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

[Miklos Szeredi]

On Wed, Jan 27, 2021 at 4:35 PM Greg Kurz <groug@kaod.org> wrote:
>
> 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.

Generally there's no guarantee, however there can be certain
situations where the caller can indeed rely on the existence of the
parent (e.g. /tmp).

Thanks,
Miklos