Re: [PATCH 3/3] signal: Requeue ptrace signals

[Kees Cook]

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(&current->blocked, signr)) {
> +	if (sigismember(&current->blocked, signr) ||
> +	    fatal_signal_pending(current)) {
>  		send_signal(signr, info, current, type);
>  		signr = 0;
>  	}
> -- 
> 2.20.1
> 

-- 
Kees Cook