fix SIGNAL_UNKILLABLE && SIGKILL interaction

I need to write the changelog, and perhaps even split this small patch for better documentation.

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.

Reported-by Dmitry Vyukov <[email protected]>

Reported-by Kyle Huey <[email protected]>

Signed-off-by Oleg Nesterov <[email protected]>

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

kernel/signal.c +2 -3

index 8fc0182..7e15b56 100644
@@ -931,9 +931,9 @@ static void complete_signal(int sig, struct task_struct *p, int group)
         * 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.
                 */
--

2.5.0

Quoted 3 lines · Oleg Nesterov

I need to write the changelog, and perhaps even split this small patch for better documentation.

OK, it is not clear if I answered Eric's concerns or not, let me send the patches for review anyway. I tried to document every change in signal.c.

Oleg.

 kernel/signal.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

ping...

Dmitry confirms this actually fixes the problem reported by syzkaller we discussed in another thread.

Quoted 13 lines · Oleg Nesterov

Quoted 3 lines · Oleg Nesterov

I need to write the changelog, and perhaps even split this small patch for better documentation.

OK, it is not clear if I answered Eric's concerns or not, let me send the patches for review anyway. I tried to document every change in signal.c.

Oleg.

 kernel/signal.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)