diff mbox series

[v8,1/2] Add a "nosymfollow" mount option.

Message ID 20200819164317.637421-1-zwisler@google.com
State New
Headers show
Series [v8,1/2] Add a "nosymfollow" mount option. | expand

Commit Message

Ross Zwisler Aug. 19, 2020, 4:43 p.m. UTC
From: Mattias Nissler <mnissler@chromium.org>

For mounts that have the new "nosymfollow" option, don't follow symlinks
when resolving paths. The new option is similar in spirit to the
existing "nodev", "noexec", and "nosuid" options, as well as to the
LOOKUP_NO_SYMLINKS resolve flag in the openat2(2) syscall. Various BSD
variants have been supporting the "nosymfollow" mount option for a long
time with equivalent implementations.

Note that symlinks may still be created on file systems mounted with
the "nosymfollow" option present. readlink() remains functional, so
user space code that is aware of symlinks can still choose to follow
them explicitly.

Setting the "nosymfollow" mount option helps prevent privileged
writers from modifying files unintentionally in case there is an
unexpected link along the accessed path. The "nosymfollow" option is
thus useful as a defensive measure for systems that need to deal with
untrusted file systems in privileged contexts.

More information on the history and motivation for this patch can be
found here:

https://sites.google.com/a/chromium.org/dev/chromium-os/chromiumos-design-docs/hardening-against-malicious-stateful-data#TOC-Restricting-symlink-traversal

Signed-off-by: Mattias Nissler <mnissler@chromium.org>
Signed-off-by: Ross Zwisler <zwisler@google.com>
Reviewed-by: Aleksa Sarai <cyphar@cyphar.com>
---
Changes since v7 [1]:
 * Rebased onto v5.9-rc1.
 * Added selftest in second patch.
 * Added Aleska's Reviewed-By tag.  Thank you for the review!

After this lands I will upstream changes to util-linux[2] and man-pages
[3].

[1]: https://lkml.org/lkml/2020/8/11/896
[2]: https://github.com/rzwisler/util-linux/commit/7f8771acd85edb70d97921c026c55e1e724d4e15
[3]: https://github.com/rzwisler/man-pages/commit/b8fe8079f64b5068940c0144586e580399a71668
---
 fs/namei.c                 | 3 ++-
 fs/namespace.c             | 2 ++
 fs/proc_namespace.c        | 1 +
 fs/statfs.c                | 2 ++
 include/linux/mount.h      | 3 ++-
 include/linux/statfs.h     | 1 +
 include/uapi/linux/mount.h | 1 +
 7 files changed, 11 insertions(+), 2 deletions(-)

Comments

Ross Zwisler Aug. 26, 2020, 8:48 p.m. UTC | #1
O Wed, Aug 19, 2020 at 10:43:16AM -0600, Ross Zwisler wrote:
> From: Mattias Nissler <mnissler@chromium.org>
> 
> For mounts that have the new "nosymfollow" option, don't follow symlinks
> when resolving paths. The new option is similar in spirit to the
> existing "nodev", "noexec", and "nosuid" options, as well as to the
> LOOKUP_NO_SYMLINKS resolve flag in the openat2(2) syscall. Various BSD
> variants have been supporting the "nosymfollow" mount option for a long
> time with equivalent implementations.
> 
> Note that symlinks may still be created on file systems mounted with
> the "nosymfollow" option present. readlink() remains functional, so
> user space code that is aware of symlinks can still choose to follow
> them explicitly.
> 
> Setting the "nosymfollow" mount option helps prevent privileged
> writers from modifying files unintentionally in case there is an
> unexpected link along the accessed path. The "nosymfollow" option is
> thus useful as a defensive measure for systems that need to deal with
> untrusted file systems in privileged contexts.
> 
> More information on the history and motivation for this patch can be
> found here:
> 
> https://sites.google.com/a/chromium.org/dev/chromium-os/chromiumos-design-docs/hardening-against-malicious-stateful-data#TOC-Restricting-symlink-traversal
> 
> Signed-off-by: Mattias Nissler <mnissler@chromium.org>
> Signed-off-by: Ross Zwisler <zwisler@google.com>
> Reviewed-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
> Changes since v7 [1]:
>  * Rebased onto v5.9-rc1.
>  * Added selftest in second patch.
>  * Added Aleska's Reviewed-By tag.  Thank you for the review!
> 
> After this lands I will upstream changes to util-linux[2] and man-pages
> [3].
> 
> [1]: https://lkml.org/lkml/2020/8/11/896
> [2]: https://github.com/rzwisler/util-linux/commit/7f8771acd85edb70d97921c026c55e1e724d4e15
> [3]: https://github.com/rzwisler/man-pages/commit/b8fe8079f64b5068940c0144586e580399a71668
> ---

Friendly ping on this.

Al, now that the changes to fs/namei.c have landed and we're past the merge
window for v5.9, what are your thoughts on this patch and the associated test?
Al Viro Aug. 27, 2020, 1:59 a.m. UTC | #2
On Wed, Aug 26, 2020 at 02:48:19PM -0600, Ross Zwisler wrote:

> Al, now that the changes to fs/namei.c have landed and we're past the merge
> window for v5.9, what are your thoughts on this patch and the associated test?

Humm...  should that be nd->path.mnt->mnt_flags or link->mnt->mnt_flags?
Usually it's the same thing, but they might differ.  IOW, is that about the
directory we'd found it in, or is it about the link itself?
Aleksa Sarai Aug. 27, 2020, 3:41 p.m. UTC | #3
On 2020-08-27, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Wed, Aug 26, 2020 at 02:48:19PM -0600, Ross Zwisler wrote:
> 
> > Al, now that the changes to fs/namei.c have landed and we're past the merge
> > window for v5.9, what are your thoughts on this patch and the associated test?
> 
> Humm...  should that be nd->path.mnt->mnt_flags or link->mnt->mnt_flags?
> Usually it's the same thing, but they might differ.  IOW, is that about the
> directory we'd found it in, or is it about the link itself?

Now that you mention it, I think link->mnt->mnt_flags makes more sense.
The restriction should apply in the context of whatever filesystem
contains the symlink, and that would matches FreeBSD's semantics (at
least as far as I can tell from a quick look at sys/kern/vfs_lookup.c).
Ross Zwisler Aug. 27, 2020, 4:48 p.m. UTC | #4
On Fri, Aug 28, 2020 at 01:41:39AM +1000, Aleksa Sarai wrote:
> On 2020-08-27, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > On Wed, Aug 26, 2020 at 02:48:19PM -0600, Ross Zwisler wrote:
> > 
> > > Al, now that the changes to fs/namei.c have landed and we're past the merge
> > > window for v5.9, what are your thoughts on this patch and the associated test?
> > 
> > Humm...  should that be nd->path.mnt->mnt_flags or link->mnt->mnt_flags?
> > Usually it's the same thing, but they might differ.  IOW, is that about the
> > directory we'd found it in, or is it about the link itself?
> 
> Now that you mention it, I think link->mnt->mnt_flags makes more sense.
> The restriction should apply in the context of whatever filesystem
> contains the symlink, and that would matches FreeBSD's semantics (at
> least as far as I can tell from a quick look at sys/kern/vfs_lookup.c).

Yep, changing this to link->mnt->mnt_flags makes sense to me, as you're right
that we care about the link itself and not the link's parent directory.  Thank
you for the review, and I'll send out v9 momentarily.
diff mbox series

Patch

diff --git a/fs/namei.c b/fs/namei.c
index e99e2a9da0f7d..12d92af2e2ca7 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1626,7 +1626,8 @@  static const char *pick_link(struct nameidata *nd, struct path *link,
 			return ERR_PTR(error);
 	}
 
-	if (unlikely(nd->flags & LOOKUP_NO_SYMLINKS))
+	if (unlikely(nd->flags & LOOKUP_NO_SYMLINKS) ||
+			unlikely(nd->path.mnt->mnt_flags & MNT_NOSYMFOLLOW))
 		return ERR_PTR(-ELOOP);
 
 	if (!(nd->flags & LOOKUP_RCU)) {
diff --git a/fs/namespace.c b/fs/namespace.c
index bae0e95b3713a..6408788a649e1 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3160,6 +3160,8 @@  int path_mount(const char *dev_name, struct path *path,
 		mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
 	if (flags & MS_RDONLY)
 		mnt_flags |= MNT_READONLY;
+	if (flags & MS_NOSYMFOLLOW)
+		mnt_flags |= MNT_NOSYMFOLLOW;
 
 	/* The default atime for remount is preservation */
 	if ((flags & MS_REMOUNT) &&
diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c
index 3059a9394c2d6..e59d4bb3a89e4 100644
--- a/fs/proc_namespace.c
+++ b/fs/proc_namespace.c
@@ -70,6 +70,7 @@  static void show_mnt_opts(struct seq_file *m, struct vfsmount *mnt)
 		{ MNT_NOATIME, ",noatime" },
 		{ MNT_NODIRATIME, ",nodiratime" },
 		{ MNT_RELATIME, ",relatime" },
+		{ MNT_NOSYMFOLLOW, ",nosymfollow" },
 		{ 0, NULL }
 	};
 	const struct proc_fs_opts *fs_infop;
diff --git a/fs/statfs.c b/fs/statfs.c
index 2616424012ea7..59f33752c1311 100644
--- a/fs/statfs.c
+++ b/fs/statfs.c
@@ -29,6 +29,8 @@  static int flags_by_mnt(int mnt_flags)
 		flags |= ST_NODIRATIME;
 	if (mnt_flags & MNT_RELATIME)
 		flags |= ST_RELATIME;
+	if (mnt_flags & MNT_NOSYMFOLLOW)
+		flags |= ST_NOSYMFOLLOW;
 	return flags;
 }
 
diff --git a/include/linux/mount.h b/include/linux/mount.h
index de657bd211fa6..aaf343b38671c 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -30,6 +30,7 @@  struct fs_context;
 #define MNT_NODIRATIME	0x10
 #define MNT_RELATIME	0x20
 #define MNT_READONLY	0x40	/* does the user want this to be r/o? */
+#define MNT_NOSYMFOLLOW	0x80
 
 #define MNT_SHRINKABLE	0x100
 #define MNT_WRITE_HOLD	0x200
@@ -46,7 +47,7 @@  struct fs_context;
 #define MNT_SHARED_MASK	(MNT_UNBINDABLE)
 #define MNT_USER_SETTABLE_MASK  (MNT_NOSUID | MNT_NODEV | MNT_NOEXEC \
 				 | MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME \
-				 | MNT_READONLY)
+				 | MNT_READONLY | MNT_NOSYMFOLLOW)
 #define MNT_ATIME_MASK (MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME )
 
 #define MNT_INTERNAL_FLAGS (MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL | \
diff --git a/include/linux/statfs.h b/include/linux/statfs.h
index 9bc69edb8f188..fac4356ea1bfc 100644
--- a/include/linux/statfs.h
+++ b/include/linux/statfs.h
@@ -40,6 +40,7 @@  struct kstatfs {
 #define ST_NOATIME	0x0400	/* do not update access times */
 #define ST_NODIRATIME	0x0800	/* do not update directory access times */
 #define ST_RELATIME	0x1000	/* update atime relative to mtime/ctime */
+#define ST_NOSYMFOLLOW	0x2000	/* do not follow symlinks */
 
 struct dentry;
 extern int vfs_get_fsid(struct dentry *dentry, __kernel_fsid_t *fsid);
diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index 96a0240f23fed..dd8306ea336c1 100644
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -16,6 +16,7 @@ 
 #define MS_REMOUNT	32	/* Alter flags of a mounted FS */
 #define MS_MANDLOCK	64	/* Allow mandatory locks on an FS */
 #define MS_DIRSYNC	128	/* Directory modifications are synchronous */
+#define MS_NOSYMFOLLOW	256	/* Do not follow symlinks */
 #define MS_NOATIME	1024	/* Do not update access times. */
 #define MS_NODIRATIME	2048	/* Do not update directory access times */
 #define MS_BIND		4096