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

Peter Zijlstra
2025-02-19T16:36:50Z

Scott notes that non-taken branches are faster. Abuse overlapping code
that traps instead of explicit UD2 instructions.

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>
---
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;