[PATCH] mm: oom: Fix race condition between oom_badness and do_exit of task

[Gaurav Kohli]

oom_badness access real_cred of task for calculation without increasing
the usage counter of task struct. This may create a race if do_exit of
same task runs and free the real_cred. So using get_task_struct which
blocks the freeing until oom_badness is executing.

el1_da+0x24/0x84
security_capable_noaudit+0x64/0x94
has_capability_noaudit+0x38/0x58
oom_badness.part.21+0x114/0x1c0
oom_badness+0x50/0x5c
proc_oom_score+0x48/0x80
proc_single_show+0x5c/0xb8

Signed-off-by: Gaurav Kohli <gkohli@xxxxxxxxxxxxxx>

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 6fd9773..5f4cc4b 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -114,9 +114,11 @@ struct task_struct *find_lock_task_mm(struct task_struct *p)
 
 	for_each_thread(p, t) {
 		task_lock(t);
+		get_task_struct(t);
 		if (likely(t->mm))
 			goto found;
 		task_unlock(t);
+		put_task_struct(t);
 	}
 	t = NULL;
 found:
@@ -191,6 +193,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 			test_bit(MMF_OOM_SKIP, &p->mm->flags) ||
 			in_vfork(p)) {
 		task_unlock(p);
+		put_task_struct(p);
 		return 0;
 	}
 
@@ -208,7 +211,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 	 */
 	if (has_capability_noaudit(p, CAP_SYS_ADMIN))
 		points -= (points * 3) / 100;
-
+	put_task_struct(p);
 	/* Normalize to oom_score_adj units */
 	adj *= totalpages / 1000;
 	points += adj;
-- 
1.9.1