Syzbot instance running on upstream kernel found a use-after-free bug in oom_kill_process. On further inspection it seems like the process selected to be oom-killed has exited even before reaching read_lock(&tasklist_lock) in oom_kill_process(). More specifically the tsk->usage is 1 which is due to get_task_struct() in oom_evaluate_task() and the put_task_struct within for_each_thread() frees the tsk and for_each_thread() tries to access the tsk. The easiest fix is to do get/put across the for_each_thread() on the selected task. Now the next question is should we continue with the oom-kill as the previously selected task has exited? However before adding more complexity and heuristics, let's answer why we even look at the children of oom-kill selected task? The select_bad_process() has already selected the worst process in the system/memcg. Due to race, the selected process might not be the worst at the kill time but does that matter? The userspace can use the oom_score_adj interface to prefer children to be killed before the parent. I looked at the history but it seems like this is there before git history.
Unified · Split
| @@ -975,6 +975,13 @@ static void oom_kill_process(struct oom_control *oc, const char *message) · 974 unmodified lines | ||
| 975 | 975 | * still freeing memory. |
| 976 | 976 | */ |
| 977 | 977 | read_lock(&tasklist_lock); |
| 978 | + | |
| 979 | + /* | |
| 980 | + * The task 'p' might have already exited before reaching here. The | |
| 981 | + * put_task_struct() will free task_struct 'p' while the loop still try | |
| 982 | + * to access the field of 'p', so, get an extra reference. | |
| 983 | + */ | |
| 984 | + get_task_struct(p); | |
| 978 | 985 | for_each_thread(p, t) { |
| 979 | 986 | list_for_each_entry(child, &t->children, sibling) { |
| 980 | 987 | unsigned int child_points; |
| @@ -994,6 +1001,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message) · 13 unmodified lines | ||
| 994 | 1001 | } |
| 995 | 1002 | } |
| 996 | 1003 | } |
| 1004 | + put_task_struct(p); | |
| 997 | 1005 | read_unlock(&tasklist_lock); |
| 998 | 1006 | |
| 999 | 1007 | /* |