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From:   Emil Velikov <emil.l.velikov@gmail.com>
Date:   Mon, 3 Apr 2023 14:22:20 +0100
Message-ID: <CACvgo53bkvDm7CXN_zFETZeuWas50tJys6u+nc5DO_MWK4SZfw@mail.gmail.com>
Subject: Re: [PATCH v4 2/2] drm/virtio: Support sync objects
To:     Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc:     Emil Velikov <emil.velikov@collabora.com>,
        Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>,
        =?UTF-8?B?TWFyZWsgT2zFocOhaw==?= <maraeo@gmail.com>,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Gurchetan Singh <gurchetansingh@chromium.org>,
        Gerd Hoffmann <kraxel@redhat.com>,
        David Airlie <airlied@redhat.com>, kernel@collabora.com,
        virtualization@lists.linux-foundation.org
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On Mon, 3 Apr 2023 at 14:00, Emil Velikov <emil.l.velikov@gmail.com> 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