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

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Date:   Wed, 17 Nov 2021 10:31:20 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Kees Cook <keescook@...omium.org>
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

Kees Cook <keescook@...omium.org> writes:

> 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"?

Yes.  Potential problem.  I will update the comment.

>> 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
>>