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powerpc/module_64: Fix livepatching for RO modules

Joe Lawrence requested to merge joe.lawrence/centos-stream-9:bz2019205 into main

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2019205

Upstream status: https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git

commit 8734b41b3efe0fc6082c1937b0e88556c396dc96 Author: Russell Currey ruscur@russell.cc Date: Tue Nov 23 18:15:20 2021 +1000

powerpc/module_64: Fix livepatching for RO modules

Livepatching a loaded module involves applying relocations through
apply_relocate_add(), which attempts to write to read-only memory when
CONFIG_STRICT_MODULE_RWX=y.  Work around this by performing these
writes through the text poke area by using patch_instruction().

R_PPC_REL24 is the only relocation type generated by the kpatch-build
userspace tool or klp-convert kernel tree that I observed applying a
relocation to a post-init module.

A more comprehensive solution is planned, but using patch_instruction()
for R_PPC_REL24 on should serve as a sufficient fix.

This does have a performance impact, I observed ~15% overhead in
module_load() on POWER8 bare metal with checksum verification off.

Fixes: c35717c71e98 ("powerpc: Set ARCH_HAS_STRICT_MODULE_RWX")
Cc: stable@vger.kernel.org # v5.14+
Reported-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Russell Currey <ruscur@russell.cc>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
[mpe: Check return codes from patch_instruction()]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211214121248.777249-1-mpe@ellerman.id.au

Signed-off-by: Joe Lawrence joe.lawrence@redhat.com

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