[RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special files if caller has CAP_SYS_RESOURCE

[Dr. David Alan Gilbert]

* Schaufler, Casey (casey.schaufler at intel.com) wrote:
> > -----Original Message-----
> > From: Vivek Goyal <vgoyal at redhat.com>
> > Sent: Friday, June 25, 2021 12:12 PM
> > To: linux-fsdevel at vger.kernel.org; linux-kernel at vger.kernel.org;
> > viro at zeniv.linux.org.uk
> > Cc: virtio-fs at redhat.com; dwalsh at redhat.com; dgilbert at redhat.com;
> > berrange at redhat.com; vgoyal at redhat.com
> 
> Please include Linux Security Module list <linux-security-module at vger.kernel.org>
> and selinux at vger.kernel.org on this topic.
> 
> > Subject: [RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special files if
> > caller has CAP_SYS_RESOURCE
> > 
> > Hi,
> > 
> > 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 would seem to provide mechanism whereby a user can violate
> SELinux policy quite easily. 
> 
> > 
> > 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.
> 
> Can you please provide some rationale for this assertion?
> I have been working with security xattrs longer than anyone
> and have trouble accepting the statement.

There seem to be a few very different ways of using SELinux in
containers/guests, and many ways of using shared filesystems.

A common request is that we share a host filesystem into the guest (a
VM), and then the guest can do with it whatever it likes, preferably
without making the guest privileged in any way, and with having as few
priviliges on the daemons running on behalf of the guest ('virtiofd'
which is a fuse implementation daemon that runs on the host).

By remapping all guests xattr to add a "user.virtiofsd." prefix,
the guest can label it's filesystem and implement it's own SELinux
policy, but because it's using "user." on the host, it can neither
bypass nor change the hosts SELinux labelling or policies.

(It also means that the guest can set capabilities and other xattr's,
again without confusing the host).

> > 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.
> > 
> > "man xattr" seems to suggest that primary reason to disallow is
> > that arbitrary users can set unlimited amount of "user.*" xattrs
> > on these files and bypass quota check.
> > 
> > If that's the primary reason, I am wondering is it possible to relax
> > the restrictions if caller has CAP_SYS_RESOURCE. This capability
> > allows caller to bypass quota checks. So it should not be
> > a problem atleast from quota perpective.
> > 
> > That will allow me to give CAP_SYS_RESOURCE to virtiofs deamon
> > and remap xattrs arbitrarily.
> 
> On a Smack system you should require CAP_MAC_ADMIN to remap
> security. xattrs. I sounds like you're in serious danger of running afoul
> of LSM attribute policy on a reasonable general level.

Note that the remapping is done by the userspace daemon running on the
host (and takes parameters saying what remapping is required); as
such it's still bound by whatever LSM policies the host wants; we're
just giving the guest the ability to add it's own policies without
breaking the hosts.

Of course if you want the guest kernel to see the host xattrs
then you don't want the remapping; there are even some cases where you
might want to allow the guest to set those xattrs; but then you really
do have to start worrying about what the guest could do to your
filesystem.

The only thing getting in the way of the guest being able to do a full
relabel seems to be the limitation on user.* on non-files.

Dave

> > 
> > Thanks
> > Vivek
> > 
> > Vivek Goyal (1):
> >   xattr: Allow user.* xattr on symlink/special files with
> >     CAP_SYS_RESOURCE
> > 
> >  fs/xattr.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > --
> > 2.25.4
> 
-- 
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK