linux-kernel - Re: Fix issue with alternatives/paravirt patches

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Message-ID: <20160708165740.GA20248@packer-debian-8-amd64.digitalocean.com>
Date:	Fri, 8 Jul 2016 12:57:40 -0400
From:	Jessica Yu <jeyu@...hat.com>
To:	Christopher Arges <chris.j.arges@...onical.com>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Miroslav Benes <mbenes@...e.cz>,
	Petr Mladek <pmladek@...e.com>, Jiri Kosina <jikos@...nel.org>,
	Eugene Shatokhin <eugene.shatokhin@...alab.ru>,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Fix issue with alternatives/paravirt patches

+++ Christopher Arges [08/07/16 00:22 -0500]:
>On Tue, Jul 05, 2016 at 10:34:58PM -0400, Jessica Yu wrote:
>> Hi,
>>
>> A few months ago, Chris Arges reported a bug involving alternatives/paravirt
>> patching that was discussed here [1] and here [2]. To briefly summarize the
>> bug, patch modules that contained .altinstructions or .parainstructions
>> sections would break because these alternative/paravirt patches would be
>> applied first by the module loader (see x86 module_finalize()), then
>> livepatch would later clobber these patches when applying per-object
>> relocations. This lead to crashes and unpredictable behavior.
>>
>> One conclusion we reached from our last discussion was that we will
>> need to introduce some arch-specific code to address this problem.
>> This patchset presents a possible fix for the bug by adding a new
>> arch-specific arch_klp_init_object_loaded() function that by default
>> does nothing but can be overridden by different arches.
>>
>> To fix this issue for x86, since we can access a patch module's Elf
>> sections through mod->klp_info, we can simply delay the calls to
>> apply_paravirt() and apply_alternatives() to arch_klp_init_object_loaded(),
>> which is called after relocations have been written for an object.
>> In addition, for patch modules, .parainstructions and .altinstructions are
>> prefixed by ".klp.arch.${objname}" so that the module loader ignores them
>> and livepatch can apply them manually.
>>
>> Currently for kpatch, we don't support including jump table sections in
>> the patch module, and supporting .smp_locks is currently broken, so we
>> don't consider those sections (for now).
>>
>> I did some light testing with some patches to kvm and verified that the
>> original issue reported in [2] was fixed.
>>
>> Based on linux-next.
>>
>
>Jessica,
>
>I was able to test these patches on top of linux-next. I took your kpatch
>branch and hacked it a bit to get it working and was able to
>apply a patch to 'kvm_arch_vm_ioctl' while running a VM workload.
>
>Great job!
>
>Tested-by: Chris J Arges <chris.j.arges@...onical.com>

Excellent, thanks for testing it out Chris!

Jessica