Re: [PATCH v4 2/2] drm/virtio: Support sync objects

[Emil Velikov]

On Mon, 3 Apr 2023 at 14:00, Emil Velikov <emil.l.velikov@xxxxxxxxx> wrote:

> > > I think we should zero num_(in|out)_syncobjs when the respective parse
> > > fails. Otherwise we get one "cleanup" within the parse function itself
> > > and a second during the cleanup_submit. Haven't looked at it too closely
> > > but I suspect that will trigger an UAF or two.
> >
> > There are checks for NULL pointers in the code that will prevent the
> > UAF.  I'll add zeroing of the nums for more consistency.
> >
>
> Riiiight the drm_syncobj is attached to the encapsulating struct
> virtio_gpu_submit _only_ on success.
> By clearing the num variables,  the NULL checks will no longer be
> needed ... in case you'd want to drop that.
>
> Either way - even as-is the code is safe.
>

Err or not. The NULL check itself will cause NULL pointer deref.

In more detail: in/out syncobjs are memset() to NULL in
virtio_gpu_init_submit(). The virtio_gpu_parse_(|post_)deps() will
fail and leave them unchanged. Then virtio_gpu_free_syncobjs() and
virtio_gpu_reset_syncobjs() will trigger a NULL ptr deref, because
they are accessing the elements of a the (NULL) array.

Apart from the num_(in|out)_syncobjcs=0, I would drop the NULL checks
- they give a false sense of security IMHO.

-Emil