From: Shakeel Butt shakeelb@google.com Subject: 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/20190121215850.221745-1-shakeelb@google.com Reported-by: syzbot+7fbbfa368521945f0e3d@syzkaller.appspotmail.com Fixes: 6b0c81b3be11 ("mm, oom: reduce dependency on tasklist_lock") Signed-off-by: Shakeel Butt shakeelb@google.com Reviewed-by: Roman Gushchin guro@fb.com Acked-by: Michal Hocko mhocko@suse.com Cc: David Rientjes rientjes@google.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Tetsuo Handa penguin-kernel@i-love.sakura.ne.jp Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/oom_kill.c | 8 ++++++++ 1 file changed, 8 insertions(+)

--- a/mm/oom_kill.c~mm-oom-fix-use-after-free-in-oom_kill_process +++ a/mm/oom_kill.c @@ -975,6 +975,13 @@ 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; @@ -994,6 +1001,7 @@ static void oom_kill_process(struct oom_ } } } + put_task_struct(p); read_unlock(&tasklist_lock);

/* _