[PATCH v3 0/1] Relax restrictions on user.* xattr

[Andreas Gruenbacher]

Hi,

On Thu, Sep 2, 2021 at 5:22 PM Vivek Goyal <vgoyal at redhat.com> wrote:
> This is V3 of the patch. Previous versions were posted here.
>
> v2: https://lore.kernel.org/linux-fsdevel/20210708175738.360757-1-vgoyal@redhat.com/
> v1: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.com/
>
> Changes since v2
> ----------------
> - Do not call inode_permission() for special files as file mode bits
>   on these files represent permissions to read/write from/to device
>   and not necessarily permission to read/write xattrs. In this case
>   now user.* extended xattrs can be read/written on special files
>   as long as caller is owner of file or has CAP_FOWNER.
>
> - Fixed "man xattr". Will post a patch in same thread little later. (J.
>   Bruce Fields)
>
> - Fixed xfstest 062. Changed it to run only on older kernels where
>   user extended xattrs are not allowed on symlinks/special files. Added
>   a new replacement test 648 which does exactly what 062. Just that
>   it is supposed to run on newer kernels where user extended xattrs
>   are allowed on symlinks and special files. Will post patch in
>   same thread (Ted Ts'o).
>
> Testing
> -------
> - Ran xfstest "./check -g auto" with and without patches and did not
>   notice any new failures.
>
> - Tested setting "user.*" xattr with ext4/xfs/btrfs/overlay/nfs
>   filesystems and it works.
>
> Description
> ===========
>
> Right now we don't allow setting user.* xattrs on symlinks and special
> files at all. Initially I thought that real reason behind this
> restriction is quota limitations but from last conversation it seemed
> that real reason is that permission bits on symlink and special files
> are special and different from regular files and directories, hence
> this restriction is in place. (I tested with xfs user quota enabled and
> quota restrictions kicked in on symlink).
>
> This version of patch allows reading/writing user.* xattr on symlink and
> special files if caller is owner or priviliged (has CAP_FOWNER) w.r.t inode.

the idea behind user.* xattrs is that they behave similar to file
contents as far as permissions go. It follows from that that symlinks
and special files cannot have user.* xattrs. This has been the model
for many years now and applications may be expecting these semantics,
so we cannot simply change the behavior. So NACK from me.

> Who wants to set user.* xattr on symlink/special files
> -----------------------------------------------------
> I have primarily two users at this point of time.
>
> - virtiofs daemon.
>
> - fuse-overlay. Giuseppe, seems to set user.* xattr attrs on unpriviliged
>   fuse-overlay as well and he ran into similar issue. So fuse-overlay
>   should benefit from this change as well.
>
> Why virtiofsd wants to set user.* xattr on symlink/special files
> ----------------------------------------------------------------
> In virtiofs, actual file server is virtiosd daemon running on host.
> There we have a mode where xattrs can be remapped to something else.
> For example security.selinux can be remapped to
> user.virtiofsd.securit.selinux on the host.
>
> This remapping is useful when SELinux is enabled in guest and virtiofs
> as being used as rootfs. Guest and host SELinux policy might not match
> and host policy might deny security.selinux xattr setting by guest
> onto host. Or host might have SELinux disabled and in that case to
> be able to set security.selinux xattr, virtiofsd will need to have
> CAP_SYS_ADMIN (which we are trying to avoid). Being able to remap
> guest security.selinux (or other xattrs) on host to something else
> is also better from security point of view.
>
> But when we try this, we noticed that SELinux relabeling in guest
> is failing on some symlinks. When I debugged a little more, I
> came to know that "user.*" xattrs are not allowed on symlinks
> or special files.
>
> So if we allow owner (or CAP_FOWNER) to set user.* xattr, it will
> allow virtiofs to arbitrarily remap guests's xattrs to something
> else on host and that solves this SELinux issue nicely and provides
> two SELinux policies (host and guest) to co-exist nicely without
> interfering with each other.

The fact that user.* xattrs don't work in this remapping scenario
should have told you that you're doing things wrong; the user.*
namespace seriously was never meant to be abused in this way.

You may be able to get away with using trusted.* xattrs which support
roughly the kind of daemon use I think you're talking about here, but
I'm not sure selinux will be happy with labels that aren't fully under
its own control. I really wonder why this wasn't obvious enough.

Thanks,
Andreas

> Thanks
> Vivek
>
> Vivek Goyal (1):
>   xattr: Allow user.* xattr on symlink and special files
>
>  fs/xattr.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
>
> --
> 2.31.1
>