wait_for_helper: SIGCHLD from user-space can lead to use-after-free
1. wait_for_helper() calls 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>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 file changed
