c:             inode_init_owner(&init_user_ns, inode, dir, mode);
>>
>> All of the above filesystems end up calling inode_init_owner() when new
>> filesystem objects are created through the following ->mkdir(),
>> ->symlink(), ->mknod(), ->create(), ->tmpfile(), ->rename() inode
>> operations.
>>
>> Since directories always inherit the S_ISGID bit with the exception of
>> xfs when irix_sgid_inherit mode is turned on S_ISGID stripping doesn't
>> apply. The ->symlink() inode operation trivially inherit the mode from
>> the target and the ->rename() inode operation inherits the mode from the
>> source inode.
>>
>> All other inode operations will have the S_ISGID bit stripped once in
>> vfs_prepare_mode() before.
>>
>> In addition to this there are filesystems which allow the creation of
>> filesystem objects through ioctl()s or - in the case of spufs -
>> circumventing the vfs in other ways. If filesystem objects are created
>> through ioctl()s the vfs doesn't know about it and can't apply regular
>> permission checking including S_ISGID logic. Therfore, a filesystem
>> relying on S_ISGID stripping in inode_init_owner() in their ioctl()
>> callpath will be affected by moving this logic into the vfs.
>>
>> So we did our best to audit all filesystems in this regard:
>>
>> * btrfs allows the creation of filesystem objects through various
>>    ioctls(). Snapshot creation literally takes a snapshot and so the mode
>>    is fully preserved and S_ISGID stripping doesn't apply.
>>
>>    Creating a new subvolum relies on inode_init_owner() in
>>    btrfs_new_inode() but only creates directories and doesn't raise
>>    S_ISGID.
>>
>> * ocfs2 has a peculiar implementation of reflinks. In contrast to e.g.
>>    xfs and btrfs FICLONE/FICLONERANGE ioctl() that is only concerned with
>>    the actual extents ocfs2 uses a separate ioctl() that also creates the
>>    target file.
>>
>>    Iow, ocfs2 circumvents the vfs entirely here and did indeed rely on
>>    inode_init_owner() to strip the S_ISGID bit. This is the only place
>>    where a filesystem needs to call mode_strip_sgid() directly but this
>>    is self-inflicted pain tbh.
>>
>> * spufs doesn't go through the vfs at all and doesn't use ioctl()s
>>    either. Instead it has a dedicated system call spufs_create() which
>>    allows the creation of filesystem objects. But spufs only creates
>>    directories and doesn't allo S_SIGID bits, i.e. it specifically only
>>    allows 0777 bits.
>>
>> * bpf uses vfs_mkobj() but also doesn't allow S_ISGID bits to be created.
>>
>> While we did our best to audit everything there's a risk of regressions
>> in here. However, for the sake of maintenance and given that we've seen
>> a range of bugs years after S_ISGID inheritance issues were fixed (see
>> [1]-[3]) the risk seems worth taking. In the worst case we will have to
>> revert.
>>
>> Associated with this change is a new set of fstests to enforce the
>> semantics for all new filesystems.
>>
>> Link: e014f37db1a2 ("xfs: use setattr_copy to set vfs inode attributes") [1]
>> Link: 01ea173e103e ("xfs: fix up non-directory creation in SGID directories") [2]
>> Link: fd84bfdddd16 ("ceph: fix up non-directory creation in SGID directories") [3]
>> Reviewed-by: Darrick J. Wong<djwong@kernel.org>
>> Suggested-by: Dave Chinner<david@fromorbit.com>
>> Signed-off-by: Yang Xu<xuyang2018.jy@fujitsu.com>
>> ---
>>   fs/inode.c         |  2 --
>>   fs/namei.c         | 22 +++++++++-------------
>>   fs/ocfs2/namei.c   |  1 +
>>   include/linux/fs.h | 11 +++++++++++
>>   4 files changed, 21 insertions(+), 15 deletions(-)
>>
>> diff --git a/fs/inode.c b/fs/inode.c
>> index e9a5f2ec2f89..dd357f4b556d 100644
>> --- a/fs/inode.c
>> +++ b/fs/inode.c
>> @@ -2246,8 +2246,6 @@ void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
>>   		/* Directories are special, and always inherit S_ISGID */
>>   		if (S_ISDIR(mode))
>>   			mode |= S_ISGID;
>> -		else
>> -			mode = mode_strip_sgid(mnt_userns, dir, mode);
>>   	} else
>>   		inode_fsgid_set(inode, mnt_userns);
>>   	inode->i_mode = mode;
>> diff --git a/fs/namei.c b/fs/namei.c
>> index 73646e28fae0..5dbf00704ae8 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -3287,8 +3287,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
>>   	if (open_flag&  O_CREAT) {
>>   		if (open_flag&  O_EXCL)
>>   			open_flag&= ~O_TRUNC;
>> -		if (!IS_POSIXACL(dir->d_inode))
>> -			mode&= ~current_umask();
>> +		mode = vfs_prepare_mode(mnt_userns, dir->d_inode, mode);
>>   		if (likely(got_write))
>>   			create_error = may_o_create(mnt_userns,&nd->path,
>>   						    dentry, mode);
>> @@ -3521,8 +3520,7 @@ struct dentry *vfs_tmpfile(struct user_namespace *mnt_userns,
>>   	child = d_alloc(dentry,&slash_name);
>>   	if (unlikely(!child))
>>   		goto out_err;
>> -	if (!IS_POSIXACL(dir))
>> -		mode&= ~current_umask();
>> +	mode = vfs_prepare_mode(mnt_userns, dir, mode);
>>   	error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
>>   	if (error)
>>   		goto out_err;
>> @@ -3850,13 +3848,12 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
>>   	if (IS_ERR(dentry))
>>   		goto out1;
>>
>> -	if (!IS_POSIXACL(path.dentry->d_inode))
>> -		mode&= ~current_umask();
>> +	mnt_userns = mnt_user_ns(path.mnt);
>> +	mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
>>   	error = security_path_mknod(&path, dentry, mode, dev);
>>   	if (error)
>>   		goto out2;
>>
>> -	mnt_userns = mnt_user_ns(path.mnt);
>>   	switch (mode&  S_IFMT) {
>>   		case 0: case S_IFREG:
>>   			error = vfs_create(mnt_userns, path.dentry->d_inode,
>> @@ -3943,6 +3940,7 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
>>   	struct path path;
>>   	int error;
>>   	unsigned int lookup_flags = LOOKUP_DIRECTORY;
>> +	struct user_namespace *mnt_userns;
>>
>>   retry:
>>   	dentry = filename_create(dfd, name,&path, lookup_flags);
>> @@ -3950,15 +3948,13 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
>>   	if (IS_ERR(dentry))
>>   		goto out_putname;
>>
>> -	if (!IS_POSIXACL(path.dentry->d_inode))
>> -		mode&= ~current_umask();
>> +	mnt_userns = mnt_user_ns(path.mnt);
>> +	mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
>>   	error = security_path_mkdir(&path, dentry, mode);
>> -	if (!error) {
>> -		struct user_namespace *mnt_userns;
>> -		mnt_userns = mnt_user_ns(path.mnt);
>> +	if (!error)
>>   		error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry,
>>   				  mode);
>> -	}
>> +
>>   	done_path_create(&path, dentry);
>>   	if (retry_estale(error, lookup_flags)) {
>>   		lookup_flags |= LOOKUP_REVAL;
>> diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
>> index c75fd54b9185..961d1cf54388 100644
>> --- a/fs/ocfs2/namei.c
>> +++ b/fs/ocfs2/namei.c
>> @@ -197,6 +197,7 @@ static struct inode *ocfs2_get_init_inode(struct inode *dir, umode_t mode)
>>   	 * callers. */
>>   	if (S_ISDIR(mode))
>>   		set_nlink(inode, 2);
>> +	mode = mode_strip_sgid(&init_user_ns, dir, mode);
>>   	inode_init_owner(&init_user_ns, inode, dir, mode);
>>   	status = dquot_initialize(inode);
>>   	if (status)
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 98b44a2732f5..914c8f28bb02 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -3459,6 +3459,17 @@ static inline bool dir_relax_shared(struct inode *inode)
>>   	return !IS_DEADDIR(inode);
>>   }
>>
>> +static inline umode_t vfs_prepare_mode(struct user_namespace *mnt_userns,
>> +				   const struct inode *dir, umode_t mode)
>
> Sorry, since you're only calling the helper in fs/namei.c you don't need
> to expose it in fs.h; just keep it local to fs/namei.c.

Oh, yes, will move it into fs/namei.c.
Christian Brauner April 27, 2022, 9:22 a.m. UTC | #7
On Tue, Apr 26, 2022 at 04:53:49PM +0200, Christian Brauner wrote:
> On Tue, Apr 26, 2022 at 01:52:11PM +0200, Miklos Szeredi wrote:
> > On Tue, 26 Apr 2022 at 13:21, Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > On Tue, Apr 26, 2022 at 1:38 PM Christian Brauner <brauner@kernel.org> wrote:
> > 
> > > > One thing that I just remembered and which I think I haven't mentioned
> > > > so far is that moving S_ISGID stripping from filesystem callpaths into
> > > > the vfs callpaths means that we're hoisting this logic out of vfs_*()
> > > > helpers implicitly.
> > > >
> > > > So filesystems that call vfs_*() helpers directly can't rely on S_ISGID
> > > > stripping being done in vfs_*() helpers anymore unless they pass the
> > > > mode on from a prior run through the vfs.
> > > >
> > > > This mostly affects overlayfs which calls vfs_*() functions directly. So
> > > > a typical overlayfs callstack would be (roughly - I'm omw to lunch):
> > > >
> > > > sys_mknod()
> > > > -> do_mknodat(mode) // calls vfs_prepare_mode()
> > > >    -> .mknod = ovl_mknod(mode)
> > > >       -> ovl_create(mode)
> > > >          -> vfs_mknod(mode)
> > > >
> > > > I think we are safe as overlayfs passes on the mode on from its own run
> > > > through the vfs and then via vfs_*() to the underlying filesystem but it
> > > > is worth point that out.
> > > >
> > > > Ccing Amir just for confirmation.
> > >
> > > Looks fine to me, but CC Miklos ...
> > 
> > Looks fine to me as well.  Overlayfs should share the mode (including
> > the suid and sgid bits), owner, group and ACL's with the underlying
> > filesystem, so clearing sgid based on overlay parent directory should
> > result in the same mode as if it was done based on the parent
> > directory on the underlying layer.
> 
> Ah yes, good point.
> 
> > 
> > AFAIU this logic is not affected by userns or mnt_userns, but
> > Christian would be best to confirm that.
> 
> It does depend on it as S_ISGID stripping requires knowledge about
> whether the caller has CAP_FSETID and is capable over the parent
> directory or if they are in the group the file is owned by.
> 
> I think ultimately it might just come down to moving vfs_prepare_mode()
> into vfs_*() helpers and not into the do_*at() helpers.
> 
> That would be cleaner anyway as right now we have this weird disconnect
> between vfs_tmpfile() and vfs_{create,mknod,mkdir}(). IOW, vfs_tmpfile()
> doesn't even have an associated do_*() wrapper where we could call
> vfs_prepare_mode() from.
> 
> So ultimately it might be nicer if we do it in vfs_*() helpers anyway.
> 
> The less pretty thing about it will be that the security_path_*() hooks
> also want a mode.
> 
> Right now these hooks receive the mode as it's passed in from userspace
> minus umask but before S_ISGID stripping happens.
> 
> Whereas I think they should really see what the filesystem sees and
> currently it's a bug that they see something else.
> 
> I need to think about this a bit.

So on top of that series (though it should just be folded in), does that
look reasonable?

From e993f81caae60fee4f77b40d46ad3863ea383493 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Wed, 27 Apr 2022 10:53:35 +0200
Subject: [PATCH] UNTESTED UNTESTED UNTESTED

As I realized yesterday we need to move mode preparation into the vfs_*()
instead of do_*() helpers as filesystems like overlayfs have the following
callstacks:

sys_mknod(ovl_path, mode)
-> do_mknodat(ovl_path, mode)
   -> .mknod = ovl_mknod(ovl_path, mode)
      -> vfs_mknod(xfs_path, mode)
	 -> .mknod = xfs_vn_mknod(xfs_path, mode)

and the requirement that this will yield the same mode as:

sys_mknod(xfs_path, mode)
-> do_mknodat(xfs_path, mode)
   -> .mknod = xfs_vn_mknod(xfs_path, mode)

By moving setgid stripping into vfs_*() helpers we achieve:

- Moving setgid stripping out of the individual filesystem's responsibility.
- Ensure that callers of vfs_*() helpers continue to get correct setgid
  stripping.

Another thing I realized while looking at this yesterday was the entanglement
with security hooks. Security hooks currently see a different mode than the
actual filesystem sees when it calls into inode_init_owner(). This patch
doesn't change that!

I originally thought that we might be able to make the security hooks see the
same mode that the filesystem will see. However, I have doubts. First, I don't
think that is achievable without more restructuring. Second, I don't think it's
required as the hooks have clearly been placed before any vfs_*() calls and
thereby have committed themselves to see the mode as passed in from userspace
(minus the umask). We will simply continue doing just exactly that
side-stepping the issue for now.

Sketched-by: Christian Brauner (Microsoft) <brauner@kernel.org>
---
 fs/namei.c         | 103 +++++++++++++++++++++++++++++++++++++++------
 include/linux/fs.h |  11 -----
 2 files changed, 90 insertions(+), 24 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 5dbf00704ae8..8b83db15ae5f 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2998,6 +2998,71 @@ void unlock_rename(struct dentry *p1, struct dentry *p2)
 }
 EXPORT_SYMBOL(unlock_rename);
 
+/**
+ * mode_strip_umask - handle vfs umask stripping
+ * @dir:	parent directory of the new inode
+ * @mode:	mode of the new inode to be created in @dir
+ *
+ * Umask stripping depends on whether or not the filesystem supports POSIX
+ * ACLs. If the filesystem doesn't support it umask stripping is done directly
+ * in here. If the filesystem does support POSIX ACLs umask stripping is
+ * deferred until the filesystem calls posix_acl_create().
+ *
+ * Returns: mode
+ */
+static inline umode_t mode_strip_umask(const struct inode *dir, umode_t mode)
+{
+	if (!IS_POSIXACL(dir))
+		mode &= ~current_umask();
+	return mode;
+}
+
+/**
+ * vfs_prepare_mode - prepare the mode to be used for a new inode
+ * @mnt_userns:	user namespace of the mount the inode was found from
+ * @dir:	parent directory of the new inode
+ * @mode:	mode of the new inode
+ * @mask_perms:	allowed permission by the vfs
+ * @type:	type of file to be created
+ *
+ * This helper consolidates and enforces vfs restrictions on the @mode of a new
+ * object to be created.
+ *
+ * Umask stripping depends on whether the filesystem supports POSIX ACLs (see
+ * the kernel documentation for mode_strip_umask()). Moving umask stripping
+ * after setgid stripping allows the same ordering for both non-POSIX ACL and
+ * POSIX ACL supporting filesystems.
+ *
+ * Note that it's currently valid for @type to be 0 if a directory is created.
+ * Filesystems raise that flag individually and we need to check whether each
+ * filesystem can deal with receiving S_IFDIR from the vfs before we enforce a
+ * non-zero type.
+ *
+ * Returns: mode to be passed to the filesystem
+ */
+static inline umode_t vfs_prepare_mode(struct user_namespace *mnt_userns,
+				       const struct inode *dir, umode_t mode,
+				       umode_t mask_perms, umode_t type)
+{
+	/*
+	 * S_ISGID stripping depends on the mode of the new file so make sure
+	 * that the caller gives us this information and splat if we miss it.
+	 */
+	WARN_ON_ONCE((mode & S_IFMT) == 0);
+
+	mode = mode_strip_sgid(mnt_userns, dir, mode);
+	mode = mode_strip_umask(dir, mode);
+
+	/*
+	 * Apply the vfs mandated allowed permission mask and set the type of
+	 * file to be created before we call into the filesystem.
+	 */
+	mode &= (mask_perms & ~S_IFMT);
+	mode |= (type & S_IFMT);
+
+	return mode;
+}
+
 /**
  * vfs_create - create new file
  * @mnt_userns:	user namespace of the mount the inode was found from
@@ -3023,8 +3088,9 @@ int vfs_create(struct user_namespace *mnt_userns, struct inode *dir,
 
 	if (!dir->i_op->create)
 		return -EACCES;	/* shouldn't it be ENOSYS? */
-	mode &= S_IALLUGO;
-	mode |= S_IFREG;
+
+	mode = vfs_prepare_mode(mnt_userns, d_inode(path.dentry), mode,
+				S_IALLUGO, S_IFREG);
 	error = security_inode_create(dir, dentry, mode);
 	if (error)
 		return error;
@@ -3287,7 +3353,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 	if (open_flag & O_CREAT) {
 		if (open_flag & O_EXCL)
 			open_flag &= ~O_TRUNC;
-		mode = vfs_prepare_mode(mnt_userns, dir->d_inode, mode);
+		mode = vfs_prepare_mode(mnt_userns, dir->d_inode, mode, mode, mode);
 		if (likely(got_write))
 			create_error = may_o_create(mnt_userns, &nd->path,
 						    dentry, mode);
@@ -3520,7 +3586,7 @@ struct dentry *vfs_tmpfile(struct user_namespace *mnt_userns,
 	child = d_alloc(dentry, &slash_name);
 	if (unlikely(!child))
 		goto out_err;
-	mode = vfs_prepare_mode(mnt_userns, dir, mode);
+	mode = vfs_prepare_mode(mnt_userns, dir, mode, mode, mode);
 	error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
 	if (error)
 		goto out_err;
@@ -3798,6 +3864,8 @@ int vfs_mknod(struct user_namespace *mnt_userns, struct inode *dir,
 	if (!dir->i_op->mknod)
 		return -EPERM;
 
+	mode = vfs_prepare_mode(mnt_userns, d_inode(path.dentry),
+				mode, mode, mode);
 	error = devcgroup_inode_mknod(mode, dev);
 	if (error)
 		return error;
@@ -3848,12 +3916,13 @@ static int do_mknodat(int dfd, struct filename *name, umode_t mode,
 	if (IS_ERR(dentry))
 		goto out1;
 
-	mnt_userns = mnt_user_ns(path.mnt);
-	mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
-	error = security_path_mknod(&path, dentry, mode, dev);
+	error = security_path_mknod(&path, dentry,
+				    mode_strip_umask(d_inode(path.dentry), mode),
+				    dev);
 	if (error)
 		goto out2;
 
+	mnt_userns = mnt_user_ns(path.mnt);
 	switch (mode & S_IFMT) {
 		case 0: case S_IFREG:
 			error = vfs_create(mnt_userns, path.dentry->d_inode,
@@ -3919,7 +3988,13 @@ int vfs_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
 	if (!dir->i_op->mkdir)
 		return -EPERM;
 
-	mode &= (S_IRWXUGO|S_ISVTX);
+	/*
+	 * Filesystems currently raise S_IFDIR individually. We should try and
+	 * fix that going forward passing it in from the vfs as we do for all
+	 * other files going forward.
+	 */
+	mode = vfs_prepare_mode(mnt_userns, d_inode(path.dentry),
+				mode, S_IRWXUGO | S_ISVTX, 0);
 	error = security_inode_mkdir(dir, dentry, mode);
 	if (error)
 		return error;
@@ -3940,7 +4015,6 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
 	struct path path;
 	int error;
 	unsigned int lookup_flags = LOOKUP_DIRECTORY;
-	struct user_namespace *mnt_userns;
 
 retry:
 	dentry = filename_create(dfd, name, &path, lookup_flags);
@@ -3948,12 +4022,15 @@ int do_mkdirat(int dfd, struct filename *name, umode_t mode)
 	if (IS_ERR(dentry))
 		goto out_putname;
 
-	mnt_userns = mnt_user_ns(path.mnt);
-	mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
-	error = security_path_mkdir(&path, dentry, mode);
-	if (!error)
+	error = security_path_mkdir(&path, dentry,
+				    mode_strip_umask(d_inode(path.dentry), mode));
+	if (!error) {
+		struct user_namespace *mnt_userns;
+
+		mnt_userns = mnt_user_ns(path.mnt);
 		error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry,
 				  mode);
+	}
 
 	done_path_create(&path, dentry);
 	if (retry_estale(error, lookup_flags)) {
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 914c8f28bb02..98b44a2732f5 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3459,17 +3459,6 @@ static inline bool dir_relax_shared(struct inode *inode)
 	return !IS_DEADDIR(inode);
 }
 
-static inline umode_t vfs_prepare_mode(struct user_namespace *mnt_userns,
-				   const struct inode *dir, umode_t mode)
-{
-	mode = mode_strip_sgid(mnt_userns, dir, mode);
-
-	if (!IS_POSIXACL(dir))
-		mode &= ~current_umask();
-
-	return mode;
-}
-
 extern bool path_noexec(const struct path *path);
 extern void inode_nohighmem(struct inode *inode);
Al Viro April 28, 2022, 4:45 a.m. UTC | #8
On Wed, Apr 27, 2022 at 11:22:01AM +0200, Christian Brauner wrote:

> +static inline umode_t vfs_prepare_mode(struct user_namespace *mnt_userns,
> +				       const struct inode *dir, umode_t mode,
> +				       umode_t mask_perms, umode_t type)
> +{
> +	/*
> +	 * S_ISGID stripping depends on the mode of the new file so make sure
> +	 * that the caller gives us this information and splat if we miss it.
> +	 */
> +	WARN_ON_ONCE((mode & S_IFMT) == 0);

<blink>

First of all, what happens if you call mknod("/tmp/blah", 0, 0)?  And the only
thing about type bits we care about is "is it a directory" - the sensitive
stuff is in the low 12 bits...  What is that check about?

> +	mode = mode_strip_sgid(mnt_userns, dir, mode);
> +	mode = mode_strip_umask(dir, mode);
> +
> +	/*
> +	 * Apply the vfs mandated allowed permission mask and set the type of
> +	 * file to be created before we call into the filesystem.
> +	 */
> +	mode &= (mask_perms & ~S_IFMT);
> +	mode |= (type & S_IFMT);
> +
> +	return mode;
Christian Brauner April 28, 2022, 8:07 a.m. UTC | #9
On Thu, Apr 28, 2022 at 04:45:05AM +0000, Al Viro wrote:
> On Wed, Apr 27, 2022 at 11:22:01AM +0200, Christian Brauner wrote:
> 
> > +static inline umode_t vfs_prepare_mode(struct user_namespace *mnt_userns,
> > +				       const struct inode *dir, umode_t mode,
> > +				       umode_t mask_perms, umode_t type)
> > +{
> > +	/*
> > +	 * S_ISGID stripping depends on the mode of the new file so make sure
> > +	 * that the caller gives us this information and splat if we miss it.
> > +	 */
> > +	WARN_ON_ONCE((mode & S_IFMT) == 0);
> 
> <blink>
> 
> First of all, what happens if you call mknod("/tmp/blah", 0, 0)?  And the only
> thing about type bits we care about is "is it a directory" - the sensitive
> stuff is in the low 12 bits...  What is that check about?

Do note that this is just an untested rough sketch to illustrate how to
move it into vfs_*() helpers.
diff mbox series

Patch

diff --git a/fs/inode.c b/fs/inode.c
index e9a5f2ec2f89..dd357f4b556d 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2246,8 +2246,6 @@  void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
 		/* Directories are special, and always inherit S_ISGID */
 		if (S_ISDIR(mode))
 			mode |= S_ISGID;
-		else
-			mode = mode_strip_sgid(mnt_userns, dir, mode);
 	} else
 		inode_fsgid_set(inode, mnt_userns);
 	inode->i_mode = mode;
diff --git a/fs/namei.c b/fs/namei.c
index 73646e28fae0..5dbf00704ae8 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3287,8 +3287,7 @@  static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 	if (open_flag & O_CREAT) {
 		if (open_flag & O_EXCL)
 			open_flag &= ~O_TRUNC;
-		if (!IS_POSIXACL(dir->d_inode))
-			mode &= ~current_umask();
+		mode = vfs_prepare_mode(mnt_userns, dir->d_inode, mode);
 		if (likely(got_write))
 			create_error = may_o_create(mnt_userns, &nd->path,
 						    dentry, mode);
@@ -3521,8 +3520,7 @@  struct dentry *vfs_tmpfile(struct user_namespace *mnt_userns,
 	child = d_alloc(dentry, &slash_name);
 	if (unlikely(!child))
 		goto out_err;
-	if (!IS_POSIXACL(dir))
-		mode &= ~current_umask();
+	mode = vfs_prepare_mode(mnt_userns, dir, mode);
 	error = dir->i_op->tmpfile(mnt_userns, dir, child, mode);
 	if (error)
 		goto out_err;
@@ -3850,13 +3848,12 @@  static int do_mknodat(int dfd, struct filename *name, umode_t mode,
 	if (IS_ERR(dentry))
 		goto out1;
 
-	if (!IS_POSIXACL(path.dentry->d_inode))
-		mode &= ~current_umask();
+	mnt_userns = mnt_user_ns(path.mnt);
+	mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
 	error = security_path_mknod(&path, dentry, mode, dev);
 	if (error)
 		goto out2;
 
-	mnt_userns = mnt_user_ns(path.mnt);
 	switch (mode & S_IFMT) {
 		case 0: case S_IFREG:
 			error = vfs_create(mnt_userns, path.dentry->d_inode,
@@ -3943,6 +3940,7 @@  int do_mkdirat(int dfd, struct filename *name, umode_t mode)
 	struct path path;
 	int error;
 	unsigned int lookup_flags = LOOKUP_DIRECTORY;
+	struct user_namespace *mnt_userns;
 
 retry:
 	dentry = filename_create(dfd, name, &path, lookup_flags);
@@ -3950,15 +3948,13 @@  int do_mkdirat(int dfd, struct filename *name, umode_t mode)
 	if (IS_ERR(dentry))
 		goto out_putname;
 
-	if (!IS_POSIXACL(path.dentry->d_inode))
-		mode &= ~current_umask();
+	mnt_userns = mnt_user_ns(path.mnt);
+	mode = vfs_prepare_mode(mnt_userns, path.dentry->d_inode, mode);
 	error = security_path_mkdir(&path, dentry, mode);
-	if (!error) {
-		struct user_namespace *mnt_userns;
-		mnt_userns = mnt_user_ns(path.mnt);
+	if (!error)
 		error = vfs_mkdir(mnt_userns, path.dentry->d_inode, dentry,
 				  mode);
-	}
+
 	done_path_create(&path, dentry);
 	if (retry_estale(error, lookup_flags)) {
 		lookup_flags |= LOOKUP_REVAL;
diff --git a/fs/ocfs2/namei.c b/fs/ocfs2/namei.c
index c75fd54b9185..961d1cf54388 100644
--- a/fs/ocfs2/namei.c
+++ b/fs/ocfs2/namei.c
@@ -197,6 +197,7 @@  static struct inode *ocfs2_get_init_inode(struct inode *dir, umode_t mode)
 	 * callers. */
 	if (S_ISDIR(mode))
 		set_nlink(inode, 2);
+	mode = mode_strip_sgid(&init_user_ns, dir, mode);
 	inode_init_owner(&init_user_ns, inode, dir, mode);
 	status = dquot_initialize(inode);
 	if (status)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 98b44a2732f5..914c8f28bb02 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3459,6 +3459,17 @@  static inline bool dir_relax_shared(struct inode *inode)
 	return !IS_DEADDIR(inode);
 }
 
+static inline umode_t vfs_prepare_mode(struct user_namespace *mnt_userns,
+				   const struct inode *dir, umode_t mode)
+{
+	mode = mode_strip_sgid(mnt_userns, dir, mode);
+
+	if (!IS_POSIXACL(dir))
+		mode &= ~current_umask();
+
+	return mode;
+}
+
 extern bool path_noexec(const struct path *path);
 extern void inode_nohighmem(struct inode *inode);