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

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(a)kernel.org&gt; > Assisted-by: Gemini <gemini(a)google.com&gt; # 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(a)amd.com&gt; > Cc: Tvrtko Ursulin <tvrtko.ursulin(a)igalia.com&gt; > Cc: Dave Airlie <airlied(a)redhat.com&gt; > Cc: Gerd Hoffmann <kraxel(a)redhat.com&gt; > Cc: Maarten Lankhorst <maarten.lankhorst(a)linux.intel.com&gt; > Cc: Maxime Ripard <mripard(a)kernel.org&gt; > Cc: Thomas Zimmermann <tzimmermann(a)suse.de&gt; > Cc: David Airlie <airlied(a)gmail.com&gt; > Cc: Simona Vetter <simona(a)ffwll.ch&gt; > Cc: stable(a)vger.kernel.org > --- > Cc: virtualization(a)lists.linux.dev > Cc: spice-devel(a)lists.freedesktop.org > Cc: dri-devel(a)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?