[PATCH] drm/i915/gem: Fix request use-after-free in active_engine()

active_engine() walks timeline->requests in reverse under RCU and takes a
temporary reference before inspecting each request. However, it drops that
reference in the loop body before list_for_each_entry_reverse() advances
the cursor.

Concurrent retirement can unlink the same request and drop its base
reference while active_engine() holds the temporary reference. The put in
active_engine() may then be final, freeing or recycling the request before
the loop step reads rq->link.prev. SLAB_TYPESAFE_BY_RCU does not defer that
reuse.

Open-code the reverse walk and cache the previous request while the current
request is still referenced. The next request remains protected by
i915_request_get_rcu() and validated against the timeline before use.

An i915 mock selftest forced retirement between the active check and cursor
advance. The vulnerable tree reached the final request release and
kmem_cache_free(), while the fixed tree completed the same ordering without
accessing rq after the put.

Fixes: 3cfea8c97c93 ("drm/i915/gem: Hold request reference for canceling an active context")
Cc: [email protected] # v5.10+
Signed-off-by: Shuangpeng Bai <[email protected]>
---
 drivers/gpu/drm/i915/gem/i915_gem_context.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index c58ffa5a8fa6..ff5c892a0176 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -1361,7 +1361,7 @@ static bool __cancel_engine(struct intel_engine_cs *engine)
 static struct intel_engine_cs *active_engine(struct intel_context *ce)
 {
 	struct intel_engine_cs *engine = NULL;
-	struct i915_request *rq;
+	struct i915_request *rq, *prev;
 
 	if (intel_context_has_inflight(ce))
 		return intel_context_inflight(ce);
@@ -1375,7 +1375,8 @@ static struct intel_engine_cs *active_engine(struct intel_context *ce)
 	 * (and onto a new timeline->requests list).
 	 */
 	rcu_read_lock();
-	list_for_each_entry_reverse(rq, &ce->timeline->requests, link) {
+	rq = list_last_entry(&ce->timeline->requests, typeof(*rq), link);
+	while (!list_entry_is_head(rq, &ce->timeline->requests, link)) {
 		bool found;
 
 		/* timeline is already completed upto this point? */
@@ -1387,9 +1388,14 @@ static struct intel_engine_cs *active_engine(struct intel_context *ce)
 		if (likely(rcu_access_pointer(rq->timeline) == ce->timeline))
 			found = i915_request_active_engine(rq, &engine);
 
+		/* Cache the cursor before the put, which may release rq. */
+		if (!found)
+			prev = list_prev_entry(rq, link);
 		i915_request_put(rq);
 		if (found)
 			break;
+
+		rq = prev;
 	}
 	rcu_read_unlock();
 

base-commit: a59f57e2aa127c5354168d2ec4bac920df1be4f4
-- 
2.43.0