linux-kernel - [PATCH v2 1/2] mm, oom: fix use-after-free in oom_kill

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Date:   Mon, 21 Jan 2019 10:50:31 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Roman Gushchin <guro@...com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Shakeel Butt <shakeelb@...gle.com>,
        syzbot+7fbbfa368521945f0e3d@...kaller.appspotmail.com,
        Michal Hocko <mhocko@...e.com>, stable@...nel.org
Subject: [PATCH v2 1/2] 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.

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: Andrew Morton <akpm@...ux-foundation.org>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc: stable@...nel.org
Cc: linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org
---
Changelog since v1:
- Improved the commit message and added the Reported-by and Fixes tags.

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

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 0930b4365be7..1a007dae1e8f 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -981,6 +981,13 @@ 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;
@@ -1000,6 +1007,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 			}
 		}
 	}
+	put_task_struct(p);
 	read_unlock(&tasklist_lock);

 	/*
-- 
2.20.1.321.g9e740568ce-goog