Date:   Wed, 17 Nov 2021 10:44:55 -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 3/3] signal: Requeue ptrace signals

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

> On Mon, Nov 15, 2021 at 11:34:33PM -0600, Eric W. Biederman wrote:
>> 
>> Kyle Huey <me@...ehuey.com> writes:
>> 
>> > rr, a userspace record and replay debugger[0], uses the recorded register
>> > state at PTRACE_EVENT_EXIT to find the point in time at which to cease
>> > executing the program during replay.
>> >
>> > If a SIGKILL races with processing another signal in get_signal, it is
>> > possible for the kernel to decline to notify the tracer of the original
>> > signal. But if the original signal had a handler, the kernel proceeds
>> > with setting up a signal handler frame as if the tracer had chosen to
>> > deliver the signal unmodified to the tracee. When the kernel goes to
>> > execute the signal handler that it has now modified the stack and registers
>> > for, it will discover the pending SIGKILL, and terminate the tracee
>> > without executing the handler. When PTRACE_EVENT_EXIT is delivered to
>> > the tracer, however, the effects of handler setup will be visible to
>> > the tracer.
>> >
>> > Because rr (the tracer) was never notified of the signal, it is not aware
>> > that a signal handler frame was set up and expects the state of the program
>> > at PTRACE_EVENT_EXIT to be a state that will be reconstructed naturally
>> > by allowing the program to execute from the last event. When that fails
>> > to happen during replay, rr will assert and die.
>> >
>> > The following patches add an explicit check for a newly pending SIGKILL
>> > after the ptracer has been notified and the siglock has been reacquired.
>> > If this happens, we stop processing the current signal and proceed
>> > immediately to handling the SIGKILL. This makes the state reported at
>> > PTRACE_EVENT_EXIT the unmodified state of the program, and also avoids the
>> > work to set up a signal handler frame that will never be used.
>> >
>> > [0] https://rr-project.org/
>> 
>> The problem is that while the traced process makes it into ptrace_stop,
>> the tracee is killed before the tracer manages to wait for the
>> tracee and discover which signal was about to be delivered.
>> 
>> More generally the problem is that while siglock was dropped a signal
>> with process wide effect is short cirucit delivered to the entire
>> process killing it, but the process continues to try and deliver another
>> signal.
>> 
>> In general it impossible to avoid all cases where work is performed
>> after the process has been killed.  In particular if the process is
>> killed after get_signal returns the code will simply not know it has
>> been killed until after delivering the signal frame to userspace.
>> 
>> On the other hand when the code has already discovered the process
>> has been killed and taken user space visible action that shows
>> the kernel knows the process has been killed, it is just silly
>> to then write the signal frame to the user space stack.
>> 
>> Instead of being silly detect the process has been killed
>> in ptrace_signal and requeue the signal so the code can pretend
>> it was simply never dequeued for delivery.
>> 
>> To test the process has been killed I use fatal_signal_pending rather
>> than signal_group_exit to match the test in signal_pending_state which
>> is used in schedule which is where ptrace_stop detects the process has
>> been killed.
>> 
>> Requeuing the signal so the code can pretend it was simply never
>> dequeued improves the user space visible behavior that has been
>> present since ebf5ebe31d2c ("[PATCH] signal-fixes-2.5.59-A4").
>> 
>> Kyle Huey verified that this change in behavior and makes rr happy.
>> 
>> Reported-by: Kyle Huey <khuey@...ehuey.com>
>> Reported-by: Marko Mäkelä <marko.makela@...iadb.com>
>> History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.gi
>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>
> Yay pre-git-history! :)

One of these days we might finish removing the rough edges and
fixing the corner case bugs in the original linux pthreads support.

Eric


> Reviewed-by: Kees Cook <keescook@...omium.org>
>
>> ---
>>  kernel/signal.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/kernel/signal.c b/kernel/signal.c
>> index 43e8b7e362b0..621401550f0f 100644
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -2565,7 +2565,8 @@ static int ptrace_signal(int signr, kernel_siginfo_t *info, enum pid_type type)
>>  	}
>>  
>>  	/* If the (new) signal is now blocked, requeue it.  */
>> -	if (sigismember(&current->blocked, signr)) {
>> +	if (sigismember(&current->blocked, signr) ||
>> +	    fatal_signal_pending(current)) {
>>  		send_signal(signr, info, current, type);
>>  		signr = 0;
>>  	}
>> -- 
>> 2.20.1
>> 

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