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

["Jiri Slaby (SUSE)" <jirislaby@kernel.org>]

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()` 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
---
 drivers/gpu/drm/qxl/qxl_release.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/qxl/qxl_release.c b/drivers/gpu/drm/qxl/qxl_release.c
index 06979d0e8a9f..92c88f9c17a8 100644
--- a/drivers/gpu/drm/qxl/qxl_release.c
+++ b/drivers/gpu/drm/qxl/qxl_release.c
@@ -91,15 +91,12 @@ qxl_release_alloc(struct qxl_device *qdev, int type,
 	int handle;
 	size_t size = sizeof(*release);
 
-	release = kmalloc(size, GFP_KERNEL);
+	release = kzalloc(size, GFP_KERNEL);
 	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);
-- 
2.55.0