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Subject[PATCH 2/2] wait_task_zombie: fix 2/3 races vs forget_original_parent()
Two threads, T1 and T2. T2 ptraces P, and P is not a child of ptracer's
thread group. P exits and goes to TASK_ZOMBIE.

T1 does wait_task_zombie(P):

P->exit_state = TASK_DEAD;
...
read_unlock(&tasklist_lock);

T2 does exit(), takes tasklist,
forget_original_parent() does
__ptrace_unlink(P) but doesn't
call do_notify_parent(P) because
p->exit_state == EXIT_DEAD.

Now, P is not visible to our process: __ptrace_unlink() removed it from
->children. We should send notification to P->parent and release P if and
only if SIGCHLD is ignored.

And we have 3 bugs:

1. P->parent does do_wait() and gets -ECHILD (P is on ->parent->children,
but its state is TASK_DEAD).

2. // wait_task_zombie() continues
if (put_user(...)) {
// TODO: is this safe?
p->exit_state = EXIT_ZOMBIE;
return;
}
we return without notification/release, task_struct leaked.

Solution: ignore -EFAULT and proceed. It is an application's bug if
we can't fill infop/stat_addr (in case of VM_FAULT_OOM we have much
more problems).

3. // wait_task_zombie() continues
if (p->real_parent != p->parent) {
// Not taken, it was untraced'ed
...
}
release_task(p);

we released the task which we shouldn't.

Solution: check ->real_parent != ->parent before, under tasklist_lock,
but use ptrace_unlink() instead of __ptrace_unlink() to check ->ptrace.


This patch hopefully solves 2 and 3, the 1st bug will be fixed later, we need
some cleanups in forget_original_parent/reparent_thread.

However, the first race is very unlikely and not critical, so I hope it makes
sense to fix 1 and 2 for now.

4. Small cleanup: don't "restore" EXIT_ZOMBIE unless we know we are not going
to realease the child.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- t/kernel/exit.c~2_RACES 2007-08-05 18:46:00.000000000 +0400
+++ t/kernel/exit.c 2007-08-05 18:54:09.000000000 +0400
@@ -1167,8 +1167,7 @@ static int wait_task_zombie(struct task_
int __user *stat_addr, struct rusage __user *ru)
{
unsigned long state;
- int retval;
- int status;
+ int retval, status, traced;

if (unlikely(noreap)) {
pid_t pid = p->pid;
@@ -1210,7 +1209,10 @@ static int wait_task_zombie(struct task_
return 0;
}

- if (likely(p->real_parent == p->parent)) {
+ /* traced means p->ptrace, but not vice versa */
+ traced = (p->real_parent != p->parent);
+
+ if (likely(!traced)) {
struct signal_struct *psig;
struct signal_struct *sig;

@@ -1292,35 +1294,30 @@ static int wait_task_zombie(struct task_
retval = put_user(p->pid, &infop->si_pid);
if (!retval && infop)
retval = put_user(p->uid, &infop->si_uid);
- if (retval) {
- // TODO: is this safe?
- p->exit_state = EXIT_ZOMBIE;
- return retval;
- }
- retval = p->pid;
- if (p->real_parent != p->parent) {
+ if (!retval)
+ retval = p->pid;
+
+ if (traced) {
write_lock_irq(&tasklist_lock);
- /* Double-check with lock held. */
- if (p->real_parent != p->parent) {
- __ptrace_unlink(p);
- // TODO: is this safe?
- p->exit_state = EXIT_ZOMBIE;
- /*
- * If this is not a detached task, notify the parent.
- * If it's still not detached after that, don't release
- * it now.
- */
+ /* We dropped tasklist, ptracer could die and untrace */
+ ptrace_unlink(p);
+ /*
+ * If this is not a detached task, notify the parent.
+ * If it's still not detached after that, don't release
+ * it now.
+ */
+ if (p->exit_signal != -1) {
+ do_notify_parent(p, p->exit_signal);
if (p->exit_signal != -1) {
- do_notify_parent(p, p->exit_signal);
- if (p->exit_signal != -1)
- p = NULL;
+ p->exit_state = EXIT_ZOMBIE;
+ p = NULL;
}
}
write_unlock_irq(&tasklist_lock);
}
if (p != NULL)
release_task(p);
- BUG_ON(!retval);
+
return retval;
}

-
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