This is a note to let you know that I've just added the patch titled
exit: fix race between wait_consider_task() and wait_task_zombie()
to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
exit-fix-race-between-wait_consider_task-and-wait_task_zombie.patch
and it can be found in the queue-3.18 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From 3245d6acab981a2388ffb877c7ecc97e763c59d4 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <[email protected]>
Date: Thu, 8 Jan 2015 14:32:12 -0800
Subject: exit: fix race between wait_consider_task() and wait_task_zombie()
From: Oleg Nesterov <[email protected]>
commit 3245d6acab981a2388ffb877c7ecc97e763c59d4 upstream.
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 commit
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]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
kernel/exit.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1302,9 +1302,15 @@ static int wait_task_continued(struct wa
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);
@@ -1325,7 +1331,7 @@ static int wait_consider_task(struct wai
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.
@@ -1352,7 +1358,7 @@ static int wait_consider_task(struct wai
}
/* 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)) {
/*
Patches currently in stable-queue which might be from [email protected] are
queue-3.18/exit-fix-race-between-wait_consider_task-and-wait_task_zombie.patch
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