From: Kees Cook <keescook@chromium.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
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: Tue, 16 Nov 2021 10:31:15 -0800 [thread overview]
Message-ID: <202111161031.57764153B@keescook> (raw)
In-Reply-To: <87tugcd5p2.fsf_-_@email.froward.int.ebiederm.org>
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! :)
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
>
--
Kees Cook
next prev parent reply other threads:[~2021-11-16 18:31 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 [this message]
2021-11-17 16:44 ` Eric W. Biederman
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=202111161031.57764153B@keescook \
--to=keescook@chromium.org \
--cc=axboe@kernel.dk \
--cc=ebiederm@xmission.com \
--cc=elver@google.com \
--cc=legion@kernel.org \
--cc=linux-api@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=marko.makela@mariadb.com \
--cc=me@kylehuey.com \
--cc=oleg@redhat.com \
--cc=pcc@google.com \
--cc=peterz@infradead.org \
--cc=rocallahan@gmail.com \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).