Re: [PATCH] drm/syncobj: Fix use-after-free

["Christian König" <christian.koenig@amd.com>]

Am 19.01.21 um 14:03 schrieb Daniel Vetter:
> While reviewing Christian's annotation patch I noticed that we have a
> user-after-free for the WAIT_FOR_SUBMIT case: We drop the syncobj
> reference before we've completed the waiting.
>
> Of course usually there's nothing bad happening here since userspace
> keeps the reference, but we can't rely on userspace to play nice here!
>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Fixes: bc9c80fe01a2 ("drm/syncobj: use the timeline point in drm_syncobj_find_fence v4")
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: dri-devel@lists.freedesktop.org
> Cc: <stable@vger.kernel.org> # v5.2+

Reviewed-by: Christian König <christian.koenig@amd.com>

> ---
>   drivers/gpu/drm/drm_syncobj.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> index 6e74e6745eca..349146049849 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -388,19 +388,18 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
>   		return -ENOENT;
>   
>   	*fence = drm_syncobj_fence_get(syncobj);
> -	drm_syncobj_put(syncobj);
>   
>   	if (*fence) {
>   		ret = dma_fence_chain_find_seqno(fence, point);
>   		if (!ret)
> -			return 0;
> +			goto out;
>   		dma_fence_put(*fence);
>   	} else {
>   		ret = -EINVAL;
>   	}
>   
>   	if (!(flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT))
> -		return ret;
> +		goto out;
>   
>   	memset(&wait, 0, sizeof(wait));
>   	wait.task = current;
> @@ -432,6 +431,9 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
>   	if (wait.node.next)
>   		drm_syncobj_remove_wait(syncobj, &wait);
>   
> +out:
> +	drm_syncobj_put(syncobj);
> +
>   	return ret;
>   }
>   EXPORT_SYMBOL(drm_syncobj_find_fence);

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