[PATCH v2 0/2] Fix issue with alternatives/paravirt patches

From: Jessica Yu
Date: Thu Jul 21 2016 - 01:24:25 EST


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.

v1 here:

- 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 (2):
livepatch: use arch_klp_init_object_loaded() to finish arch-specific tasks
livepatch/x86: apply alternatives and paravirt patches after relocations

arch/x86/kernel/Makefile | 1 +
arch/x86/kernel/livepatch.c | 65 +++++++++++++++++++++++++++++++++++++++++++++
include/linux/livepatch.h | 3 +++
kernel/livepatch/core.c | 12 +++++++--
4 files changed, 79 insertions(+), 2 deletions(-)
create mode 100644 arch/x86/kernel/livepatch.c