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authorShakeel Butt <shakeelb@google.com>2019-02-01 14:20:54 -0800
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2019-02-06 17:27:44 +0100
commit54adc36a438bba64c19ec5d4334a0993614f899d (patch)
parent9ee7c007088272d0887dd172a3460b1cb0c10cc4 (diff)
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: 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> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
1 files changed, 8 insertions, 0 deletions
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 12049a6f0172..cc317efdcb43 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -962,6 +962,13 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
* still freeing memory.
+ /*
+ * 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;
@@ -981,6 +988,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
+ put_task_struct(p);