Re: [PATCH 1/3] signal: In get_signal test for signal_group_exit every time through the loop

Kees Cook
2021-11-16T18:23:58Z

On Mon, Nov 15, 2021 at 11:32:50PM -0600, Eric W. Biederman wrote:
>
> Recently while investigating a problem with rr and signals I noticed
> that siglock is dropped in ptrace_signal and get_signal does not jump
> to relock.
>
> Looking farther to see if the problem is anywhere else I see that
> do_signal_stop also returns if signal_group_exit is true. I believe
> that test can now never be true, but it is a bit hard to trace
> through and be certain.
>
> Testing signal_group_exit is not expensive, so move the test for
> signal_group_exit into the for loop inside of get_signal to ensure
> the test is never skipped improperly.

Seems reasonable.

>
> This has been a potential since I added the test for signal_group_exit

nit: missing word after "potential"? "bug", "problem"?

> was added.
>
> Fixes: 35634ffa1751 ("signal: Always notice exiting tasks")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

> ---
> kernel/signal.c | 20 ++++++++++----------
> 1 file changed, 10 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 7c4b7ae714d4..986fa69c15c5 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2662,19 +2662,19 @@ bool get_signal(struct ksignal *ksig)
> goto relock;
> }
>
> - /* Has this task already been marked for death? */
> - if (signal_group_exit(signal)) {
> - ksig->info.si_signo = signr = SIGKILL;
> - sigdelset(&current->pending.signal, SIGKILL);
> - trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,
> - &sighand->action[SIGKILL - 1]);
> - recalc_sigpending();
> - goto fatal;
> - }
> -
> for (;;) {
> struct k_sigaction *ka;
>
> + /* Has this task already been marked for death? */
> + if (signal_group_exit(signal)) {
> + ksig->info.si_signo = signr = SIGKILL;
> + sigdelset(&current->pending.signal, SIGKILL);
> + trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,
> + &sighand->action[SIGKILL - 1]);
> + recalc_sigpending();
> + goto fatal;
> + }
> +
> if (unlikely(current->jobctl & JOBCTL_STOP_PENDING) &&
> do_signal_stop(0))
> goto relock;
> --
> 2.20.1
>

--
Kees Cook