[Intel-gfx] [PATCH v2] drm/i915/gt: update request engine before removing virtual GuC engine

[Andrzej Hajda]

GuC virtual engines can be removed before request removal. On the other
side driver expects rq->engine to be a valid pointer for a whole life of
request. Setting rq->engine to an always valid engine should solve
the issue.

The patch fixes bug detected by KASAN with following signature:
i915 0000:00:02.0: [drm:i915_drop_caches_set [i915]] Dropping caches: 0x0000005c [0x0000005c]
BUG: KASAN: slab-use-after-free in i915_fence_release+0x2a2/0x2f0 [i915]
Read of size 4 at addr ffff88813ffda6e8 by task kworker/u32:10/157
...
Allocated by task 1184:
...
guc_create_virtual+0x4d/0x1160 [i915]
i915_gem_create_context+0x11ee/0x18c0 [i915]
...
Freed by task 151:
...
intel_guc_deregister_done_process_msg+0x324/0x6d0 [i915]
...

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7926
Signed-off-by: Andrzej Hajda <andrzej.hajda at intel.com>
---
 drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
index a0e3ef1c65d246..2c877ea5eda6f0 100644
--- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
+++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
@@ -3461,6 +3461,8 @@ static void guc_prio_fini(struct i915_request *rq, struct intel_context *ce)
 static void remove_from_context(struct i915_request *rq)
 {
 	struct intel_context *ce = request_to_scheduling_context(rq);
+	struct intel_engine_cs *engine;
+	intel_engine_mask_t tmp;
 
 	GEM_BUG_ON(intel_context_is_child(ce));
 
@@ -3478,6 +3480,15 @@ static void remove_from_context(struct i915_request *rq)
 
 	atomic_dec(&ce->guc_id.ref);
 	i915_request_notify_execute_cb_imm(rq);
+
+	/*
+	 * GuC virtual engine can disappear after this call, so let's assign
+	 * something valid, as driver expects this to be always valid pointer.
+	 */
+	for_each_engine_masked(engine, rq->engine->gt, rq->execution_mask, tmp) {
+		rq->engine = engine;
+		break;
+	}
 }
 
 static const struct intel_context_ops guc_context_ops = {
-- 
2.34.1