From: ebiederm@xmission.com (Eric W. Biederman)
To: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org, "Kyle Huey" <me@kylehuey.com>,
"Jens Axboe" <axboe@kernel.dk>,
"Peter Zijlstra" <peterz@infradead.org>,
"Marco Elver" <elver@google.com>,
"Oleg Nesterov" <oleg@redhat.com>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Peter Collingbourne" <pcc@google.com>,
"Alexey Gladkov" <legion@kernel.org>,
"Robert O'Callahan" <rocallahan@gmail.com>,
"Marko Mäkelä" <marko.makela@mariadb.com>,
linux-api@vger.kernel.org, "Al Viro" <viro@zeniv.linux.org.uk>,
"Linus Torvalds" <torvalds@linux-foundation.org>
Subject: Re: [PATCH 3/3] signal: Requeue ptrace signals
Date: Wed, 17 Nov 2021 10:44:55 -0600 [thread overview]
Message-ID: <87a6i2afzs.fsf@email.froward.int.ebiederm.org> (raw)
In-Reply-To: <202111161031.57764153B@keescook> (Kees Cook's message of "Tue, 16 Nov 2021 10:31:15 -0800")
Kees Cook <keescook@chromium.org> writes:
> On Mon, Nov 15, 2021 at 11:34:33PM -0600, Eric W. Biederman 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.
>> >
>> > [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@kylehuey.com>
>> Reported-by: Marko Mäkelä <marko.makela@mariadb.com>
>> History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.gi
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>
> 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@chromium.org>
>
>> ---
>> 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
>>
next prev parent reply other threads:[~2021-11-17 16:45 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-01 3:41 [PATCH] signal: SIGKILL can cause signal effects to appear at PTRACE_EVENT_EXIT without tracer notification Kyle Huey
2021-11-01 3:41 ` [PATCH 1/2] signal: factor out SIGKILL generation in get_signal Kyle Huey
2021-11-01 3:41 ` [PATCH 2/2] signal: after notifying a ptracer of a signal, recheck for pending SIGKILLs Kyle Huey
2021-11-02 14:08 ` [PATCH] signal: SIGKILL can cause signal effects to appear at PTRACE_EVENT_EXIT without tracer notification Eric W. Biederman
2021-11-02 16:01 ` Kyle Huey
2021-11-02 18:06 ` Eric W. Biederman
2021-11-02 19:09 ` Kyle Huey
2021-11-08 23:58 ` Kyle Huey
2021-11-14 17:19 ` Eric W. Biederman
2021-11-16 5:29 ` [PATCH 0/3] signal: requeuing undeliverable signals Eric W. Biederman
2021-11-16 5:32 ` [PATCH 1/3] signal: In get_signal test for signal_group_exit every time through the loop Eric W. Biederman
2021-11-16 18:23 ` Kees Cook
2021-11-17 16:31 ` Eric W. Biederman
2021-11-16 5:33 ` [PATCH 2/3] signal: Requeue signals in the appropriate queue Eric W. Biederman
2021-11-16 18:30 ` Kees Cook
2021-11-17 16:42 ` Eric W. Biederman
2021-11-16 5:34 ` [PATCH 3/3] signal: Requeue ptrace signals Eric W. Biederman
2021-11-16 18:31 ` Kees Cook
2021-11-17 16:44 ` Eric W. Biederman [this message]
2021-11-17 16:24 ` [PATCH 0/3] signal: requeuing undeliverable signals Kyle Huey
2021-11-17 16:51 ` Eric W. Biederman
2021-11-18 6:12 ` Marko Mäkelä
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