4.9-stable review patch. If anyone has any objections, please let me know. ------------------ From: Shakeel Butt <shakeelb@xxxxxxxxxx> commit cefc7ef3c87d02fc9307835868ff721ea12cc597 upstream. 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. Link: http://lkml.kernel.org/r/20190121215850.221745-1-shakeelb@xxxxxxxxxx Reported-by: syzbot+7fbbfa368521945f0e3d@xxxxxxxxxxxxxxxxxxxxxxxxx Fixes: 6b0c81b3be11 ("mm, oom: reduce dependency on tasklist_lock") Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx> Reviewed-by: Roman Gushchin <guro@xxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/oom_kill.c | 8 ++++++++ 1 file changed, 8 insertions(+) --- a/mm/oom_kill.c +++ b/mm/oom_kill.c @@ -861,6 +861,13 @@ static void oom_kill_process(struct oom_ * still freeing memory. */ read_lock(&tasklist_lock); + + /* + * The task 'p' might have already exited before reaching here. The + * put_task_struct() will free task_struct 'p' while the loop still try + * to access the field of 'p', so, get an extra reference. + */ + get_task_struct(p); for_each_thread(p, t) { list_for_each_entry(child, &t->children, sibling) { unsigned int child_points; @@ -880,6 +887,7 @@ static void oom_kill_process(struct oom_ } } } + put_task_struct(p); read_unlock(&tasklist_lock); p = find_lock_task_mm(victim);
