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

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Date:	Tue, 6 May 2014 10:03:46 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Ingo Molnar <mingo@...nel.org>
cc:	David Lang <david@...g.hm>, Josh Poimboeuf <jpoimboe@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Slaby <jslaby@...e.cz>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] kpatch: dynamic kernel patching

On Tue, 6 May 2014, Ingo Molnar wrote:

> So what I'm curious about, what is the actual 'in the field' distro 
> experience, about the type of live-patches that get pushed with 
> urgency?

This is of course a very good question. We've done some very light 
preparatory analysis and went through patches which would make most sense 
to be shipped as hot/live patches without enough time for proper downtime 
scheduling (i.e. CVE severity high enough (local root), etc). Most of the 
time, these turn out to be a one-or-few liners, mostly adding extra check, 
fixing bounds, etc. There were just one or two in a few years history 
where some extra care would be needed.

Of course, I guess that most valuable input regarding this could be coming 
from kSplice guys, as they have the biggest in-field experience with this, 
and are shipping a lot of non-trivial patches this way, not just the 
"super-critical" ones. But I am not really sure whether we can expect any 
input from them, unfortunately.

> My guess would be that the overwhelming majority of live-patches don't 
> change data structures - and hence the right initial model would be to 
> ensure (via tooling, and via review) that 'v1' and 'v2' data is exactly 
> the same.

I fully agree ... that's why we are not pro-actively dealing with that in 
kGraft, for the sake of keeping it stupid at simple (at least in the early 
stages).

> The most abstract way to 'live patch' a kernel is to do a checkpoint 
> save on all application state, to reboot the kernel, boot into the 
> patched kernel and then restore all application state seemlessly.

In some sense yes, but it seems to still be rather costly operation 
(especially checkpointing on a HUGE machine might take some time, if I 
understand the technology correctly) compared to just flipping a few bytes 
(while maintaining overall correctness, of course) in the kernel memory.

Basically, CR might be the best solution for very complex and large kernel 
updates. What I am currently most concerned about though, are scenarios 
where datacenter owners need to urgently apply a local root one-liner fix 
with as little hassle as possible.

-- 
Jiri Kosina
SUSE Labs
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