From: Oleg Nesterov <oleg@xxxxxxxxxx>
Subject: 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@xxxxxxxxxx>
Reported-by: Arne Goedeke <el@xxxxxxxxxxxx>
Tested-by: Arne Goedeke <el@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [3.15+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/exit.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
diff -puN kernel/exit.c~exit-fix-race-between-wait_consider_task-and-wait_task_zombie kernel/exit.c
--- a/kernel/exit.c~exit-fix-race-between-wait_consider_task-and-wait_task_zombie
+++ a/kernel/exit.c
@@ -1287,9 +1287,15 @@ 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);
@@ -1310,7 +1316,7 @@ 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.
@@ -1337,7 +1343,7 @@ 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)) {
/*
_
--
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