x86/kcfi: Decode the kCFI check from the trap address

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Since commit b2250d63e6 ("x86/kcfi: Optimize call sequence"), a
CONFIG_FINEIBT=y kernel running in kCFI mode rewrites each kCFI call
site at boot so that a failed check traps on the 0xd6 byte of
"test $0xd6, %al", one byte past where the compiler's "ud2" was.
handle_cfi_failure() steps "addr" back one byte to find the trap in
.kcfi_traps, but decode_cfi_insn() still decodes from regs->ip, so it
reads each instruction starting at its second byte.
"movl $-<id>, %r10d" then decodes as "movl $-<id>, %edx", so the
reported type is right by accident, but "addl -15(%r11), %r10d" decodes
as "addl -15(%rbx), %edx", so the reported target is whatever %rbx holds:

  CFI failure at lkdtm_indirect_call+0x16/0x20 (target: 0x12; expected type: 0xc79a2d1b)

Pass the adjusted address to decode_cfi_insn() and decode from it.

Tested ARCH=x86_64 defconfig with CONFIG_CFI=y and CONFIG_LKDTM=y under
QEMU, with LKDTM's CFI_FORWARD_PROTO and both Clang 24.0.0git and GCC
17.0.0 (experimental, with the kCFI series). With CONFIG_FINEIBT=y in
kCFI mode, the report gives %rbx as the target without this change, and
the expected lkdtm_increment_int() with it. With cfi=fineibt, with
cfi=fineibt,paranoid, and with CONFIG_FINEIBT=n, it gives
lkdtm_increment_int() both with and without this change.

Fixes: b2250d63e6 ("x86/kcfi: Optimize call sequence")
Assisted-by: LLM
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20261012090436.1676525-1-kees@kernel.org

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Kees Cook 2026-10-12 02:04:24 -07:00 • committed by Peter Zijlstra

parent 24a19b953e

commit 4505a262de

1 changed files with 7 additions and 7 deletions

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arch/x86/kernel/cfi.c

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