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

[Schaufler, Casey]

> -----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.

> 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.

> 
> 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