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exit: fix race between wait_consider_task() and wait_task_zombie()

PATCH New Oleg Nesterov 1 participant 2 messages Dec 15, 2014

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Oleg Nesterov

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#0 lore raw

cover exit: fix race between wait_consider_task() and wait_task_zombie()

Andrew,

I think this is v3.19/stable material. Just in case, this bug is old, it wasn't introduced by the recent changes.

Oleg.

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#1 lore raw

PATCH 1/1 exit: fix race between wait_consider_task() and wait_task_zombie()

wait_consider_task() checks EXIT_ZOMBIE after EXIT_DEAD/EXIT_TRACE and both checks can fail if we race with EXIT_ZOMBIE -> EXIT_DEAD/EXIT_TRACE change in between, gcc needs to reload p->exit_state after security_task_wait(). In this case ->notask_error will be wrongly cleared and do_wait() can hang forever if it was the last eligible child.

Many thanks to Arne who carefully investigated the problem.

Note: this bug is very old but it was pure theoretical until b3ab03160dfa "wait: completely ignore the EXIT_DEAD tasks". Before this commit "-O2" was probably enough to guarantee that compiler won't read ->exit_state twice.

Signed-off-by Oleg Nesterov <[email protected]>
Reported-by Arne Goedeke <[email protected]>
Tested-by Arne Goedeke <[email protected]>
Cc <[email protected]> # v3.15+
---
 kernel/exit.c |   12 +++++++++---
 1 files changed, 9 insertions(+), 3 deletions(-)
kernel/exit.c +9 -3
index 43394f7..37efc8e 100644
@@ -1303,9 +1303,15 @@ static int wait_task_continued(struct wait_opts *wo, struct task_struct *p)
 static int wait_consider_task(struct wait_opts *wo, int ptrace,
 				struct task_struct *p)
 {
+	/*
+	 * We can race with wait_task_zombie() from another thread.
+	 * Ensure that EXIT_ZOMBIE -> EXIT_DEAD/EXIT_TRACE transition
+	 * can't confuse the checks below.
+	 */
+	int exit_state = ACCESS_ONCE(p->exit_state);
 	int ret;
 
-	if (unlikely(p->exit_state == EXIT_DEAD))
+	if (unlikely(exit_state == EXIT_DEAD))
 		return 0;
 
 	ret = eligible_child(wo, p);

@@ -1326,7 +1332,7 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
 		return 0;
 	}
 
-	if (unlikely(p->exit_state == EXIT_TRACE)) {
+	if (unlikely(exit_state == EXIT_TRACE)) {
 		/*
 		 * ptrace == 0 means we are the natural parent. In this case
 		 * we should clear notask_error, debugger will notify us.

@@ -1353,7 +1359,7 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
 	}
 
 	/* slay zombie? */
-	if (p->exit_state == EXIT_ZOMBIE) {
+	if (exit_state == EXIT_ZOMBIE) {
 		/* we don't reap group leaders with subthreads */
 		if (!delay_group_leader(p)) {
 			/*
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