mm, oom: fix use-after-free in oom_kill_process

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/[email protected]
Reported-by: [email protected]
Fixes: 6b0c81b ("mm, oom: reduce dependency on tasklist_lock")
Signed-off-by: Shakeel Butt <[email protected]>
Reviewed-by: Roman Gushchin <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/oom_kill.c

@@ -975,6 +975,13 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 	 * 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;
@@ -994,6 +1001,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 			}
 		}
 	}
+	put_task_struct(p);
 	read_unlock(&tasklist_lock);
 
 	/*