Re: xattr names for unprivileged stacking?

Greg Kurz
2020-07-28T14:27:11Z

On Tue, 28 Jul 2020 11:55:03 +0100
"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> Hi,
> Are there any standards for mapping xattr names/classes when
> a restricted view of the filesystem needs to think it's root?
>
> e.g. VMs that mount host filesystems, remote filesystems etc and the
> client kernel tries to set a trusted. or security. xattr and you want
> to store that on an underlying normal filesystem, but your
> VM system doesn't want to have CAP_SYS_ADMIN and/or doesn't want to
> interfere with the real hosts security.
>
> I can see some existing examples:
>
> 9p in qemu
> maps system.posix_acl_* to user.virtfs.system.posix_acl_*
> stops the guest accessing any user.virtfs.*
>
> overlayfs
> uses trusted.overlay.* on upper layer and blocks that from
> clients
>
> fuse-overlayfs
> uses trusted.overlay.* for compatibiltiy if it has perms,
> otherwise falls back to user.fuseoverlayfs.*
>
> crosvm's virtiofs
> maps "security.sehash" to "user.virtiofs.security.sehash"
> and blocks the guest from accessing user.virtiofs.*
>
> Does anyone know of any others?
>

Hi Dave,

Sorry, I'm not aware of any other example.

Cc'ing Christian Schoenebeck, the new 9p maintainer in QEMU in case
he has some information to share in this area.

Cheers,

--
Greg

> It all seems quite adhoc; these all fall to bits when you
> stack them or when you write a filesystem using one of these
> schemes and then mount it with another.
>
> (I'm about to do a similar mapping for virtiofs's C daemon)
>
> Thanks in advance,
>
> Dave
>
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>