x86/kcfi: Emit the FineIBT paranoid "cs" prefix as a byte

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The "cs" prefix on the LEA in the FineIBT paranoid caller template is
padding: it places the LEA's 0xf0 immediate byte where it doubles as the
LOCK prefix that makes the failing Jcc raise #UD. GNU as warns about it
in every CONFIG_FINEIBT=y build:

  Warning: segment override on `lea' is ineffectual

Clang's integrated assembler does not warn, and both assemble the same
bytes. Emit the prefix with ".byte 0x2e" to silence GNU as.

Build tested ARCH=x86_64 defconfig with CONFIG_CFI=y, with GCC 17.0.0
(experimental, with the kCFI series) and GNU as 2.47, and with Clang
24.0.0git: GNU as no longer warns, and the template assembles to the
same bytes as before with both. Under QEMU with cfi=fineibt,paranoid,
the paranoid caller check catches LKDTM's CFI_FORWARD_PROTO with both.

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/20261012090436.1676525-2-kees@kernel.org

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Kees Cook 2026-10-12 02:04:25 -07:00 • committed by Peter Zijlstra

parent 24a19b953e

commit 41d1119e2f

1 changed files with 2 additions and 1 deletions

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arch/x86/kernel/alternative.c

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