[PATCH v6 1/3] perf/core: Fix NULL pmu_ctx passed to pmu->sched_task()

[Puranjay Mohan]

perf_pmu_sched_task() returns early when cpuctx->task_ctx is set, and
cpc->task_epc is only non-NULL while a task context is scheduled in on
this CPU. __perf_pmu_sched_task() therefore always passes NULL:

  Unable to handle kernel NULL pointer dereference at virtual address 00
  pc : armv8pmu_sched_task+0x14/0x50
  Call trace:
   armv8pmu_sched_task+0x14/0x50 (P)
   perf_pmu_sched_task+0xac/0x108
   __perf_event_task_sched_out+0x6c/0xe0

Pass &cpc->epc instead. __perf_init_event_pmu_context() sets its ->pmu
when the PMU is registered; ->ctx stays NULL until a CPU-wide event
attaches. That is enough here because armv8pmu_sched_task() is the only
in-tree implementation that dereferences the argument at all, and it
only reads ->pmu. The oops therefore needs BRBE, which arrived in v6.17.

Fixes: bd2756811766 ("perf: Rewrite core context handling")
Cc: stable at vger.kernel.org
Acked-by: Usama Arif <usama.arif at linux.dev>
Signed-off-by: Puranjay Mohan <puranjay at kernel.org>
---
 kernel/events/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index b282de3e7d7ca..9815894b67e77 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3907,7 +3907,8 @@ static void __perf_pmu_sched_task(struct perf_cpu_pmu_context *cpc,
 	perf_ctx_lock(cpuctx, cpuctx->task_ctx);
 	perf_pmu_disable(pmu);
 
-	pmu->sched_task(cpc->task_epc, task, sched_in);
+	pmu->sched_task(cpc->task_epc ? cpc->task_epc : &cpc->epc,
+			task, sched_in);
 
 	perf_pmu_enable(pmu);
 	perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
-- 
2.53.0-Meta