| Message ID | 20210708175738.360757-2-vgoyal@redhat.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Paul Moore |
| Headers | show |
| Series | Relax restrictions on user.* xattr | expand |
On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote: > Currently user.* xattr are not allowed on symlink and special files. > > man xattr and recent discussion suggested that primary reason for this > restriction is how file permissions for symlinks and special files > are little different from regular files and directories. > > For symlinks, they are world readable/writable and if user xattr were > to be permitted, it will allow unpriviliged users to dump a huge amount > of user.* xattrs on symlinks without any control. > > For special files, permissions typically control capability to read/write > from devices (and not necessarily from filesystem). So if a user can > write to device (/dev/null), does not necessarily mean it should be allowed > to write large number of user.* xattrs on the filesystem device node is > residing in. > > This patch proposes to relax the restrictions a bit and allow file owner > or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs > on symlink and special files. > > virtiofs daemon has a need to store user.* xatrrs on all the files > (including symlinks and special files), and currently that fails. This > patch should help. > > Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.com/ > Signed-off-by: Vivek Goyal <vgoyal@redhat.com> > --- Seems reasonable and useful. Acked-by: Christian Brauner <christian.brauner@ubuntu.com> One question, do all filesystem supporting xattrs deal with setting them on symlinks/device files correctly? > fs/xattr.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/fs/xattr.c b/fs/xattr.c > index 5c8c5175b385..2f1855c8b620 100644 > --- a/fs/xattr.c > +++ b/fs/xattr.c > @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode, > } > > /* > - * In the user.* namespace, only regular files and directories can have > - * extended attributes. For sticky directories, only the owner and > - * privileged users can write attributes. > + * In the user.* namespace, for symlinks and special files, only > + * the owner and priviliged users can read/write attributes. > + * For sticky directories, only the owner and privileged users can > + * write attributes. > */ > if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) { > - if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode)) > + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) && > + !inode_owner_or_capable(mnt_userns, inode)) > return (mask & MAY_WRITE) ? -EPERM : -ENODATA; > if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) && > (mask & MAY_WRITE) && > -- > 2.25.4 >
On Fri, Jul 09, 2021 at 11:19:15AM +0200, Christian Brauner wrote: > On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote: > > Currently user.* xattr are not allowed on symlink and special files. > > > > man xattr and recent discussion suggested that primary reason for this > > restriction is how file permissions for symlinks and special files > > are little different from regular files and directories. > > > > For symlinks, they are world readable/writable and if user xattr were > > to be permitted, it will allow unpriviliged users to dump a huge amount > > of user.* xattrs on symlinks without any control. > > > > For special files, permissions typically control capability to read/write > > from devices (and not necessarily from filesystem). So if a user can > > write to device (/dev/null), does not necessarily mean it should be allowed > > to write large number of user.* xattrs on the filesystem device node is > > residing in. > > > > This patch proposes to relax the restrictions a bit and allow file owner > > or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs > > on symlink and special files. > > > > virtiofs daemon has a need to store user.* xatrrs on all the files > > (including symlinks and special files), and currently that fails. This > > patch should help. > > > > Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.com/ > > Signed-off-by: Vivek Goyal <vgoyal@redhat.com> > > --- > > Seems reasonable and useful. > Acked-by: Christian Brauner <christian.brauner@ubuntu.com> > > One question, do all filesystem supporting xattrs deal with setting them > on symlinks/device files correctly? Wrote a simple bash script to do setfattr/getfattr user.foo xattr on symlink and device node on ext4, xfs and btrfs and it works fine. https://github.com/rhvgoyal/misc/blob/master/generic-programs/user-xattr-special-files.sh I probably can add some more filesystems to test. Thanks Vivek > > > fs/xattr.c | 10 ++++++---- > > 1 file changed, 6 insertions(+), 4 deletions(-) > > > > diff --git a/fs/xattr.c b/fs/xattr.c > > index 5c8c5175b385..2f1855c8b620 100644 > > --- a/fs/xattr.c > > +++ b/fs/xattr.c > > @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode, > > } > > > > /* > > - * In the user.* namespace, only regular files and directories can have > > - * extended attributes. For sticky directories, only the owner and > > - * privileged users can write attributes. > > + * In the user.* namespace, for symlinks and special files, only > > + * the owner and priviliged users can read/write attributes. > > + * For sticky directories, only the owner and privileged users can > > + * write attributes. > > */ > > if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) { > > - if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode)) > > + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) && > > + !inode_owner_or_capable(mnt_userns, inode)) > > return (mask & MAY_WRITE) ? -EPERM : -ENODATA; > > if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) && > > (mask & MAY_WRITE) && > > -- > > 2.25.4 > > >
On 7/9/2021 8:27 AM, Vivek Goyal wrote: > On Fri, Jul 09, 2021 at 11:19:15AM +0200, Christian Brauner wrote: >> On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote: >>> Currently user.* xattr are not allowed on symlink and special files. >>> >>> man xattr and recent discussion suggested that primary reason for this >>> restriction is how file permissions for symlinks and special files >>> are little different from regular files and directories. >>> >>> For symlinks, they are world readable/writable and if user xattr were >>> to be permitted, it will allow unpriviliged users to dump a huge amount >>> of user.* xattrs on symlinks without any control. >>> >>> For special files, permissions typically control capability to read/write >>> from devices (and not necessarily from filesystem). So if a user can >>> write to device (/dev/null), does not necessarily mean it should be allowed >>> to write large number of user.* xattrs on the filesystem device node is >>> residing in. >>> >>> This patch proposes to relax the restrictions a bit and allow file owner >>> or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs >>> on symlink and special files. >>> >>> virtiofs daemon has a need to store user.* xatrrs on all the files >>> (including symlinks and special files), and currently that fails. This >>> patch should help. >>> >>> Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.com/ >>> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> >>> --- >> Seems reasonable and useful. >> Acked-by: Christian Brauner <christian.brauner@ubuntu.com> >> >> One question, do all filesystem supporting xattrs deal with setting them >> on symlinks/device files correctly? > Wrote a simple bash script to do setfattr/getfattr user.foo xattr on > symlink and device node on ext4, xfs and btrfs and it works fine. How about nfs, tmpfs, overlayfs and/or some of the other less conventional filesystems? > > https://github.com/rhvgoyal/misc/blob/master/generic-programs/user-xattr-special-files.sh > > I probably can add some more filesystems to test. > > Thanks > Vivek > >>> fs/xattr.c | 10 ++++++---- >>> 1 file changed, 6 insertions(+), 4 deletions(-) >>> >>> diff --git a/fs/xattr.c b/fs/xattr.c >>> index 5c8c5175b385..2f1855c8b620 100644 >>> --- a/fs/xattr.c >>> +++ b/fs/xattr.c >>> @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode, >>> } >>> >>> /* >>> - * In the user.* namespace, only regular files and directories can have >>> - * extended attributes. For sticky directories, only the owner and >>> - * privileged users can write attributes. >>> + * In the user.* namespace, for symlinks and special files, only >>> + * the owner and priviliged users can read/write attributes. >>> + * For sticky directories, only the owner and privileged users can >>> + * write attributes. >>> */ >>> if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) { >>> - if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode)) >>> + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) && >>> + !inode_owner_or_capable(mnt_userns, inode)) >>> return (mask & MAY_WRITE) ? -EPERM : -ENODATA; >>> if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) && >>> (mask & MAY_WRITE) && >>> -- >>> 2.25.4 >>>
On Fri, Jul 09, 2021 at 08:34:41AM -0700, Casey Schaufler wrote: > On 7/9/2021 8:27 AM, Vivek Goyal wrote: > > On Fri, Jul 09, 2021 at 11:19:15AM +0200, Christian Brauner wrote: > >> On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote: > >>> Currently user.* xattr are not allowed on symlink and special files. > >>> > >>> man xattr and recent discussion suggested that primary reason for this > >>> restriction is how file permissions for symlinks and special files > >>> are little different from regular files and directories. > >>> > >>> For symlinks, they are world readable/writable and if user xattr were > >>> to be permitted, it will allow unpriviliged users to dump a huge amount > >>> of user.* xattrs on symlinks without any control. > >>> > >>> For special files, permissions typically control capability to read/write > >>> from devices (and not necessarily from filesystem). So if a user can > >>> write to device (/dev/null), does not necessarily mean it should be allowed > >>> to write large number of user.* xattrs on the filesystem device node is > >>> residing in. > >>> > >>> This patch proposes to relax the restrictions a bit and allow file owner > >>> or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs > >>> on symlink and special files. > >>> > >>> virtiofs daemon has a need to store user.* xatrrs on all the files > >>> (including symlinks and special files), and currently that fails. This > >>> patch should help. > >>> > >>> Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.com/ > >>> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> > >>> --- > >> Seems reasonable and useful. > >> Acked-by: Christian Brauner <christian.brauner@ubuntu.com> > >> > >> One question, do all filesystem supporting xattrs deal with setting them > >> on symlinks/device files correctly? > > Wrote a simple bash script to do setfattr/getfattr user.foo xattr on > > symlink and device node on ext4, xfs and btrfs and it works fine. > > How about nfs, tmpfs, overlayfs and/or some of the other less conventional > filesystems? tmpfs does not support user.* xattr at all on any kind of files. overlayfs works fine. I updated my test too. nfs seems to have some issues. - I can set user.foo xattr on symlink and query it back using xattr name. getfattr -h -n user.foo foo-link.txt But when I try to dump all xattrs on this file, user.foo is being filtered out it looks like. Not sure why. - I can't set "user.foo" xattr on a device node on nfs and I get "Permission denied". I am assuming nfs server is returning this. I am using knfsd with following in /etc/exports. /mnt/test/nfs-server 127.0.0.1(insecure,no_root_squash,rw,async) Copying Bruce. He might have an idea. Thanks Vivek > > > > > https://github.com/rhvgoyal/misc/blob/master/generic-programs/user-xattr-special-files.sh > > > > I probably can add some more filesystems to test. > > > > Thanks > > Vivek > > > >>> fs/xattr.c | 10 ++++++---- > >>> 1 file changed, 6 insertions(+), 4 deletions(-) > >>> > >>> diff --git a/fs/xattr.c b/fs/xattr.c > >>> index 5c8c5175b385..2f1855c8b620 100644 > >>> --- a/fs/xattr.c > >>> +++ b/fs/xattr.c > >>> @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode, > >>> } > >>> > >>> /* > >>> - * In the user.* namespace, only regular files and directories can have > >>> - * extended attributes. For sticky directories, only the owner and > >>> - * privileged users can write attributes. > >>> + * In the user.* namespace, for symlinks and special files, only > >>> + * the owner and priviliged users can read/write attributes. > >>> + * For sticky directories, only the owner and privileged users can > >>> + * write attributes. > >>> */ > >>> if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) { > >>> - if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode)) > >>> + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) && > >>> + !inode_owner_or_capable(mnt_userns, inode)) > >>> return (mask & MAY_WRITE) ? -EPERM : -ENODATA; > >>> if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) && > >>> (mask & MAY_WRITE) && > >>> -- > >>> 2.25.4 > >>> >
On Fri, Jul 9, 2021 at 1:59 PM Vivek Goyal <vgoyal@redhat.com> wrote: > nfs seems to have some issues. I'm not sure what the expected behavior is for nfs. All I have for now is some generic troubleshooting ideas, sorry: > - I can set user.foo xattr on symlink and query it back using xattr name. > > getfattr -h -n user.foo foo-link.txt > > But when I try to dump all xattrs on this file, user.foo is being > filtered out it looks like. Not sure why. Logging into the server and seeing what's set there could help confirm whether it's the client or server that's at fault. (Or watching the traffic in wireshark; there are GET/SET/LISTXATTR ops that should be easy to spot.) > - I can't set "user.foo" xattr on a device node on nfs and I get > "Permission denied". I am assuming nfs server is returning this. Wireshark should tell you whether it's the server or client doing that. The RFC is https://datatracker.ietf.org/doc/html/rfc8276, and I don't see any explicit statement about what the server should do in the case of symlinks or device nodes, but I do see "Any regular file or directory may have a set of extended attributes", so that was clearly the assumption. Also, NFS4ERR_WRONG_TYPE is listed as a possible error return for the xattr ops. But on a quick skim I don't see any explicit checks in the nfsd code, so I *think* it's just relying on the vfs for any file type checks. --b.
On Fri, Jul 09, 2021 at 08:34:41AM -0700, Casey Schaufler wrote: > >> One question, do all filesystem supporting xattrs deal with setting them > >> on symlinks/device files correctly? > > Wrote a simple bash script to do setfattr/getfattr user.foo xattr on > > symlink and device node on ext4, xfs and btrfs and it works fine. > > How about nfs, tmpfs, overlayfs and/or some of the other less conventional > filesystems? As a suggestion, perhaps you could take your bash script and turn it into an xfstests test so we can more easily test various file systems, both now and once the commit is accepted, to look for regressions? Cheers, - Ted
On Fri, 9 Jul 2021 08:34:41 -0700 Casey Schaufler <casey@schaufler-ca.com> wrote: > On 7/9/2021 8:27 AM, Vivek Goyal wrote: > > On Fri, Jul 09, 2021 at 11:19:15AM +0200, Christian Brauner wrote: > >> On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote: > >>> Currently user.* xattr are not allowed on symlink and special files. > >>> > >>> man xattr and recent discussion suggested that primary reason for this > >>> restriction is how file permissions for symlinks and special files > >>> are little different from regular files and directories. > >>> > >>> For symlinks, they are world readable/writable and if user xattr were > >>> to be permitted, it will allow unpriviliged users to dump a huge amount > >>> of user.* xattrs on symlinks without any control. > >>> > >>> For special files, permissions typically control capability to read/write > >>> from devices (and not necessarily from filesystem). So if a user can > >>> write to device (/dev/null), does not necessarily mean it should be allowed > >>> to write large number of user.* xattrs on the filesystem device node is > >>> residing in. > >>> > >>> This patch proposes to relax the restrictions a bit and allow file owner > >>> or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs > >>> on symlink and special files. > >>> > >>> virtiofs daemon has a need to store user.* xatrrs on all the files > >>> (including symlinks and special files), and currently that fails. This > >>> patch should help. > >>> > >>> Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.com/ > >>> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> > >>> --- > >> Seems reasonable and useful. > >> Acked-by: Christian Brauner <christian.brauner@ubuntu.com> > >> > >> One question, do all filesystem supporting xattrs deal with setting them > >> on symlinks/device files correctly? > > Wrote a simple bash script to do setfattr/getfattr user.foo xattr on > > symlink and device node on ext4, xfs and btrfs and it works fine. > > How about nfs, tmpfs, overlayfs and/or some of the other less conventional > filesystems? > How about virtiofs then ? :-) > > > > https://github.com/rhvgoyal/misc/blob/master/generic-programs/user-xattr-special-files.sh > > > > I probably can add some more filesystems to test. > > > > Thanks > > Vivek > > > >>> fs/xattr.c | 10 ++++++---- > >>> 1 file changed, 6 insertions(+), 4 deletions(-) > >>> > >>> diff --git a/fs/xattr.c b/fs/xattr.c > >>> index 5c8c5175b385..2f1855c8b620 100644 > >>> --- a/fs/xattr.c > >>> +++ b/fs/xattr.c > >>> @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode, > >>> } > >>> > >>> /* > >>> - * In the user.* namespace, only regular files and directories can have > >>> - * extended attributes. For sticky directories, only the owner and > >>> - * privileged users can write attributes. > >>> + * In the user.* namespace, for symlinks and special files, only > >>> + * the owner and priviliged users can read/write attributes. > >>> + * For sticky directories, only the owner and privileged users can > >>> + * write attributes. > >>> */ > >>> if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) { > >>> - if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode)) > >>> + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) && > >>> + !inode_owner_or_capable(mnt_userns, inode)) > >>> return (mask & MAY_WRITE) ? -EPERM : -ENODATA; > >>> if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) && > >>> (mask & MAY_WRITE) && > >>> -- > >>> 2.25.4 > >>> > > _______________________________________________ > Virtio-fs mailing list > Virtio-fs@redhat.com > https://listman.redhat.com/mailman/listinfo/virtio-fs >
On Fri, Jul 09, 2021 at 04:10:16PM -0400, Bruce Fields wrote: > On Fri, Jul 9, 2021 at 1:59 PM Vivek Goyal <vgoyal@redhat.com> wrote: > > nfs seems to have some issues. > > I'm not sure what the expected behavior is for nfs. All I have for > now is some generic troubleshooting ideas, sorry: > > > - I can set user.foo xattr on symlink and query it back using xattr name. > > > > getfattr -h -n user.foo foo-link.txt > > > > But when I try to dump all xattrs on this file, user.foo is being > > filtered out it looks like. Not sure why. > > Logging into the server and seeing what's set there could help confirm > whether it's the client or server that's at fault. (Or watching the > traffic in wireshark; there are GET/SET/LISTXATTR ops that should be > easy to spot.) > > > - I can't set "user.foo" xattr on a device node on nfs and I get > > "Permission denied". I am assuming nfs server is returning this. > > Wireshark should tell you whether it's the server or client doing that. > > The RFC is https://datatracker.ietf.org/doc/html/rfc8276, and I don't > see any explicit statement about what the server should do in the case > of symlinks or device nodes, but I do see "Any regular file or > directory may have a set of extended attributes", so that was clearly > the assumption. Also, NFS4ERR_WRONG_TYPE is listed as a possible > error return for the xattr ops. But on a quick skim I don't see any > explicit checks in the nfsd code, so I *think* it's just relying on > the vfs for any file type checks. Hi Bruce, Thanks for the response. I am just trying to do set a user.foo xattr on a device node on nfs. setfattr -n "user.foo" -v "bar" /mnt/nfs/test-dev and I get -EACCESS. I put some printk() statements and EACCESS is being returned from here. nfs4_xattr_set_nfs4_user() { if (!nfs_access_get_cached(inode, current_cred(), &cache, true)) { if (!(cache.mask & NFS_ACCESS_XAWRITE)) { return -EACCES; } } } Value of cache.mask=0xd at the time of error. Thanks Vivek
On Mon, Jul 12, 2021 at 10:02:47AM -0400, Vivek Goyal wrote: > On Fri, Jul 09, 2021 at 04:10:16PM -0400, Bruce Fields wrote: > > On Fri, Jul 9, 2021 at 1:59 PM Vivek Goyal <vgoyal@redhat.com> wrote: > > > nfs seems to have some issues. > > > > I'm not sure what the expected behavior is for nfs. All I have for > > now is some generic troubleshooting ideas, sorry: > > > > > - I can set user.foo xattr on symlink and query it back using xattr name. > > > > > > getfattr -h -n user.foo foo-link.txt > > > > > > But when I try to dump all xattrs on this file, user.foo is being > > > filtered out it looks like. Not sure why. > > > > Logging into the server and seeing what's set there could help confirm > > whether it's the client or server that's at fault. (Or watching the > > traffic in wireshark; there are GET/SET/LISTXATTR ops that should be > > easy to spot.) > > > > > - I can't set "user.foo" xattr on a device node on nfs and I get > > > "Permission denied". I am assuming nfs server is returning this. > > > > Wireshark should tell you whether it's the server or client doing that. > > > > The RFC is https://datatracker.ietf.org/doc/html/rfc8276, and I don't > > see any explicit statement about what the server should do in the case > > of symlinks or device nodes, but I do see "Any regular file or > > directory may have a set of extended attributes", so that was clearly > > the assumption. Also, NFS4ERR_WRONG_TYPE is listed as a possible > > error return for the xattr ops. But on a quick skim I don't see any > > explicit checks in the nfsd code, so I *think* it's just relying on > > the vfs for any file type checks. > > Hi Bruce, > > Thanks for the response. I am just trying to do set a user.foo xattr on > a device node on nfs. > > setfattr -n "user.foo" -v "bar" /mnt/nfs/test-dev > > and I get -EACCESS. > > I put some printk() statements and EACCESS is being returned from here. > > nfs4_xattr_set_nfs4_user() { > if (!nfs_access_get_cached(inode, current_cred(), &cache, true)) { > if (!(cache.mask & NFS_ACCESS_XAWRITE)) { > return -EACCES; > } > } > } > > Value of cache.mask=0xd at the time of error. Looks like 0xd is what the server returns to access on a device node with mode bits rw- for the caller. Commit c11d7fd1b317 "nfsd: take xattr bits into account for permission checks" added the ACCESS_X* bits for regular files and directories but not others. But you don't want to determine permission from the mode bits anyway, you want it to depend on the owner, so I guess we should be calling xattr_permission somewhere if we want that behavior. The RFC assumes user xattrs are for regular files and directories, without, as far as I can tell, actually explicitly forbidding them on other objects. We should also raise this with the working group if we want to increase the chances that you'll get the behavior you want on non-Linux servers. The "User extended attributes" section of the xattr(7) man page will need updating. --b.
On Mon, Jul 12, 2021 at 11:41:06AM -0400, J. Bruce Fields wrote: > On Mon, Jul 12, 2021 at 10:02:47AM -0400, Vivek Goyal wrote: > > On Fri, Jul 09, 2021 at 04:10:16PM -0400, Bruce Fields wrote: > > > On Fri, Jul 9, 2021 at 1:59 PM Vivek Goyal <vgoyal@redhat.com> wrote: > > > > nfs seems to have some issues. > > > > > > I'm not sure what the expected behavior is for nfs. All I have for > > > now is some generic troubleshooting ideas, sorry: > > > > > > > - I can set user.foo xattr on symlink and query it back using xattr name. > > > > > > > > getfattr -h -n user.foo foo-link.txt > > > > > > > > But when I try to dump all xattrs on this file, user.foo is being > > > > filtered out it looks like. Not sure why. > > > > > > Logging into the server and seeing what's set there could help confirm > > > whether it's the client or server that's at fault. (Or watching the > > > traffic in wireshark; there are GET/SET/LISTXATTR ops that should be > > > easy to spot.) > > > > > > > - I can't set "user.foo" xattr on a device node on nfs and I get > > > > "Permission denied". I am assuming nfs server is returning this. > > > > > > Wireshark should tell you whether it's the server or client doing that. > > > > > > The RFC is https://datatracker.ietf.org/doc/html/rfc8276, and I don't > > > see any explicit statement about what the server should do in the case > > > of symlinks or device nodes, but I do see "Any regular file or > > > directory may have a set of extended attributes", so that was clearly > > > the assumption. Also, NFS4ERR_WRONG_TYPE is listed as a possible > > > error return for the xattr ops. But on a quick skim I don't see any > > > explicit checks in the nfsd code, so I *think* it's just relying on > > > the vfs for any file type checks. > > > > Hi Bruce, > > > > Thanks for the response. I am just trying to do set a user.foo xattr on > > a device node on nfs. > > > > setfattr -n "user.foo" -v "bar" /mnt/nfs/test-dev > > > > and I get -EACCESS. > > > > I put some printk() statements and EACCESS is being returned from here. > > > > nfs4_xattr_set_nfs4_user() { > > if (!nfs_access_get_cached(inode, current_cred(), &cache, true)) { > > if (!(cache.mask & NFS_ACCESS_XAWRITE)) { > > return -EACCES; > > } > > } > > } > > > > Value of cache.mask=0xd at the time of error. > > Looks like 0xd is what the server returns to access on a device node > with mode bits rw- for the caller. > > Commit c11d7fd1b317 "nfsd: take xattr bits into account for permission > checks" added the ACCESS_X* bits for regular files and directories but > not others. > > But you don't want to determine permission from the mode bits anyway, > you want it to depend on the owner, Thinking more about this part. Current implementation of my patch is effectively doing both the checks. It checks that you are owner or have CAP_FOWNER in xattr_permission() and then goes on to call inode_permission(). And that means file mode bits will also play a role. If caller does not have write permission on the file, it will be denied setxattr(). If I don't call inode_permission(), and just return 0 right away for file owner (for symlinks and special files), then just being owner is enough to write user.* xattr. And then even security modules will not get a chance to block that operation. IOW, if you are owner of a symlink or special file, you can write as many user.* xattr as you like and except quota does not look like anything else can block it. I am wondering if this approach is ok? > so I guess we should be calling > xattr_permission somewhere if we want that behavior. > > The RFC assumes user xattrs are for regular files and directories, > without, as far as I can tell, actually explicitly forbidding them on > other objects. We should also raise this with the working group if we > want to increase the chances that you'll get the behavior you want on > non-Linux servers. Ok. I am hoping once this patch merges in some form, then I can follow it up with relevant working group. > > The "User extended attributes" section of the xattr(7) man page will > need updating. Agreed. I will take care of that in a separate patch. Right now, I am not too sure if being owner should be the only check and I should skip calling inode_permission() entirely or not. Thanks Vivek > > --b. >
On Fri, Jul 09, 2021 at 04:36:33PM -0400, Theodore Ts'o wrote: > On Fri, Jul 09, 2021 at 08:34:41AM -0700, Casey Schaufler wrote: > > >> One question, do all filesystem supporting xattrs deal with setting them > > >> on symlinks/device files correctly? > > > Wrote a simple bash script to do setfattr/getfattr user.foo xattr on > > > symlink and device node on ext4, xfs and btrfs and it works fine. > > > > How about nfs, tmpfs, overlayfs and/or some of the other less conventional > > filesystems? > > As a suggestion, perhaps you could take your bash script and turn it > into an xfstests test so we can more easily test various file systems, > both now and once the commit is accepted, to look for regressions? Sounds good. I see there is already an xattr test (generic/062) which is broken after my patch. Current test expects that user.* xattrs will fail on symlink/special device. I will probably have to query kernel version and modify test so that expect failure before a certain version and success otherwise. Thanks Vivek
On Mon, Jul 12, 2021 at 01:47:59PM -0400, Vivek Goyal wrote: > On Mon, Jul 12, 2021 at 11:41:06AM -0400, J. Bruce Fields wrote: > > Looks like 0xd is what the server returns to access on a device node > > with mode bits rw- for the caller. > > > > Commit c11d7fd1b317 "nfsd: take xattr bits into account for permission > > checks" added the ACCESS_X* bits for regular files and directories but > > not others. > > > > But you don't want to determine permission from the mode bits anyway, > > you want it to depend on the owner, > > Thinking more about this part. Current implementation of my patch is > effectively doing both the checks. It checks that you are owner or > have CAP_FOWNER in xattr_permission() and then goes on to call > inode_permission(). And that means file mode bits will also play a > role. If caller does not have write permission on the file, it will > be denied setxattr(). > > If I don't call inode_permission(), and just return 0 right away for > file owner (for symlinks and special files), then just being owner > is enough to write user.* xattr. And then even security modules will > not get a chance to block that operation. IOW, if you are owner of > a symlink or special file, you can write as many user.* xattr as you > like and except quota does not look like anything else can block > it. I am wondering if this approach is ok? Yeah, I'd expect security modules to get a say, and I wouldn't expect mode bits on device nodes to be useful for deciding whether it makes sense for xattrs to be readable or writeable. But, I don't really know. Do we have any other use cases besides this case of storing security labels in user xattrs? --b.
On Mon, Jul 12, 2021 at 03:31:39PM -0400, J. Bruce Fields wrote: > On Mon, Jul 12, 2021 at 01:47:59PM -0400, Vivek Goyal wrote: > > On Mon, Jul 12, 2021 at 11:41:06AM -0400, J. Bruce Fields wrote: > > > Looks like 0xd is what the server returns to access on a device node > > > with mode bits rw- for the caller. > > > > > > Commit c11d7fd1b317 "nfsd: take xattr bits into account for permission > > > checks" added the ACCESS_X* bits for regular files and directories but > > > not others. > > > > > > But you don't want to determine permission from the mode bits anyway, > > > you want it to depend on the owner, > > > > Thinking more about this part. Current implementation of my patch is > > effectively doing both the checks. It checks that you are owner or > > have CAP_FOWNER in xattr_permission() and then goes on to call > > inode_permission(). And that means file mode bits will also play a > > role. If caller does not have write permission on the file, it will > > be denied setxattr(). > > > > If I don't call inode_permission(), and just return 0 right away for > > file owner (for symlinks and special files), then just being owner > > is enough to write user.* xattr. And then even security modules will > > not get a chance to block that operation. IOW, if you are owner of > > a symlink or special file, you can write as many user.* xattr as you > > like and except quota does not look like anything else can block > > it. I am wondering if this approach is ok? > > Yeah, I'd expect security modules to get a say, and I wouldn't expect > mode bits on device nodes to be useful for deciding whether it makes > sense for xattrs to be readable or writeable. Actually, calling inode_permission() for symlinks probably should be fine. Its the device node which is problematic. Because we started with the assumption that mode bits there represent access writes for read/writing to device (and not to the filesystem). > > But, I don't really know. > > Do we have any other use cases besides this case of storing security > labels in user xattrs? Storing security label was one example. In case of virtiofs, there is a good chance that we will end up remapping all the guest xattrs and prefix these with "user.virtiofsd". fuse-overlay is another use case. They are storing real uid/gid in user.* xattrs for files over NFS. I think overlayfs can be another benefeciary in some form. Now there is support for unpriviliged mouting of overlayfs from inside a user namespace. And that uses xattrs "user.overlay" on upper files for overlayfs specific metadata. Device nodes are not copied up. But they might have an issue with symlinks. Miklos, will know more. Thanks Vivek
On 7/12/2021 10:47 AM, Vivek Goyal wrote: > On Mon, Jul 12, 2021 at 11:41:06AM -0400, J. Bruce Fields wrote: >> On Mon, Jul 12, 2021 at 10:02:47AM -0400, Vivek Goyal wrote: >>> On Fri, Jul 09, 2021 at 04:10:16PM -0400, Bruce Fields wrote: >>>> On Fri, Jul 9, 2021 at 1:59 PM Vivek Goyal <vgoyal@redhat.com> wrote: >>>>> nfs seems to have some issues. >>>> I'm not sure what the expected behavior is for nfs. All I have for >>>> now is some generic troubleshooting ideas, sorry: >>>> >>>>> - I can set user.foo xattr on symlink and query it back using xattr name. >>>>> >>>>> getfattr -h -n user.foo foo-link.txt >>>>> >>>>> But when I try to dump all xattrs on this file, user.foo is being >>>>> filtered out it looks like. Not sure why. >>>> Logging into the server and seeing what's set there could help confirm >>>> whether it's the client or server that's at fault. (Or watching the >>>> traffic in wireshark; there are GET/SET/LISTXATTR ops that should be >>>> easy to spot.) >>>> >>>>> - I can't set "user.foo" xattr on a device node on nfs and I get >>>>> "Permission denied". I am assuming nfs server is returning this. >>>> Wireshark should tell you whether it's the server or client doing that. >>>> >>>> The RFC is https://datatracker.ietf.org/doc/html/rfc8276, and I don't >>>> see any explicit statement about what the server should do in the case >>>> of symlinks or device nodes, but I do see "Any regular file or >>>> directory may have a set of extended attributes", so that was clearly >>>> the assumption. Also, NFS4ERR_WRONG_TYPE is listed as a possible >>>> error return for the xattr ops. But on a quick skim I don't see any >>>> explicit checks in the nfsd code, so I *think* it's just relying on >>>> the vfs for any file type checks. >>> Hi Bruce, >>> >>> Thanks for the response. I am just trying to do set a user.foo xattr on >>> a device node on nfs. >>> >>> setfattr -n "user.foo" -v "bar" /mnt/nfs/test-dev >>> >>> and I get -EACCESS. >>> >>> I put some printk() statements and EACCESS is being returned from here. >>> >>> nfs4_xattr_set_nfs4_user() { >>> if (!nfs_access_get_cached(inode, current_cred(), &cache, true)) { >>> if (!(cache.mask & NFS_ACCESS_XAWRITE)) { >>> return -EACCES; >>> } >>> } >>> } >>> >>> Value of cache.mask=0xd at the time of error. >> Looks like 0xd is what the server returns to access on a device node >> with mode bits rw- for the caller. >> >> Commit c11d7fd1b317 "nfsd: take xattr bits into account for permission >> checks" added the ACCESS_X* bits for regular files and directories but >> not others. >> >> But you don't want to determine permission from the mode bits anyway, >> you want it to depend on the owner, > Thinking more about this part. Current implementation of my patch is > effectively doing both the checks. It checks that you are owner or > have CAP_FOWNER in xattr_permission() and then goes on to call > inode_permission(). And that means file mode bits will also play a > role. If caller does not have write permission on the file, it will > be denied setxattr(). > > If I don't call inode_permission(), and just return 0 right away for > file owner (for symlinks and special files), then just being owner > is enough to write user.* xattr. And then even security modules will > not get a chance to block that operation. That isn't going to fly. SELinux and Smack don't rely on ownership as a criteria for access. Being the owner of a symlink conveys no special privilege. The LSM must be consulted to determine if the module's policy allows the access. > IOW, if you are owner of > a symlink or special file, you can write as many user.* xattr as you > like and except quota does not look like anything else can block > it. I am wondering if this approach is ok? > > > >> so I guess we should be calling >> xattr_permission somewhere if we want that behavior. >> The RFC assumes user xattrs are for regular files and directories, >> without, as far as I can tell, actually explicitly forbidding them on >> other objects. We should also raise this with the working group if we >> want to increase the chances that you'll get the behavior you want on >> non-Linux servers. > Ok. I am hoping once this patch merges in some form, then I can > follow it up with relevant working group. > >> The "User extended attributes" section of the xattr(7) man page will >> need updating. > Agreed. I will take care of that in a separate patch. > > Right now, I am not too sure if being owner should be the only check > and I should skip calling inode_permission() entirely or not. > > Thanks > Vivek > >> --b. >>
On 7/12/2021 5:49 AM, Greg Kurz wrote: > On Fri, 9 Jul 2021 08:34:41 -0700 > Casey Schaufler <casey@schaufler-ca.com> wrote: > >> On 7/9/2021 8:27 AM, Vivek Goyal wrote: >>> On Fri, Jul 09, 2021 at 11:19:15AM +0200, Christian Brauner wrote: >>>> On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote: >>>>> Currently user.* xattr are not allowed on symlink and special files. >>>>> >>>>> man xattr and recent discussion suggested that primary reason for this >>>>> restriction is how file permissions for symlinks and special files >>>>> are little different from regular files and directories. >>>>> >>>>> For symlinks, they are world readable/writable and if user xattr were >>>>> to be permitted, it will allow unpriviliged users to dump a huge amount >>>>> of user.* xattrs on symlinks without any control. >>>>> >>>>> For special files, permissions typically control capability to read/write >>>>> from devices (and not necessarily from filesystem). So if a user can >>>>> write to device (/dev/null), does not necessarily mean it should be allowed >>>>> to write large number of user.* xattrs on the filesystem device node is >>>>> residing in. >>>>> >>>>> This patch proposes to relax the restrictions a bit and allow file owner >>>>> or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs >>>>> on symlink and special files. >>>>> >>>>> virtiofs daemon has a need to store user.* xatrrs on all the files >>>>> (including symlinks and special files), and currently that fails. This >>>>> patch should help. >>>>> >>>>> Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.com/ >>>>> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> >>>>> --- >>>> Seems reasonable and useful. >>>> Acked-by: Christian Brauner <christian.brauner@ubuntu.com> >>>> >>>> One question, do all filesystem supporting xattrs deal with setting them >>>> on symlinks/device files correctly? >>> Wrote a simple bash script to do setfattr/getfattr user.foo xattr on >>> symlink and device node on ext4, xfs and btrfs and it works fine. >> How about nfs, tmpfs, overlayfs and/or some of the other less conventional >> filesystems? >> > How about virtiofs then ? :-) One of the "less conventional filesystems", surely. �
On Tue, Jul 13, 2021 at 07:17:00AM -0700, Casey Schaufler wrote: > On 7/12/2021 10:47 AM, Vivek Goyal wrote: > > On Mon, Jul 12, 2021 at 11:41:06AM -0400, J. Bruce Fields wrote: > >> On Mon, Jul 12, 2021 at 10:02:47AM -0400, Vivek Goyal wrote: > >>> On Fri, Jul 09, 2021 at 04:10:16PM -0400, Bruce Fields wrote: > >>>> On Fri, Jul 9, 2021 at 1:59 PM Vivek Goyal <vgoyal@redhat.com> wrote: > >>>>> nfs seems to have some issues. > >>>> I'm not sure what the expected behavior is for nfs. All I have for > >>>> now is some generic troubleshooting ideas, sorry: > >>>> > >>>>> - I can set user.foo xattr on symlink and query it back using xattr name. > >>>>> > >>>>> getfattr -h -n user.foo foo-link.txt > >>>>> > >>>>> But when I try to dump all xattrs on this file, user.foo is being > >>>>> filtered out it looks like. Not sure why. > >>>> Logging into the server and seeing what's set there could help confirm > >>>> whether it's the client or server that's at fault. (Or watching the > >>>> traffic in wireshark; there are GET/SET/LISTXATTR ops that should be > >>>> easy to spot.) > >>>> > >>>>> - I can't set "user.foo" xattr on a device node on nfs and I get > >>>>> "Permission denied". I am assuming nfs server is returning this. > >>>> Wireshark should tell you whether it's the server or client doing that. > >>>> > >>>> The RFC is https://datatracker.ietf.org/doc/html/rfc8276, and I don't > >>>> see any explicit statement about what the server should do in the case > >>>> of symlinks or device nodes, but I do see "Any regular file or > >>>> directory may have a set of extended attributes", so that was clearly > >>>> the assumption. Also, NFS4ERR_WRONG_TYPE is listed as a possible > >>>> error return for the xattr ops. But on a quick skim I don't see any > >>>> explicit checks in the nfsd code, so I *think* it's just relying on > >>>> the vfs for any file type checks. > >>> Hi Bruce, > >>> > >>> Thanks for the response. I am just trying to do set a user.foo xattr on > >>> a device node on nfs. > >>> > >>> setfattr -n "user.foo" -v "bar" /mnt/nfs/test-dev > >>> > >>> and I get -EACCESS. > >>> > >>> I put some printk() statements and EACCESS is being returned from here. > >>> > >>> nfs4_xattr_set_nfs4_user() { > >>> if (!nfs_access_get_cached(inode, current_cred(), &cache, true)) { > >>> if (!(cache.mask & NFS_ACCESS_XAWRITE)) { > >>> return -EACCES; > >>> } > >>> } > >>> } > >>> > >>> Value of cache.mask=0xd at the time of error. > >> Looks like 0xd is what the server returns to access on a device node > >> with mode bits rw- for the caller. > >> > >> Commit c11d7fd1b317 "nfsd: take xattr bits into account for permission > >> checks" added the ACCESS_X* bits for regular files and directories but > >> not others. > >> > >> But you don't want to determine permission from the mode bits anyway, > >> you want it to depend on the owner, > > Thinking more about this part. Current implementation of my patch is > > effectively doing both the checks. It checks that you are owner or > > have CAP_FOWNER in xattr_permission() and then goes on to call > > inode_permission(). And that means file mode bits will also play a > > role. If caller does not have write permission on the file, it will > > be denied setxattr(). > > > > If I don't call inode_permission(), and just return 0 right away for > > file owner (for symlinks and special files), then just being owner > > is enough to write user.* xattr. And then even security modules will > > not get a chance to block that operation. > > That isn't going to fly. SELinux and Smack don't rely on ownership > as a criteria for access. Being the owner of a symlink conveys no > special privilege. The LSM must be consulted to determine if the > module's policy allows the access. Getting back to this thread after a while. Sorry got busy in other things. I noticed that if we skip calling inode_permission() for special files, then we will skip calling security_inode_permission() but we will still call security hooks for setxattr/getxattr/removexattr etc. security_inode_setxattr() security_inode_getxattr() security_inode_removexattr() So LSMs will still get a chance whether to allow/disallow this operation or not. And skipping security_inode_permission() kind of makes sense that for special files, I am not writing to device. So taking permission from LSMs, will not make much sense. Thanks Vivek
diff --git a/fs/xattr.c b/fs/xattr.c index 5c8c5175b385..2f1855c8b620 100644 --- a/fs/xattr.c +++ b/fs/xattr.c @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode, } /* - * In the user.* namespace, only regular files and directories can have - * extended attributes. For sticky directories, only the owner and - * privileged users can write attributes. + * In the user.* namespace, for symlinks and special files, only + * the owner and priviliged users can read/write attributes. + * For sticky directories, only the owner and privileged users can + * write attributes. */ if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) { - if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode)) + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) && + !inode_owner_or_capable(mnt_userns, inode)) return (mask & MAY_WRITE) ? -EPERM : -ENODATA; if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) && (mask & MAY_WRITE) &&
Currently user.* xattr are not allowed on symlink and special files. man xattr and recent discussion suggested that primary reason for this restriction is how file permissions for symlinks and special files are little different from regular files and directories. For symlinks, they are world readable/writable and if user xattr were to be permitted, it will allow unpriviliged users to dump a huge amount of user.* xattrs on symlinks without any control. For special files, permissions typically control capability to read/write from devices (and not necessarily from filesystem). So if a user can write to device (/dev/null), does not necessarily mean it should be allowed to write large number of user.* xattrs on the filesystem device node is residing in. This patch proposes to relax the restrictions a bit and allow file owner or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs on symlink and special files. virtiofs daemon has a need to store user.* xatrrs on all the files (including symlinks and special files), and currently that fails. This patch should help. Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.com/ Signed-off-by: Vivek Goyal <vgoyal@redhat.com> --- fs/xattr.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)