linux-kernel - [PATCH v3 0/3] Fix issue with alternatives/paravirt patches

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Date:	Wed, 17 Aug 2016 20:58:27 -0400
From:	Jessica Yu <jeyu@...hat.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Miroslav Benes <mbenes@...e.cz>,
	Petr Mladek <pmladek@...e.com>, Jiri Kosina <jikos@...nel.org>,
	Chris J Arges <chris.j.arges@...onical.com>,
	Eugene Shatokhin <eugene.shatokhin@...alab.ru>
Cc:	live-patching@...r.kernel.org, x86@...nel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jessica Yu <jeyu@...hat.com>
Subject: [PATCH v3 0/3] Fix issue with alternatives/paravirt patches

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.

v2 here:
http://lkml.kernel.org/g/1469078640-26798-1-git-send-email-jeyu@redhat.com

v3: 
 - Add documentation about arch-specific code
 - Make sure to call module_enable_ro() when returning on error

v2:
 - add BUILD_BUG_ON() check in arch_klp_init_object_loaded (x86)

[1] http://thread.gmane.org/gmane.linux.kernel/2185604/
[2] https://github.com/dynup/kpatch/issues/580

Jessica Yu (3):
  livepatch: use arch_klp_init_object_loaded() to finish arch-specific tasks
  livepatch/x86: apply alternatives and paravirt patches after relocations
  Documentation: livepatch: add section about arch-specific code

 Documentation/livepatch/module-elf-format.txt | 20 +++++++--
 arch/x86/kernel/Makefile                      |  1 +
 arch/x86/kernel/livepatch.c                   | 65 +++++++++++++++++++++++++++
 include/linux/livepatch.h                     |  3 ++
 kernel/livepatch/core.c                       | 16 +++++--
 5 files changed, 98 insertions(+), 7 deletions(-)
 create mode 100644 arch/x86/kernel/livepatch.c

-- 
2.5.5