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

Kyle Huey
2021-11-08T23:58:44Z

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?

- Kyle