Re: [PATCH 3/3] remove the no longer needed SIGNAL_UNKILLABLE check in complete_signal()

[Kees Cook]

On Fri, Nov 3, 2017 at 11:42 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> 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 <dvyukov@xxxxxxxxxx>
> Reported-by: Kyle Huey <me@xxxxxxxxxxxx>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

Thanks for digging through this! Two birds, one stone, etc. :)

-Kees

> ---
>  kernel/signal.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 8fc0182..7e15b56 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -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
>
>



-- 
Kees Cook
Pixel Security