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. ]