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Subject[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 <oleg@redhat.com>
Reported-by: Arne Goedeke <el@laramies.com>
Tested-by: Arne Goedeke <el@laramies.com>
Cc: <stable@vger.kernel.org> # v3.15+
---
kernel/exit.c | 12 +++++++++---
1 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 43394f7..37efc8e 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -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)) {
/*
--
1.5.5.1



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