Commit c3ef8a44 authored by Shakeel Butt's avatar Shakeel Butt Committed by Greg Kroah-Hartman

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

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@google.com
Reported-by: syzbot+7fbbfa368521945f0e3d@syzkaller.appspotmail.com
Fixes: 6b0c81b3 ("mm, oom: reduce dependency on tasklist_lock")
Signed-off-by: default avatarShakeel Butt <shakeelb@google.com>
Reviewed-by: default avatarRoman Gushchin <guro@fb.com>
Acked-by: default avatarMichal 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent e790eeab
......@@ -544,6 +544,13 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
* 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;
......@@ -563,6 +570,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
}
}
}
put_task_struct(p);
read_unlock(&tasklist_lock);
p = find_lock_task_mm(victim);
......
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