Re: [PATCH v2] drm/qxl: fix use-after-free and NULL pointer deref

[Tvrtko Ursulin <tvrtko.ursulin@igalia.com>]

On 28/08/2026 09:51, Jiri Slaby (SUSE) wrote:
> When allocating a `qxl_release` structure with `kmalloc()`, the underlying
> memory contained uninitialized garbage. Specifically, `release->base.flags`
> (part of the embedded `dma_fence`) was not cleared.
> 
> This garbage in `base.flags` caused helper functions such as
> `dma_fence_was_initialized()` to return true even for releases where the
> fence was never actually initialized (e.g. via `dma_fence_init()`).
> 
> Consequently, during release cleanup in `qxl_release_free()`, the driver
> attempted to put/free an uninitialized `dma_fence`, leading to refcount
> underflows (`refcount_t: underflow; use-after-free`) and subsequent NULL
> pointer dereferences in `dma_fence_signal_timestamp_locked()`.
> 
> Fix this by switching from `kmalloc()` to `kzalloc_obj()` in
> `qxl_release_alloc()`, ensuring all fields (including embedded fence
> flags) are properly zero-initialized upon allocation, and remove
> redundant explicit zero-initializations.
> 
> The dumps in question:
>   refcount_t: underflow; use-after-free.
>   WARNING: lib/refcount.c:28 at refcount_warn_saturate+0x59/0x90, CPU#0: kworker/0:0/1534
>   Modules linked in: af_packet nft_fib_inet ...
>   CPU: 0 UID: 0 PID: 1534 Comm: kworker/0:0 Not tainted 7.1.3-1-default #1 PREEMPT(full) openSUSE Tumbleweed  b041a6527f6e58424f4cd3de0fade8d408b378fd
>   ...
>   RIP: 0010:refcount_warn_saturate+0x59/0x90
>   ...
>   Call Trace:
>    <TASK>
>    qxl_release_free+0xee/0xf0 [qxl d93e9381353e619799d56790f5f8dda6cce491f6]
>    qxl_garbage_collect+0xd1/0x1b0 [qxl d93e9381353e619799d56790f5f8dda6cce491f6]
>    process_one_work+0x19e/0x3a0
>   ...
> 
> And then of course:
>   BUG: kernel NULL pointer dereference, address: 0000000000000028
>   ...
>   RIP: 0010:dma_fence_signal_timestamp_locked+0x32/0x120
> 
> Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
> Assisted-by: Gemini <gemini@google.com> # only commit log
> Fixes: 2bcbc706dfa0 ("dma-buf: add dma_fence_was_initialized function v2")
> Closes: https://bugzilla.suse.com/show_bug.cgi?id=1271081
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> Cc: Dave Airlie <airlied@redhat.com>
> Cc: Gerd Hoffmann <kraxel@redhat.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@gmail.com>
> Cc: Simona Vetter <simona@ffwll.ch>
> Cc: stable@vger.kernel.org
> ---
> Cc: virtualization@lists.linux.dev
> Cc: spice-devel@lists.freedesktop.org
> Cc: dri-devel@lists.freedesktop.org
> 
> [v2] use kzalloc_obj() instead of bare kzalloc()
> ---
>   drivers/gpu/drm/qxl/qxl_release.c | 6 +-----
>   1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/qxl/qxl_release.c b/drivers/gpu/drm/qxl/qxl_release.c
> index 06979d0e8a9f..07dc6eafe6f7 100644
> --- a/drivers/gpu/drm/qxl/qxl_release.c
> +++ b/drivers/gpu/drm/qxl/qxl_release.c
> @@ -89,17 +89,13 @@ qxl_release_alloc(struct qxl_device *qdev, int type,
>   {
>   	struct qxl_release *release;
>   	int handle;
> -	size_t size = sizeof(*release);
>   
> -	release = kmalloc(size, GFP_KERNEL);
> +	release = kzalloc_obj(*release);
>   	if (!release) {
>   		DRM_ERROR("Out of memory\n");
>   		return -ENOMEM;
>   	}
> -	release->base.ops = NULL;
>   	release->type = type;
> -	release->release_offset = 0;
> -	release->surface_release_id = 0;
>   	INIT_LIST_HEAD(&release->bos);
>   
>   	idr_preload(GFP_KERNEL);

Looks plausible on a superficial look, albeit fragile. I am not sure why 
qxl_release_alloc wasn't calling dma_fence_init in the first place?

Regards,

Tvrtko