[PATCH bpf 1/3] perf/arm_pmuv3: Fix NULL pointer dereference in armv8pmu_sched_task()

[Puranjay Mohan <puranjay@kernel.org>]

This is easily triggered with:

  perf record -b -e cycles -a -- ls

which crashes on the first context switch with:

  Unable to handle kernel NULL pointer dereference at virtual address 00[.]
  PC is at armv8pmu_sched_task+0x14/0x50
  LR is at perf_pmu_sched_task+0xac/0x108
  Call trace:
    armv8pmu_sched_task+0x14/0x50 (P)
    perf_pmu_sched_task+0xac/0x108
    __perf_event_task_sched_out+0x6c/0xe0
    prepare_task_switch+0x120/0x268
    __schedule+0x1e8/0x828
    ...

perf_pmu_sched_task() invokes the PMU sched callback with cpc->task_epc,
which is NULL when no per-task events exist for this PMU. With CPU-wide
branch-stack events, armv8pmu_sched_task() is still registered and
dereferences pmu_ctx->pmu unconditionally, causing the crash.

The bug was introduced by commit fa9d27773873 ("perf: arm_pmu: Kill last
use of per-CPU cpu_armpmu pointer") which changed the function from
using the per-CPU cpu_armpmu pointer (always valid) to dereferencing
pmu_ctx->pmu without adding a NULL check.

Add a NULL check for pmu_ctx to avoid the crash.

Fixes: fa9d27773873 ("perf: arm_pmu: Kill last use of per-CPU cpu_armpmu pointer")
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 drivers/perf/arm_pmuv3.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
index 8014ff766cff..2d097fad9c10 100644
--- a/drivers/perf/arm_pmuv3.c
+++ b/drivers/perf/arm_pmuv3.c
@@ -1074,8 +1074,15 @@ static int armv8pmu_user_event_idx(struct perf_event *event)
 static void armv8pmu_sched_task(struct perf_event_pmu_context *pmu_ctx,
 				struct task_struct *task, bool sched_in)
 {
-	struct arm_pmu *armpmu = to_arm_pmu(pmu_ctx->pmu);
-	struct pmu_hw_events *hw_events = this_cpu_ptr(armpmu->hw_events);
+	struct arm_pmu *armpmu;
+	struct pmu_hw_events *hw_events;
+
+	/* cpc->task_epc is NULL when no per-task events exist for this PMU */
+	if (!pmu_ctx)
+		return;
+
+	armpmu = to_arm_pmu(pmu_ctx->pmu);
+	hw_events = this_cpu_ptr(armpmu->hw_events);
 
 	if (!hw_events->branch_users)
 		return;
-- 
2.52.0