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

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Date:   Mon, 3 Apr 2023 18:07:14 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Emil Velikov <emil.l.velikov@...il.com>
Cc:     Emil Velikov <emil.velikov@...labora.com>,
        Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@....com>,
        Marek Olšák <maraeo@...il.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        David Airlie <airlied@...hat.com>, kernel@...labora.com,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v4 2/2] drm/virtio: Support sync objects

On 4/3/23 16:22, Emil Velikov wrote:
> On Mon, 3 Apr 2023 at 14:00, Emil Velikov <emil.l.velikov@...il.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.

The reset/free are both under the NULL check on cleanup. I think it
should work okay on error. Will improve it anyways to make more
intuitive. Thanks!

static void virtio_gpu_cleanup_submit(struct virtio_gpu_submit *submit)
{
	if (submit->in_syncobjs) {
		virtio_gpu_reset_syncobjs(submit->in_syncobjs,
					  submit->num_in_syncobjs);

		virtio_gpu_free_syncobjs(submit->in_syncobjs,
					 submit->num_in_syncobjs);
	}

-- 
Best regards,
Dmitry