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

[Vivek Goyal]

On Fri, Jun 19, 2020 at 04:16:30PM +0200, Miklos Szeredi wrote:
> On Thu, Jun 18, 2020 at 9:08 PM Vivek Goyal <vgoyal@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.
> 
> How so?  Virtiofs isn't mounting overlayfs, is it?  Only the mounter
> requires CAP_SYS_ADMIN, not the accessor.

virtiofsd needs CAP_SYS_ADMIN, otherwise fsetxattr(trusted.overlay.opaque)
fails in lo_setxattr().

This is triggered when we mount overlayfs on top of virtiofs and overlayfs
tries to set OVL_XATTR_OPAQUE on upper to check if trusted xattrs are
supported or not.

Thanks
Vivek