Messages in this thread | | | From | Kyle Huey <> | Date | Tue, 2 Nov 2021 12:09:56 -0700 | Subject | Re: [PATCH] signal: SIGKILL can cause signal effects to appear at PTRACE_EVENT_EXIT without tracer notification |
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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(¤t->blocked, signr)) { > + if (sigismember(¤t->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
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