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Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps
From:   Olivier Langlois <olivier@trillion01.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Oleg Nesterov <oleg@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        io-uring <io-uring@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>,
        "Pavel Begunkov>" <asml.silence@gmail.com>
Date:   Fri, 18 Jun 2021 16:05:36 -0400
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On Wed, 2021-06-16 at 15:00 -0500, Eric W. Biederman wrote:
> Olivier Langlois <olivier@trillion01.com> writes:
> 
> > I redid my test but this time instead of dumping directly into a
> > file,
> > I did let the coredump be piped to the systemd coredump module and
> > the
> > coredump generation isn't working as expected when piping.
> > 
> > So your code review conclusions are correct.
> 
> Thank you for confirming that.
> 
> Do you know how your test program is using io_uring?
> 
> I have been trying to put the pieces together on what io_uring is
> doing
> that stops the coredump.? The fact that it takes a little while
> before
> it kills the coredump is a little puzzling.? The code looks like all
> of
> the io_uring operations should have been canceled before the coredump
> starts.
> 
> 
With a very simple setup, I guess that this could easily be
reproducible. Make a TCP connection with a server that is streaming
non-stop data and enter a loop where you keep initiating async
OP_IOURING_READ operations on your TCP fd.

Once you have that, manually sending a SIG_SEGV is a sure fire way to
stumble into the problem. This is how I am testing the patches.

IRL, it is possible to call io_uring_enter() to submit operations and
return from the syscall without waiting on all events to have
completed. Once the process is back in userspace, if it stumble into a
bug that triggers a coredump, any remaining pending I/O operations can
set TIF_SIGNAL_NOTIFY while the coredump is generated.

I have read the part of your previous email where you share the result
of your ongoing investigation. I didn't comment as the definitive
references in io_uring matters are Jens and Pavel but I am going to
share my opinion on the matter.

I think that you did put the finger on the code cleaning up the
io_uring instance in regards to pending operations. It seems to be
io_uring_release() which is probably called from exit_files() which
happens to be after the call to exit_mm().

At first, I did entertain the idea of considering if it could be
possible to duplicate some of the operations performed by
io_uring_release() related to the infamous TIF_SIGNAL_NOTIFY setting
into io_uring_files_cancel() which is called before exit_mm().

but the idea is useless as it is not the other threads of the group
that are causing the TIF_SIGNAL_NOTIFY problem. It is the thread
calling do_coredump() which is done by the signal handing code even
before that thread enters do_exit() and start to be cleaned up. That
thread when it enters do_coredump() is still fully loaded and
operational in terms of io_uring functionality.

I guess that this io_uring cancel all pending operations hook would
have to be called from do_coredump or from get_signal() but if it is
the way to go, I feel that this is a change major enough that wouldn't
dare going there without the blessing of the maintainers in cause....