[PATCH v3 10/10] x86/ibt: Optimize fineibt-bhi arity 1 case

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

Saves a CALL to an out-of-line thunk for the common case of 1
argument.

Suggested-by: Scott Constable <scott.d.constable@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/include/asm/ibt.h | 4 ++
arch/x86/kernel/alternative.c | 61 +++++++++++++++++++++++++++++++++++-------
2 files changed, 56 insertions(+), 9 deletions(-)

--- a/arch/x86/include/asm/ibt.h
+++ b/arch/x86/include/asm/ibt.h
@@ -70,6 +70,10 @@ static inline bool __is_endbr(u32 val)
if (val == gen_endbr_poison())
return true;

+ /* See cfi_fineibt_bhi_preamble() */
+ if (IS_ENABLED(CONFIG_FINEIBT_BHI) && val == 0x001f0ff5)
+ return true;
+
val &= ~0x01000000U; /* ENDBR32 -> ENDBR64 */
return val == gen_endbr();
}
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1299,6 +1299,55 @@ static int cfi_rand_preamble(s32 *start,
return 0;
}

+static void cfi_fineibt_bhi_preamble(void *addr, int arity)
+{
+ bool warn = IS_ENABLED(CONFIG_CFI_PREMISSIVE) || cfi_warn;
+
+ if (!arity)
+ return;
+
+ if (!warn && arity == 1) {
+ /*
+ * Crazy scheme to allow arity-1 inline:
+ *
+ * __cfi_foo:
+ * 0: f3 0f 1e fa endbr64
+ * 4: 41 81 <ea> 78 56 34 12 sub 0x12345678, %r10d
+ * b: 49 0f 45 fa cmovne %r10, %rdi
+ * f: 75 f5 jne __cfi_foo+6
+ * 11: 0f 1f 00 nopl (%rax)
+ *
+ * Code that direct calls to foo()+0, decodes the tail end as:
+ *
+ * foo:
+ * 0: f5 cmc
+ * 1: 0f 1f 00 nopl (%rax)
+ *
+ * which clobbers CF, but does not affect anything ABI
+ * wise.
+ *
+ * Notably, this scheme is incompatible with permissive CFI
+ * because the cmov is unconditional and RDI will have been
+ * clobbered.
+ */
+ const u8 magic[9] = {
+ 0x49, 0x0f, 0x45, 0xfa,
+ 0x75, 0xf5,
+ BYTES_NOP3,
+ };
+
+ text_poke_early(addr + fineibt_preamble_bhi, magic, 9);
+
+ return;
+ }
+
+ text_poke_early(addr + fineibt_preamble_bhi,
+ text_gen_insn(CALL_INSN_OPCODE,
+ addr + fineibt_preamble_bhi,
+ __bhi_args[arity]),
+ CALL_INSN_SIZE);
+}
+
static int cfi_rewrite_preamble(s32 *start, s32 *end)
{
s32 *s;
@@ -1329,14 +1378,8 @@ static int cfi_rewrite_preamble(s32 *sta
"kCFI preamble has wrong register at: %pS %*ph\n",
addr, 5, addr);

- if (!cfi_bhi || !arity)
- continue;
-
- text_poke_early(addr + fineibt_preamble_bhi,
- text_gen_insn(CALL_INSN_OPCODE,
- addr + fineibt_preamble_bhi,
- __bhi_args[arity]),
- CALL_INSN_SIZE);
+ if (cfi_bhi)
+ cfi_fineibt_bhi_preamble(addr, arity);
}

return 0;
@@ -1349,7 +1392,7 @@ static void cfi_rewrite_endbr(s32 *start
for (s = start; s < end; s++) {
void *addr = (void *)s + *s;

- if (!is_endbr(addr + 16))
+ if (!exact_endbr(addr + 16))
continue;

poison_endbr(addr + 16);