linux-kernel - Re: [PATCH 1/3] signal: In get_signal test for signal_group

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Message-ID: <202111161022.7B5720B@keescook>
Date:   Tue, 16 Nov 2021 10:23:48 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, Kyle Huey <me@...ehuey.com>,
        Jens Axboe <axboe@...nel.dk>,
        Peter Zijlstra <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Alexey Gladkov <legion@...nel.org>,
        Robert O'Callahan <rocallahan@...il.com>,
        Marko Mäkelä <marko.makela@...iadb.com>,
        linux-api@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/3] signal: In get_signal test for signal_group_exit
 every time through the loop

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@...ssion.com>

Reviewed-by: Kees Cook <keescook@...omium.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