linux-kernel - Re: Oops caused by race between livepatch and ftrace

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Date:   Mon, 20 May 2019 16:46:36 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Johannes Erdfelt <johannes@...felt.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jessica Yu <jeyu@...nel.org>, Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Oops caused by race between livepatch and ftrace

[ fixed jeyu's email address ]

On 5/20/19 3:49 PM, Johannes Erdfelt wrote:
> [ ... snip ... ]
> 
> I have put together a test case that can reproduce the crash using
> KVM. The tarball includes a minimal kernel and initramfs, along with
> a script to run qemu and the .config used to build the kernel. By
> default it will attempt to reproduce by loading multiple livepatches
> at the same time. Passing 'test=ftrace' to the script will attempt to
> reproduce by racing with ftrace.
> 
> My test setup reproduces the race and oops more reliably by loading
> multiple livepatches at the same time than with the ftrace method. It's
> not 100% reproducible, so the test case may need to be run multiple
> times.
> 
> It can be found here (not attached because of its size):
> http://johannes.erdfelt.com/5.2.0-rc1-a188339ca5-livepatch-race.tar.gz

Hi Johannes,

This is cool way to distribute the repro kernel, modules, etc!

These two testing scenarios might be interesting to add to our selftests 
suite.  Can you post or add the source(s) to livepatch-test<n>.ko to the 
tarball?

> The simple patch of extending the module_mutex lock over the entirety
> of klp_init_object_loaded fixes it from the livepatch side. This
> mostly works because set_all_modules_text_{rw,ro} acquires module_mutex
> as well, but it still leaves a hole in the ftrace code. A lock should
> probably be held over the entirety of remapping the text sections RW.
> 
> This is complicated by the fact that remapping the text section in
> ftrace is handled by arch specific code. I'm not sure what a good
> solution to this is yet.

A lock or some kind of referencing count..  I'll let other folks comment 
on that side of the bug report.

Thanks,

-- Joe