Kees Cook <keescook@xxxxxxxxxxxx> writes: > 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! :) One of these days we might finish removing the rough edges and fixing the corner case bugs in the original linux pthreads support. Eric > 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 >>