linux-kernel - Re: [PATCH 2/2] kernel: add support for live patching

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Message-ID: <20141107195233.GF2057@cerebellum.variantweb.net>
Date:	Fri, 7 Nov 2014 13:52:33 -0600
From:	Seth Jennings <sjenning@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	live-patching@...r.kernel.org, kpatch@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] kernel: add support for live patching

On Fri, Nov 07, 2014 at 11:40:38AM -0800, Andy Lutomirski wrote:
> On 11/06/2014 06:39 AM, Seth Jennings wrote:
> > This commit introduces code for the live patching core.  It implements
> > an ftrace-based mechanism and kernel interface for doing live patching
> > of kernel and kernel module functions.
> > 
> > It represents the greatest common functionality set between kpatch and
> > kgraft and can accept patches built using either method.
> > 
> > This first version does not implement any consistency mechanism that
> > ensures that old and new code do not run together.  In practice, ~90% of
> > CVEs are safe to apply in this way, since they simply add a conditional
> > check.  However, any function change that can not execute safely with
> > the old version of the function can _not_ be safely applied in this
> > version.
> > 
> 
> [...]
> 
> > +/********************************************
> > + * Sysfs Interface
> > + *******************************************/
> > +/*
> > + * /sys/kernel/livepatch
> > + * /sys/kernel/livepatch/<patch>
> > + * /sys/kernel/livepatch/<patch>/enabled
> > + * /sys/kernel/livepatch/<patch>/<object>
> > + * /sys/kernel/livepatch/<patch>/<object>/<func>
> > + * /sys/kernel/livepatch/<patch>/<object>/<func>/new_addr
> > + * /sys/kernel/livepatch/<patch>/<object>/<func>/old_addr
> > + */
> 
> Letting anyone read new_addr and old_addr is a kASLR leak, and I would
> argue that showing this information to non-root at all is probably a bad
> idea.

Also worth noting that this live patching implementation currently
doesn't support kASLR, as there is a method for the patch module to
supply the old_addr, determined at generation time by pulling from
vmlinux/System.map/etc, for a particular function to resolve symbol
ambiguity in a kallsyms lookup.  Obviously, this old_addr would be wrong
for a kernel using kASLR.

Thanks,
Seth

> 
> Can you make new_addr and old_addr have mode 0600 and
> /sys/kernel/livepatch itself have mode 0500?  For the latter, an admin
> who wants unprivileged users to be able to see it can easily chmod it.
> 
> --Andy
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