Patch "kernel/signal.c: remove the no longer needed SIGNAL_UNKILLABLE check in complete_signal()" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

kernel/signal.c: remove the no longer needed SIGNAL_UNKILLABLE check in complete_signal()

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kernel-signal.c-remove-the-no-longer-needed-signal_unkillable-check-in-complete_signal.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Oleg Nesterov <[email protected]>
Date: Fri, 17 Nov 2017 15:30:08 -0800
Subject: kernel/signal.c: remove the no longer needed SIGNAL_UNKILLABLE check in complete_signal()

From: Oleg Nesterov <[email protected]>

commit 426915796ccaf9c2bd9bb06dc5702225957bc2e5 upstream.

complete_signal() checks SIGNAL_UNKILLABLE before it starts to destroy the thread group, today this is wrong in many ways.

If nothing else, fatal_signal_pending() should always imply that the whole thread group (except ->group_exit_task if it is not NULL) is killed, this check breaks the rule.

After the previous changes we can rely on sig_task_ignored(); sig_fatal(sig) && SIGNAL_UNKILLABLE can only be true if we actually want to kill this task and sig == SIGKILL OR it is traced and debugger can intercept the signal.

This should hopefully fix the problem reported by Dmitry. This test-case

	static int init(void *arg)
	{
		for (;;)
			pause();
	}

	int main(void)
	{
		char stack[16 * 1024];

		for (;;) {
			int pid = clone(init, stack + sizeof(stack)/2,
					CLONE_NEWPID | SIGCHLD, NULL);
			assert(pid > 0);

			assert(ptrace(PTRACE_ATTACH, pid, 0, 0) == 0);
			assert(waitpid(-1, NULL, WSTOPPED) == pid);

			assert(ptrace(PTRACE_DETACH, pid, 0, SIGSTOP) == 0);
			assert(syscall(__NR_tkill, pid, SIGKILL) == 0);
			assert(pid == wait(NULL));
		}
	}

triggers the WARN_ON_ONCE(!(task->jobctl & JOBCTL_STOP_PENDING)) in task_participate_group_stop(). do_signal_stop()->signal_group_exit() checks SIGNAL_GROUP_EXIT and return false, but task_set_jobctl_pending() checks fatal_signal_pending() and does not set JOBCTL_STOP_PENDING.

And his should fix the minor security problem reported by Kyle, SECCOMP_RET_TRACE can miss fatal_signal_pending() the same way if the task is the root of a pid namespace.

---
 kernel/signal.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -970,9 +970,9 @@ static void complete_signal(int sig, str
 	 * then start taking the whole group down immediately.
 	 */
 	if (sig_fatal(p, sig) &&
-	    !(signal->flags & (SIGNAL_UNKILLABLE | SIGNAL_GROUP_EXIT)) &&
+	    !(signal->flags & SIGNAL_GROUP_EXIT) &&
 	    !sigismember(&t->real_blocked, sig) &&
-	    (sig == SIGKILL || !t->ptrace)) {
+	    (sig == SIGKILL || !p->ptrace)) {
 		/*
 		 * This signal will be fatal to the whole group.
 		 */

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/kernel-signal.c-remove-the-no-longer-needed-signal_unkillable-check-in-complete_signal.patch queue-3.18/kernel-signal.c-protect-the-signal_unkillable-tasks-from-sig_kernel_only-signals.patch queue-3.18/kernel-acct.c-fix-the-acct-needcheck-check-in-check_free_space.patch queue-3.18/kernel-signal.c-protect-the-traced-signal_unkillable-tasks-from-sigkill.patch