From 4505a262deae7df06e33617dfc104d5541eec91d Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Mon, 12 Oct 2026 02:04:24 -0700 Subject: [PATCH] x86/kcfi: Decode the kCFI check from the trap address Since commit b2250d63e6422 ("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 $-, %r10d" then decodes as "movl $-, %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: b2250d63e6422 ("x86/kcfi: Optimize call sequence") Assisted-by: LLM Signed-off-by: Kees Cook Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20261012090436.1676525-1-kees@kernel.org --- arch/x86/kernel/cfi.c | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-) diff --git a/arch/x86/kernel/cfi.c b/arch/x86/kernel/cfi.c index 1df26dfdce08..8e5c4ed264a0 100644 --- a/arch/x86/kernel/cfi.c +++ b/arch/x86/kernel/cfi.c @@ -10,11 +10,11 @@ #include /* - * Returns the target address and the expected type when regs->ip points + * Returns the target address and the expected type when addr points * to a compiler-generated CFI trap. */ -static bool decode_cfi_insn(struct pt_regs *regs, unsigned long *target, - u32 *type) +static bool decode_cfi_insn(struct pt_regs *regs, unsigned long addr, + unsigned long *target, u32 *type) { char buffer[MAX_INSN_SIZE]; struct insn insn; @@ -29,13 +29,13 @@ static bool decode_cfi_insn(struct pt_regs *regs, unsigned long *target, *   movl -, %r10d ; 6 bytes * addl -(%reg), %r10d; 4 bytes * je .Ltmp1 ; 2 bytes - * ud2 ; <- regs->ip + * ud2 ; <- addr * .Ltmp1: * * We can decode the expected type and the target address from the * movl/addl instructions. */ - if (copy_from_kernel_nofault(buffer, (void *)regs->ip - 12, MAX_INSN_SIZE)) + if (copy_from_kernel_nofault(buffer, (void *)addr - 12, MAX_INSN_SIZE)) return false; if (insn_decode_kernel(&insn, &buffer[offset])) return false; @@ -44,7 +44,7 @@ static bool decode_cfi_insn(struct pt_regs *regs, unsigned long *target, *type = -(u32)insn.immediate.value; - if (copy_from_kernel_nofault(buffer, (void *)regs->ip - 6, MAX_INSN_SIZE)) + if (copy_from_kernel_nofault(buffer, (void *)addr - 6, MAX_INSN_SIZE)) return false; if (insn_decode_kernel(&insn, &buffer[offset])) return false; @@ -82,7 +82,7 @@ enum bug_trap_type handle_cfi_failure(struct pt_regs *regs) return BUG_TRAP_TYPE_NONE; } - if (!decode_cfi_insn(regs, &target, &type)) + if (!decode_cfi_insn(regs, addr, &target, &type)) return report_cfi_failure_noaddr(regs, addr); break;