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authorOleg Nesterov <oleg@redhat.com>2015-01-08 14:32:12 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2015-01-08 15:10:51 -0800
commit3245d6acab981a2388ffb877c7ecc97e763c59d4 (patch)
tree9fcebbce2ca688948665292358d22809f80140ba /kernel/exit.c
parenteb4f73b4ca6c04f31af6f1ff1bf11b5020a1216f (diff)
downloadlinux-sh-3245d6acab981a2388ffb877c7ecc97e763c59d4.tar.gz
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 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 <oleg@redhat.com> Reported-by: Arne Goedeke <el@laramies.com> Tested-by: Arne Goedeke <el@laramies.com> Cc: <stable@vger.kernel.org> [3.15+] Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel/exit.c')
-rw-r--r--kernel/exit.c12
1 files changed, 9 insertions, 3 deletions
diff --git a/kernel/exit.c b/kernel/exit.c
index 1ea4369890a3..6806c55475ee 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1287,9 +1287,15 @@ 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);
@@ -1310,7 +1316,7 @@ 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.
@@ -1337,7 +1343,7 @@ 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)) {
/*