linux-kernel - [PATCH 3.16 083/202] mm, oom: fix use-after-free in oom_kill

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Message-ID: <lsq.1556377989.291175341@decadent.org.uk>
Date:   Sat, 27 Apr 2019 16:13:09 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "David Rientjes" <rientjes@...gle.com>,
        syzbot+7fbbfa368521945f0e3d@...kaller.appspotmail.com,
        "Roman Gushchin" <guro@...com>,
        "Shakeel Butt" <shakeelb@...gle.com>,
        "Tetsuo Handa" <penguin-kernel@...ove.sakura.ne.jp>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Michal Hocko" <mhocko@...e.com>,
        "Johannes Weiner" <hannes@...xchg.org>
Subject: [PATCH 3.16 083/202] mm, oom: fix use-after-free in oom_kill_process

3.16.66-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Shakeel Butt <shakeelb@...gle.com>

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@...kaller.appspotmail.com
Fixes: 6b0c81b3be11 ("mm, oom: reduce dependency on tasklist_lock")
Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
Reviewed-by: Roman Gushchin <guro@...com>
Acked-by: Michal Hocko <mhocko@...e.com>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/oom_kill.c | 8 ++++++++
 1 file changed, 8 insertions(+)

--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -466,6 +466,13 @@ void oom_kill_process(struct task_struct
 	 * 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;
@@ -485,6 +492,7 @@ void oom_kill_process(struct task_struct
 			}
 		}
 	}
+	put_task_struct(p);
 	read_unlock(&tasklist_lock);

 	p = find_lock_task_mm(victim);