Re: [PATCH v3 05/10] x86/ibt: Optimize FineIBT sequence

Kees Cook
2025-02-19T18:01:18Z

On Wed, Feb 19, 2025 at 05:21:12PM +0100, Peter Zijlstra wrote:
> Scott notes that non-taken branches are faster. Abuse overlapping code
> that traps instead of explicit UD2 instructions.

Some kind of commenting is needed in here to explicitly call out the
embedded EA in the "subl" instruction. There is a tiny hint of it in the
disassembly dump of fineibt_preamble_start, but it's very subtle for
someone trying to understand this fresh.

> And LEA does not modify flags and will have less dependencies.
>
> Suggested-by: Scott Constable <scott.d.constable@intel.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

But it works!

Reviewed-by: Kees Cook <kees@kernel.org>

> ---
> arch/x86/kernel/alternative.c | 58 ++++++++++++++++++++++++++----------------
> arch/x86/net/bpf_jit_comp.c | 5 +--
> 2 files changed, 39 insertions(+), 24 deletions(-)
>
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1054,9 +1054,9 @@ early_param("cfi", cfi_parse_cmdline);
> * __cfi_\func: __cfi_\func:
> * movl $0x12345678,%eax // 5 endbr64 // 4
> * nop subl $0x12345678,%r10d // 7
> - * nop jz 1f // 2
> - * nop ud2 // 2
> - * nop 1: nop // 1
> + * nop jne __cfi_\func+6 // 2
> + * nop nop3 // 3
> + * nop
> * nop
> * nop
> * nop
> @@ -1068,37 +1068,47 @@ early_param("cfi", cfi_parse_cmdline);
> *
> * caller: caller:
> * movl $(-0x12345678),%r10d // 6 movl $0x12345678,%r10d // 6
> - * addl $-15(%r11),%r10d // 4 sub $16,%r11 // 4
> + * addl $-15(%r11),%r10d // 4 lea -0x10(%r11),%r11 // 4
> * je 1f // 2 nop4 // 4
> * ud2 // 2
> - * 1: call __x86_indirect_thunk_r11 // 5 call *%r11; nop2; // 5
> + * 1: cs call __x86_indirect_thunk_r11 // 6 call *%r11; nop3; // 6
> *
> */
>
> -asm( ".pushsection .rodata \n"
> - "fineibt_preamble_start: \n"
> - " endbr64 \n"
> - " subl $0x12345678, %r10d \n"
> - " je fineibt_preamble_end \n"
> - "fineibt_preamble_ud2: \n"
> - " ud2 \n"
> - " nop \n"
> - "fineibt_preamble_end: \n"
> +/*
> + * <fineibt_preamble_start>:
> + * 0: f3 0f 1e fa endbr64
> + * 4: 41 81 <ea> 78 56 34 12 sub $0x12345678, %r10d
> + * b: 75 f9 jne 6 <fineibt_preamble_start+0x6>
> + * d: 0f 1f 00 nopl (%rax)
> + */
> +asm( ".pushsection .rodata \n"
> + "fineibt_preamble_start: \n"
> + " endbr64 \n"
> + " subl $0x12345678, %r10d \n"
> + " jne fineibt_preamble_start+6 \n"
> + ASM_NOP3
> + "fineibt_preamble_end: \n"
> ".popsection\n"
> );
>
> extern u8 fineibt_preamble_start[];
> -extern u8 fineibt_preamble_ud2[];
> extern u8 fineibt_preamble_end[];
>
> #define fineibt_preamble_size (fineibt_preamble_end - fineibt_preamble_start)
> -#define fineibt_preamble_ud2 (fineibt_preamble_ud2 - fineibt_preamble_start)
> +#define fineibt_preamble_ud 6
> #define fineibt_preamble_hash 7
>
> +/*
> + * <fineibt_caller_start>:
> + * 0: 41 ba 78 56 34 12 mov $0x12345678, %r10d
> + * 6: 4d 8d 5b f0 lea -0x10(%r11), %r11
> + * a: 0f 1f 40 00 nopl 0x0(%rax)
> + */
> asm( ".pushsection .rodata \n"
> "fineibt_caller_start: \n"
> " movl $0x12345678, %r10d \n"
> - " sub $16, %r11 \n"
> + " lea -0x10(%r11), %r11 \n"
> ASM_NOP4
> "fineibt_caller_end: \n"
> ".popsection \n"
> @@ -1429,15 +1439,15 @@ static void poison_cfi(void *addr)
> }
>
> /*
> - * regs->ip points to a UD2 instruction, return true and fill out target and
> - * type when this UD2 is from a FineIBT preamble.
> + * When regs->ip points to a 0xEA byte in the FineIBT preamble,
> + * return true and fill out target and type.
> *
> * We check the preamble by checking for the ENDBR instruction relative to the
> - * UD2 instruction.
> + * 0xEA instruction.
> */
> bool decode_fineibt_insn(struct pt_regs *regs, unsigned long *target, u32 *type)
> {
> - unsigned long addr = regs->ip - fineibt_preamble_ud2;
> + unsigned long addr = regs->ip - fineibt_preamble_ud;
> u32 hash;
>
> if (!exact_endbr((void *)addr))
> @@ -1448,6 +1458,12 @@ bool decode_fineibt_insn(struct pt_regs
> __get_kernel_nofault(&hash, addr + fineibt_preamble_hash, u32, Efault);
> *type = (u32)regs->r10 + hash;
>
> + /*
> + * Since regs->ip points to the middle of an instruction; it cannot
> + * continue with the normal fixup.
> + */
> + regs->ip = *target;
> +
> return true;
>
> Efault:
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -417,9 +417,8 @@ static void emit_fineibt(u8 **pprog, u32
>
> EMIT_ENDBR();
> EMIT3_off32(0x41, 0x81, 0xea, hash); /* subl $hash, %r10d */
> - EMIT2(0x74, 0x07); /* jz.d8 +7 */
> - EMIT2(0x0f, 0x0b); /* ud2 */
> - EMIT1(0x90); /* nop */
> + EMIT2(0x75, 0xf9); /* jne.d8 .-7 */
> + EMIT3(0x0f, 0x1f, 0x00); /* nop3 */
> EMIT_ENDBR_POISON();
>
> *pprog = prog;
>
>

--
Kees Cook