In the Linux kernel, the following vulnerability has been resolved:

perf/core: Order the PMU list to fix warning about unordered pmu_ctx_list

Syskaller triggers a warning due to prev_epc->pmu != next_epc->pmu in perf_event_swap_task_ctx_data(). vmcore shows that two lists have the same perf_event_pmu_context, but not in the same order.

The problem is that the order of pmu_ctx_list for the parent is impacted by the time when an event/PMU is added. While the order for a child is impacted by the event order in the pinned_groups and flexible_groups. So the order of pmu_ctx_list in the parent and child may be different.

To fix this problem, insert the perf_event_pmu_context to its proper place after iteration of the pmu_ctx_list.

The follow testcase can trigger above warning:

perf record -e cycles --call-graph lbr -- taskset -c 3 ./a.out &

perf stat -e cpu-clock,cs -p xxx // xxx is the pid of a.out

test.c

void main() { int count = 0; pid_t pid;

    printf("%d running\n", getpid());
    sleep(30);
    printf("running\n");

    pid = fork();
    if (pid == -1) {
            printf("fork error\n");
            return;
    }
    if (pid == 0) {
            while (1) {
                    count++;
            }
    } else {
            while (1) {
                    count++;
            }
    }

}

The testcase first opens an LBR event, so it will allocate task_ctx_data, and then open tracepoint and software events, so the parent context will have 3 different perf_event_pmu_contexts. On inheritance, child ctx will insert the perf_event_pmu_context in another order and the warning will trigger.

[ mingo: Tidied up the changelog. ]