linux-kernel - Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching

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Date:	Wed, 7 May 2014 11:43:19 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Seth Jennings <sjenning@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, Jiri Slaby <jslaby@...e.cz>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching

On Wed, May 07, 2014 at 05:57:54PM +0200, Ingo Molnar wrote:
> Live patching does not enter into this question, ever. The correctness 
> of a patch to the source does not depend on 'live patching' 
> considerations in any way, shape or form.
> 
> Any mechanism that tries to blur these lines is broken by design.
> 
> My claim is that if a patch is correct/safe in the old fashioned way, 
> then a fundamental principle is that a live patching subsystem must 
> either safely apply, or safely reject the live patching attempt, 
> independently from any user input.

That's a valiant goal, but it's not going to happen unless you want to
rewrite Linux in Haskell.  It's just not possible for a program to prove
that a patch is safe to apply to a running kernel.  There are way too
many subtle interactions with dynamically allocated data between
functions.

I think the only way to achieve that is with CRIU, but it still requires
a kexec or a reboot, so you lose all kernel state and it's much more
disruptive.

> "We think/hope it won't blow up in most cases and we automated some 
> checks halfways" or "the user must know what he is doing" is really 
> not something that I think is a good concept for something as fragile 
> as live patching.

This is a distro tool, not a general purpose one.  If distros are
careful with their patch selection, it won't blow up.  It's a valuable
way for distros to help out sysadmins who need a hot security fix but
can't reboot immediately.

-- 
Josh
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