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Message-ID: <20140220173858.GA21844@redhat.com>
Date: Thu, 20 Feb 2014 18:38:58 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Al Viro <viro@...IV.linux.org.uk>,
Jan Kratochvil <jan.kratochvil@...hat.com>,
Lennart Poettering <lpoetter@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Michal Schmidt <mschmidt@...hat.com>,
Roland McGrath <roland@...k.frob.com>,
Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH 1/5] wait: fix reparent_leader() vs EXIT_DEAD->EXIT_ZOMBIE
race
wait_task_zombie() first does EXIT_ZOMBIE->EXIT_DEAD transition and
drops tasklist_lock. If this task is not the natural child and it is
traced, we change its state back to EXIT_ZOMBIE for ->real_parent.
The last transition is racy, this is even documented in 50b8d257486a
"ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT_ZOMBIE
race". wait_consider_task() tries to detect this transition and clear
->notask_error but we can't rely on ptrace_reparented(), debugger can
exit and do ptrace_unlink() before its sub-thread sets EXIT_ZOMBIE.
And there is another problem which were missed before: this transition
can also race with reparent_leader() which doesn't reset >exit_signal
if EXIT_DEAD, assuming that this task must be reaped by someone else.
So the tracee can be re-parented with ->exit_signal != SIGCHLD, and
if /sbin/init doesn't use __WALL it becomes unreapable.
Change reparent_leader() to update ->exit_signal even if EXIT_DEAD.
Note: this is the simple temporary hack for -stable, it doesn't try
to solve all problems, it will be reverted by the next changes.
Cc: stable@...r.kernel.org
Reported-by: Jan Kratochvil <jan.kratochvil@...hat.com>
Reported-and-tested-by: Michal Schmidt <mschmidt@...hat.com>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
kernel/exit.c | 15 +++++++++++----
1 files changed, 11 insertions(+), 4 deletions(-)
diff --git a/kernel/exit.c b/kernel/exit.c
index 1e77fc6..5281522 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -560,9 +560,6 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
struct list_head *dead)
{
list_move_tail(&p->sibling, &p->real_parent->children);
-
- if (p->exit_state == EXIT_DEAD)
- return;
/*
* If this is a threaded reparent there is no need to
* notify anyone anything has happened.
@@ -570,9 +567,19 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
if (same_thread_group(p->real_parent, father))
return;
- /* We don't want people slaying init. */
+ /*
+ * We don't want people slaying init.
+ *
+ * Note: we do this even if it is EXIT_DEAD, wait_task_zombie()
+ * can change ->exit_state to EXIT_ZOMBIE. If this is the final
+ * state, do_notify_parent() was already called and ->exit_signal
+ * doesn't matter.
+ */
p->exit_signal = SIGCHLD;
+ if (p->exit_state == EXIT_DEAD)
+ return;
+
/* If it has exited notify the new parent about this child's death. */
if (!p->ptrace &&
p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {
--
1.5.5.1
--
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