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

[Dr. David Alan Gilbert]

* 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?
  b) If something nasty was to write junk into the trusted attributes,
    what would happen?
  c) I see overlayfs has a fallback check if xattr isn't supported at
all - what is the consequence?

Dave

> Thanks
> Vivek
> 
> > 
> > Stefan Hajnoczi (2):
> >   virtiofsd: only retain file system capabilities
> >   virtiofsd: drop all capabilities in the wait parent process
> > 
> >  tools/virtiofsd/passthrough_ll.c | 51 ++++++++++++++++++++++++++++++++
> >  1 file changed, 51 insertions(+)
> > 
> > -- 
> > 2.25.1
> > 
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--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK