From: Sebastian Brzezinka Date: Wed, 1 Apr 2026 10:10:07 +0000 (+0200) Subject: drm/i915/gt: fix refcount underflow in intel_engine_park_heartbeat X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4c71fd099513bfa8acab529b626e1f0097b76061;p=thirdparty%2Fkernel%2Flinux.git drm/i915/gt: fix refcount underflow in intel_engine_park_heartbeat A use-after-free / refcount underflow is possible when the heartbeat worker and intel_engine_park_heartbeat() race to release the same engine->heartbeat.systole request. The heartbeat worker reads engine->heartbeat.systole and calls i915_request_put() on it when the request is complete, but clears the pointer in a separate, non-atomic step. Concurrently, a request retirement on another CPU can drop the engine wakeref to zero, triggering __engine_park() -> intel_engine_park_heartbeat(). If the heartbeat timer is pending at that point, cancel_delayed_work() returns true and intel_engine_park_heartbeat() reads the stale non-NULL systole pointer and calls i915_request_put() on it again, causing a refcount underflow: ``` <4> [487.221889] Workqueue: i915-unordered engine_retire [i915] <4> [487.222640] RIP: 0010:refcount_warn_saturate+0x68/0xb0 ... <4> [487.222707] Call Trace: <4> [487.222711] <4> [487.222716] intel_engine_park_heartbeat.part.0+0x6f/0x80 [i915] <4> [487.223115] intel_engine_park_heartbeat+0x25/0x40 [i915] <4> [487.223566] __engine_park+0xb9/0x650 [i915] <4> [487.223973] ____intel_wakeref_put_last+0x2e/0xb0 [i915] <4> [487.224408] __intel_wakeref_put_last+0x72/0x90 [i915] <4> [487.224797] intel_context_exit_engine+0x7c/0x80 [i915] <4> [487.225238] intel_context_exit+0xf1/0x1b0 [i915] <4> [487.225695] i915_request_retire.part.0+0x1b9/0x530 [i915] <4> [487.226178] i915_request_retire+0x1c/0x40 [i915] <4> [487.226625] engine_retire+0x122/0x180 [i915] <4> [487.227037] process_one_work+0x239/0x760 <4> [487.227060] worker_thread+0x200/0x3f0 <4> [487.227068] ? __pfx_worker_thread+0x10/0x10 <4> [487.227075] kthread+0x10d/0x150 <4> [487.227083] ? __pfx_kthread+0x10/0x10 <4> [487.227092] ret_from_fork+0x3d4/0x480 <4> [487.227099] ? __pfx_kthread+0x10/0x10 <4> [487.227107] ret_from_fork_asm+0x1a/0x30 <4> [487.227141] ``` Fix this by replacing the non-atomic pointer read + separate clear with xchg() in both racing paths. xchg() is a single indivisible hardware instruction that atomically reads the old pointer and writes NULL. This guarantees only one of the two concurrent callers obtains the non-NULL pointer and performs the put, the other gets NULL and skips it. Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/work_items/15880 Fixes: 058179e72e09 ("drm/i915/gt: Replace hangcheck by heartbeats") Cc: # v5.5+ Signed-off-by: Sebastian Brzezinka Reviewed-by: Krzysztof Karas Reviewed-by: Andi Shyti Signed-off-by: Andi Shyti Link: https://lore.kernel.org/r/d4c1c14255688dd07cc8044973c4f032a8d1559e.1775038106.git.sebastian.brzezinka@intel.com (cherry picked from commit 13238dc0ee4f9ab8dafa2cca7295736191ae2f42) Signed-off-by: Joonas Lahtinen --- diff --git a/drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c b/drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c index b279878dca29..6424ecce8bcb 100644 --- a/drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c +++ b/drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c @@ -148,10 +148,12 @@ static void heartbeat(struct work_struct *wrk) /* Just in case everything has gone horribly wrong, give it a kick */ intel_engine_flush_submission(engine); - rq = engine->heartbeat.systole; - if (rq && i915_request_completed(rq)) { - i915_request_put(rq); - engine->heartbeat.systole = NULL; + rq = xchg(&engine->heartbeat.systole, NULL); + if (rq) { + if (i915_request_completed(rq)) + i915_request_put(rq); + else + engine->heartbeat.systole = rq; } if (!intel_engine_pm_get_if_awake(engine)) @@ -232,8 +234,11 @@ static void heartbeat(struct work_struct *wrk) unlock: mutex_unlock(&ce->timeline->mutex); out: - if (!engine->i915->params.enable_hangcheck || !next_heartbeat(engine)) - i915_request_put(fetch_and_zero(&engine->heartbeat.systole)); + if (!engine->i915->params.enable_hangcheck || !next_heartbeat(engine)) { + rq = xchg(&engine->heartbeat.systole, NULL); + if (rq) + i915_request_put(rq); + } intel_engine_pm_put(engine); } @@ -247,8 +252,13 @@ void intel_engine_unpark_heartbeat(struct intel_engine_cs *engine) void intel_engine_park_heartbeat(struct intel_engine_cs *engine) { - if (cancel_delayed_work(&engine->heartbeat.work)) - i915_request_put(fetch_and_zero(&engine->heartbeat.systole)); + if (cancel_delayed_work(&engine->heartbeat.work)) { + struct i915_request *rq; + + rq = xchg(&engine->heartbeat.systole, NULL); + if (rq) + i915_request_put(rq); + } } void intel_gt_unpark_heartbeats(struct intel_gt *gt)