diff options
| author | Oleg Nesterov <oleg@redhat.com> | 2015-01-08 14:32:12 -0800 |
|---|---|---|
| committer | Luis Henriques <luis.henriques@canonical.com> | 2015-01-16 14:34:30 +0000 |
| commit | d8cb06ebefb3a43cc5eb841eec3c0b9bda235490 (patch) | |
| tree | e3ef0022eed22ed881dcb64c172045ebafe1c155 | |
| parent | 5e1f4178ac93cf9b70bacc4205f6010f9e14e420 (diff) | |
exit: fix race between wait_consider_task() and wait_task_zombie()
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 <oleg@redhat.com>
Reported-by: Arne Goedeke <el@laramies.com>
Tested-by: Arne Goedeke <el@laramies.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
| -rw-r--r-- | kernel/exit.c | 12 |
1 files changed, 9 insertions, 3 deletions
diff --git a/kernel/exit.c b/kernel/exit.c index e5c4668f1799..ab2472332043 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1294,9 +1294,15 @@ 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); @@ -1317,7 +1323,7 @@ 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. @@ -1344,7 +1350,7 @@ 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)) { /* |