[PATCH v3 2/2] drm/sched/tests: add a UAF regression test for the timeline name

["Jonghyuk Kim(MalHyuk)" <malhyuk97@gmail.com>]

Add a KUnit test that reproduces the drm_sched_fence timeline-name
use-after-free fixed by the previous patch. It submits a job on the mock
scheduler, takes an independent reference on the finished fence (standing
in for a userspace sync_file), lets the job finish, frees the scheduler,
and then queries the timeline name through dma_fence_timeline_name().

Without the fix the finished fence keeps its ops attached after
signalling, so this dereferences fence->sched of the freed scheduler and
KASAN reports a slab-use-after-free read in
drm_sched_fence_get_timeline_name(); with the fix the ops are detached on
signalling and a static string is returned.

The test needs no hardware - it exercises the drm_sched core through the
existing mock scheduler under KASAN. Per review it lives in a new
tests_integration.c rather than in tests_basic.c, since it is about the
scheduler's interaction with the dma-fence API rather than scheduler
behaviour in isolation.

Signed-off-by: Jonghyuk Kim(MalHyuk) <malhyuk97@gmail.com>
---
 drivers/gpu/drm/scheduler/tests/Makefile      |  1 +
 .../drm/scheduler/tests/tests_integration.c   | 92 +++++++++++++++++++
 2 files changed, 93 insertions(+)
 create mode 100644 drivers/gpu/drm/scheduler/tests/tests_integration.c

diff --git a/drivers/gpu/drm/scheduler/tests/Makefile b/drivers/gpu/drm/scheduler/tests/Makefile
index 9ec185fbbc15..10abe07c06d2 100644
--- a/drivers/gpu/drm/scheduler/tests/Makefile
+++ b/drivers/gpu/drm/scheduler/tests/Makefile
@@ -3,6 +3,7 @@
 drm-sched-tests-y := \
         mock_scheduler.o \
         tests_basic.o \
+        tests_integration.o \
         tests_scheduler.o
 
 obj-$(CONFIG_DRM_SCHED_KUNIT_TEST) += drm-sched-tests.o
diff --git a/drivers/gpu/drm/scheduler/tests/tests_integration.c b/drivers/gpu/drm/scheduler/tests/tests_integration.c
new file mode 100644
index 000000000000..5e1ca6c6fa5a
--- /dev/null
+++ b/drivers/gpu/drm/scheduler/tests/tests_integration.c
@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/dma-fence.h>
+#include <linux/rcupdate.h>
+
+#include "sched_tests.h"
+
+/*
+ * Integration-style regression tests that exercise the interaction between the
+ * DRM scheduler and the dma-fence API, rather than scheduler behaviour in
+ * isolation.
+ */
+
+/*
+ * Reproduce the drm_sched_fence timeline-name use-after-free.
+ *
+ * drm_sched_fence_get_timeline_name() dereferences fence->sched. A driver may
+ * free a per-context/per-queue/per-VM drm_gpu_scheduler while userspace still
+ * holds the exported ->finished fence (via sync_file / drm_syncobj). Querying
+ * the timeline name afterwards must not touch the freed scheduler.
+ *
+ * The dma-fence contract only permits access to driver-provided data (which
+ * includes the memory reached through &dma_fence.ops) before the fence is
+ * signalled. dma_fence_timeline_name() enforces this by returning a static
+ * string once the ops have been detached on signalling. For that detach to
+ * happen the finished fence must not carry a .release (or .wait) callback.
+ *
+ * Without the fix (finished fence keeps a .release callback) the ops are never
+ * detached, so this reads fence->sched->name from freed slab memory and KASAN
+ * reports a slab-use-after-free. Same class as CVE-2025-38703 (drm/xe) and
+ * CVE-2025-71302 (drm/panthor).
+ */
+static void drm_sched_dma_fence_uaf(struct kunit *test)
+{
+	struct drm_mock_sched_entity *entity;
+	struct drm_mock_scheduler *sched;
+	struct drm_mock_sched_job *job;
+	struct dma_fence *finished;
+	const char __rcu *name;
+	bool done;
+
+	sched = drm_mock_sched_new(test, MAX_SCHEDULE_TIMEOUT);
+	entity = drm_mock_sched_entity_new(test, DRM_SCHED_PRIORITY_NORMAL,
+					   sched);
+	job = drm_mock_sched_job_new(test, entity);
+
+	/* The s_fence is only created by drm_sched_job_arm(). */
+	drm_mock_sched_job_submit(job);
+
+	/* Independent reference on the finished fence == userspace sync_file. */
+	finished = dma_fence_get(&job->base.s_fence->finished);
+
+	/* Let the job get scheduled (hw fence created), then signal + finish. */
+	done = drm_mock_sched_job_wait_scheduled(job, HZ);
+	KUNIT_ASSERT_TRUE(test, done);
+	drm_mock_sched_advance(sched, 1);
+	done = drm_mock_sched_job_wait_finished(job, HZ);
+	KUNIT_ASSERT_TRUE(test, done);
+
+	/*
+	 * Free the per-context scheduler while the finished fence is held.
+	 * kunit_kfree() releases the backing memory immediately (rather than at
+	 * test teardown) so that fence->sched becomes a dangling pointer now.
+	 */
+	drm_mock_sched_entity_free(entity);
+	drm_mock_sched_fini(sched);
+	kunit_kfree(test, sched);
+
+	/*
+	 * Query the timeline name of the now-stale fence. With the fix the ops
+	 * have been detached on signalling and a static string is returned;
+	 * without it this is a use-after-free read of the freed scheduler.
+	 */
+	rcu_read_lock();
+	name = dma_fence_timeline_name(finished);
+	KUNIT_EXPECT_NOT_NULL(test, name);
+	rcu_read_unlock();
+
+	dma_fence_put(finished);
+}
+
+static struct kunit_case drm_sched_dma_fence_tests[] = {
+	KUNIT_CASE(drm_sched_dma_fence_uaf),
+	{}
+};
+
+static struct kunit_suite drm_sched_dma_fence = {
+	.name = "drm-sched-dma-fence-uaf",
+	.test_cases = drm_sched_dma_fence_tests,
+};
+
+kunit_test_suite(drm_sched_dma_fence);
-- 
2.43.0