On Mon, Nov 15, 2021 at 11:34:33PM -0600, Eric W. Biederman wrote: > > Kyle Huey <me@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> > Reported-by: Marko Mäkelä <marko.makela@xxxxxxxxxxx> > History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.gi > Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> Yay pre-git-history! :) Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> > --- > 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(¤t->blocked, signr)) { > + if (sigismember(¤t->blocked, signr) || > + fatal_signal_pending(current)) { > send_signal(signr, info, current, type); > signr = 0; > } > -- > 2.20.1 > -- Kees Cook
