[Virtio-fs] [PATCH 0/2] virtiofsd: drop Linux capabilities(7)

[Dr. David Alan Gilbert]

* Vivek Goyal (vgoyal at redhat.com) wrote:
> On Thu, Jun 18, 2020 at 08:16:55PM +0100, Dr. David Alan Gilbert wrote:
> > * Vivek Goyal (vgoyal at redhat.com) wrote:
> > > On Thu, Apr 16, 2020 at 05:49:05PM +0100, Stefan Hajnoczi wrote:
> > > > virtiofsd doesn't need of all Linux capabilities(7) available to root.  Keep a
> > > > whitelisted set of capabilities that we require.  This improves security in
> > > > case virtiofsd is compromised by making it hard for an attacker to gain further
> > > > access to the system.
> > > 
> > > Hi Stefan,
> > > 
> > > I just noticed that this patch set breaks overlayfs on top of virtiofs.
> > > 
> > > overlayfs sets "trusted.overlay.*" and xattrs in trusted domain
> > > need CAP_SYS_ADMIN.
> > > 
> > > man xattr says.
> > > 
> > >    Trusted extended attributes
> > >        Trusted  extended  attributes  are  visible and accessible only to pro‐
> > >        cesses that have the  CAP_SYS_ADMIN  capability.   Attributes  in  this
> > >        class are used to implement mechanisms in user space (i.e., outside the
> > >        kernel) which keep information in extended attributes to which ordinary
> > >        processes should not have access.
> > > 
> > > There is a chance that overlay moves away from trusted xattr in future.
> > > But for now we need to make it work. This is an important use case for
> > > kata docker in docker build.
> > > 
> > > May be we can add an option to virtiofsd say "--add-cap <capability>" and
> > > ask user to pass in "--add-cap cap_sys_admin" if they need to run daemon
> > > with this capaibility.
> > 
> > I'll admit I don't like the idea of giving it cap_sys_admin.
> > Can you explain:
> >   a) What overlayfs uses trusted for?
> 
> overlayfs stores bunch of metadata and uses "trusted" xattrs for it.

Tell me more about this metadata.
Taking a juicy looking one, what does OVL_XATTR_REDIRECT do?
Or what happens if I was to write random numbers into OVL_XATTR_NLINK?

> >   b) If something nasty was to write junk into the trusted attributes,
> >     what would happen?
> 
> This directory is owned by guest. So it should be able to write
> anything it wants, as long as process in guest has CAP_SYS_ADMIN, right?

Well, we shouldn't be able to break/crash/escape into the host; how
much does overlayfs validate trusted.* it uses?

> >   c) I see overlayfs has a fallback check if xattr isn't supported at
> > all - what is the consequence?
> 
> It falls back to I think read only mode. 

It looks like the fallback is more subtle to me:
        /*
         * Check if upper/work fs supports trusted.overlay.* xattr
         */
        err = ovl_do_setxattr(ofs->workdir, OVL_XATTR_OPAQUE, "0", 1, 0);
        if (err) {
                ofs->noxattr = true;
                ofs->config.index = false;
                ofs->config.metacopy = false;
                pr_warn("upper fs does not support xattr, falling back to index=off and metacopy=off.\n");

but I don't know what index and metacopy are.

> For a moment forget about overlayfs. Say a user process in guest with
> CAP_SYS_ADMIN is writing trusted.foo. Should that succeed? Is a
> passthrough filesystem, so it should go through. But currently it
> wont.

As long as any effects of what it writes are contained to the area of
the filesystem exposed to the guest, yes - however it worries me what
the consequences of broken trusted metadata is.  If it's delicate enough
that it's guarded by CAP_SYS_ADMIN someone must have worried about it.

Dave

> Thanks
> Vivek
--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK