Date:	Thu, 8 May 2014 09:08:15 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	David Lang <david@...g.hm>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	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


* David Lang <david@...g.hm> wrote:

> On Thu, 8 May 2014, Ingo Molnar wrote:
> 
> >>>
> >>>No!
> >>>
> >>>A patch to the kernel source is 'safe' if it results in a correctly
> >>>patched kernel source. Full stop!
> >>>
> >>>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.
> >>>
> >>>It's similar to how kprobes (or ftrace) will safely reject or perform
> >>>a live patching of the kernel.
> >>>
> >>>So for example, there's this recent upstream kernel fix:
> >>>
> >>>3ca9e5d36afb agp: info leak in agpioc_info_wrap()
> >>>
> >>>which fixes an information leak. The 'patch' is Git commit
> >>>3ca9e5d36afb (i.e. it patches a very specific incoming kernel source
> >>>tree that results in a specific outgoing source tree), and we know
> >>>it's safe and correct.
> >>>
> >>>Any live patching subsystem must make sure that if this patch is
> >>>live-patched, that this attempt is either rejected safely or performed
> >>>safely.
> >>>
> >>>"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.
> >>
> >>In that case you will have to reject any kernel patch that changes
> >>any memory structure, because it's impossible as a general rule to
> >>say that changing memory structures is going to be safe (or even
> >>possible) to change.
> >>
> >>that includes any access to memory that moves around a lock
> >
> >Initially restricting it to such patches would be a good beginning -
> >most of the security fixes are just failed checks, i.e. they don't
> >typically even change any external (not on stack) memory structure,
> >right?
> 
> in terms of hit-patching kernels you are correct.
> 
> but that's a far cry from what it sounded like you were demanding 
> (that it must handle any kernel patch)

No, I was not demanding that at all, my suggestion was:

   > 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.

Note the 'if'. It could start simple and stupid, and only allow cases 
where we know the patch must be trivially safe (because it does not do 
much in terms of disturbing globally visible state). That needs some 
tooling help, but apparently tooling help is in place already.

And then we can complicate it from there - but have a reasonably 
robust starting point that we _know_ works (as long as the 
implementation is correct).

Thanks,

	Ingo
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