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Subject[PATCH] wait_for_helper: SIGCHLD from user-space can lead to use-after-free
(this patch is orthogonal to other umh changes in -mm)

1. wait_for_helper() does allow_signal(SIGCHLD) to ensure the child can't
autoreap itself.

However, this means that a spurious SIGCHILD from user-space can
set TIF_SIGPENDING and:

- kernel_thread() or sys_wait4() can fail due to signal_pending()

- worse, wait4() can fail before ____call_usermodehelper() execs
or exits. In this case the caller may kfree(subprocess_info)
while the child still uses this memory.

Change the code to use SIG_DFL instead of magic "(void __user *)2"
set by allow_signal(). This means that SIGCHLD won't be delivered,
yet the child won't autoreap itsefl.

The problem is minor, only root can send a signal to this kthread.

2. If sys_wait4(&ret) fails it doesn't populate "ret", in this case
wait_for_helper() reports a random value from uninitialized var.

With this patch sys_wait4() should never fail, but still it makes
sense to initialize ret = -ECHILD so that the caller can notice
the problem.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

kernel/kmod.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

--- mm/kernel/kmod.c~4_SIGCHLD_RACE 2010-03-09 22:23:37.000000000 +0100
+++ mm/kernel/kmod.c 2010-03-10 17:36:37.000000000 +0100
@@ -194,15 +194,16 @@ static int wait_for_helper(void *data)
struct subprocess_info *sub_info = data;
pid_t pid;

- /* Install a handler: if SIGCLD isn't handled sys_wait4 won't
- * populate the status, but will return -ECHILD. */
- allow_signal(SIGCHLD);
+ /* If SIGCLD is ignored sys_wait4 won't populate the status. */
+ spin_lock_irq(&current->sighand->siglock);
+ current->sighand->action[SIGCHLD-1].sa.sa_handler = SIG_DFL;
+ spin_unlock_irq(&current->sighand->siglock);

pid = kernel_thread(____call_usermodehelper, sub_info, SIGCHLD);
if (pid < 0) {
sub_info->retval = pid;
} else {
- int ret;
+ int ret = -ECHILD;

/*
* Normally it is bogus to call wait4() from in-kernel because


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