Re: [PATCH] signal: SIGKILL can cause signal effects to appear at PTRACE_EVENT_EXIT without tracer notification

[ebiederm@xmission.com (Eric W. Biederman)]

Kyle Huey <me@kylehuey.com> writes:

> On Tue, Nov 2, 2021 at 12:09 PM Kyle Huey <me@kylehuey.com> wrote:
>>
>> On Tue, Nov 2, 2021 at 11:07 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
>> >
>> > Kyle Huey <me@kylehuey.com> writes:
>> >
>> > > On Tue, Nov 2, 2021 at 7:09 AM Eric W. Biederman <ebiederm@xmission.com> wrote:
>> > >>
>> > >> Kyle Huey <me@kylehuey.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.
>> > >> >
>> > >> > This issue was originally reported by the credited rr user.
>> > >> >
>> > >> > [0] https://rr-project.org/
>> > >>
>> > >> If I read this correctly the problem is not precisely that the rr
>> > >> debugger is never notified about the signal, but rather that the program
>> > >> is killed with SIGKILL before rr can read the notification and see which
>> > >> signal it is.
>> > >
>> > > The precise problem is that the kernel made a modification to the
>> > > tracee state (setting up the signal handler frame) without telling the
>> > > tracer about it (delivering the ptrace notification for the pending
>> > > non-SIGKILL signal).
>> >
>> > Except the kernel did make it to ptrace_stop.  The stop just did not
>> > fully happen because of SIGKILL.  I expect SIGCHLD was sent to the
>> > tracer as part of that stop that never fully happened.
>>
>> I don't know whether SIGCHLD was sent to the tracer (rr doesn't use it
>> directly) but waiting on the process does not produce a wait status
>> corresponding to the signal delivery stop for the original signal.
>> Waiting on the tracee skips immediately from whatever the preceding
>> ptrace event was to the PTRACE_EVENT_EXIT.
>>
>> (In our particular case, if it had been notified of the signal, we
>> would have chosen to suppress the signal, because the signal in
>> question is a SIGSEGV from an rdtsc instruction that has been disabled
>> via prctl(PR_SET_TSC, PR_TSC_SIGSEGV) and we emulate it in the tracer
>> due to its non-deterministic behavior. So we really don't expect to
>> see the tracee signal handler.)
>>
>> > > That can be fixed either by not modifying the
>> > > tracee state here or by telling the tracer about the signal (that will
>> > > never actually run). I suspect we'll all agree that the former seems
>> > > preferable.
>> > >
>> > >> This definitely sounds like a quality of implementation issue.
>> > >>
>> > >> The solution that is proposed in your patches simply drops the signal
>> > >> when SIGKILL is pending.
>> > >
>> > > That's right.
>> > >
>> > >> I think we can have a slightly better of quality of implementation
>> > >> than that (as well as a simpler implementation) by requeuing the
>> > >> signal instead of simply dropping it.  Something like the below.
>> > >
>> > > What is the benefit of requeueing the signal? All pending signals will
>> > > be dropped when the SIGKILL is processed, no?
>> >
>> > Not before PTRACE_EVENT_EXIT.  In fact the pending signals are not
>> > actually flushed until the thread or the entire process is reaped.
>> >
>> > Further the coredump code makes some attempt to write out the
>> > pending signals.  The code appears to predate siginfo support
>> > in the kernel so it misses a lot but it is there.
>> >
>> > The real advantage is that it keeps the logic of dealing with weird
>> > ptrace_stop logic in ptrace_signal where it belongs.  It also allows the
>> > common (and missing in this case) idiom of goto relock to be used.
>> >
>> > So I think changing ptrace_signal will be much more maintainable.
>>
>> Ok.
>>
>> > >> Can you test that and see if it works for you?
>> > >
>> > > It does not work. This triggers an infinite loop in get_signal, as we
>> > > dequeue the signal, attempt to notify the ptracer, see the pending
>> > > sigkill, requeue the signal, go around the loop, dequeue the original
>> > > signal ...
>> >
>> > Apologies I made a bit of a thinko.  That change also needs to change
>> > the handling of if (signr == 0) after ptrace_signal.
>> >
>> > Which means it would need to be something like the below.
>> >
>> > diff --git a/kernel/signal.c b/kernel/signal.c
>> > index 056a107e3cbc..eddb745b34a7 100644
>> > --- a/kernel/signal.c
>> > +++ b/kernel/signal.c
>> > @@ -2610,7 +2610,8 @@ static int ptrace_signal(int signr, kernel_siginfo_t *info)
>> >         }
>> >
>> >         /* If the (new) signal is now blocked, requeue it.  */
>> > -       if (sigismember(&current->blocked, signr)) {
>> > +       if (sigismember(&current->blocked, signr) ||
>> > +           signal_group_exit(current->signal)) {
>> >                 send_signal(signr, info, current, PIDTYPE_PID);
>> >                 signr = 0;
>> >         }
>> > @@ -2764,8 +2765,10 @@ bool get_signal(struct ksignal *ksig)
>> >                 if (unlikely(current->ptrace) && (signr != SIGKILL) &&
>> >                     !(sighand->action[signr -1].sa.sa_flags & SA_IMMUTABLE)) {
>> >                         signr = ptrace_signal(signr, &ksig->info);
>> > -                       if (!signr)
>> > -                               continue;
>> > +                       if (!signr) {
>> > +                               spin_unlock_irq(&sighand->siglock);
>> > +                               goto relock;
>> > +                       }
>> >                 }
>> >
>> >                 ka = &sighand->action[signr-1];
>> >
>> > Eric
>>
>> Yeah that appears to fix the issue.
>>
>> - Kyle
>
> Is there anything else I need to do here or are you going to take it
> from here?

No pinging me is the helpful thing to do.  I sometimes get distracted.

The merge window closes sometime later today and then v5.16-rc1 will be
out.  Then I will have a good base to work against.

Until then I can't really merge anything.

Eric