Messages in this thread | | | From | Kyle Huey <> | Date | Mon, 8 Nov 2021 15:58:21 -0800 | 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 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(¤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
Is there anything else I need to do here or are you going to take it from here?
- Kyle
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