Re: [Virtio-fs] [PATCH v2] virtiofsd: prevent opening of special files (CVE-2020-35517)

[Greg Kurz <groug@kaod.org>]

On Wed, 27 Jan 2021 16:22:49 +0100
Miklos Szeredi <mszeredi@redhat.com> wrote:

> On Wed, Jan 27, 2021 at 4:09 PM Greg Kurz <groug@kaod.org> wrote:
> >
> > On Wed, 27 Jan 2021 15:09:50 +0100
> > Miklos Szeredi <mszeredi@redhat.com> wrote:
> > > The semantics of O_CREATE are that it can fail neither because the
> > > file exists nor because it doesn't.  This doesn't matter if the
> > > exported tree is not modified outside of a single guest, because of
> > > locking provided by the guest kernel.
> > >
> >
> > Wrong. O_CREAT can legitimately fail with ENOENT if one element
> 
> Let me make my  statement more precise:
> 
> O_CREAT cannot fail with ENOENT if parent directory exists throughout
> the operation.
> 

True, but I still don't see what guarantees guest userspace that the
parent directory doesn't go away... I must have missed something.
Please elaborate.

> I'm sure this property is used all over the place in userspace code,
> and hence should be supported in strict coherence (cache=none) mode.
> 
> For relaxed coherence (cache=auto) I'm not quite sure.  NFS is usually
> the reference, we'd need to look into what guarantees it provides wrt.
> O_CREAT and remote racing unlink.
> 
> Thanks,
> Miklos
>