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Message-ID: <20250224124200.371942555@infradead.org>
Date: Mon, 24 Feb 2025 13:37:08 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
peterz@...radead.org,
alyssa.milburn@...el.com,
scott.d.constable@...el.com,
joao@...rdrivepizza.com,
andrew.cooper3@...rix.com,
jpoimboe@...nel.org,
jose.marchesi@...cle.com,
hjl.tools@...il.com,
ndesaulniers@...gle.com,
samitolvanen@...gle.com,
nathan@...nel.org,
ojeda@...nel.org,
kees@...nel.org,
alexei.starovoitov@...il.com,
mhiramat@...nel.org,
jmill@....edu
Subject: [PATCH v4 05/10] x86/ibt: Optimize FineIBT sequence
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@...el.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Kees Cook <kees@...nel.org>
---
arch/x86/kernel/alternative.c | 61 +++++++++++++++++++++++++++---------------
arch/x86/net/bpf_jit_comp.c | 5 +--
2 files changed, 42 insertions(+), 24 deletions(-)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1053,9 +1053,9 @@ 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
@@ -1067,37 +1067,50 @@ 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)
+ *
+ * Note that the JNE target is the 0xEA byte inside the SUB, this decodes as
+ * (bad) on x86_64 and raises #UD.
+ */
+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"
@@ -1428,15 +1441,15 @@ 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))
@@ -1447,6 +1460,12 @@ 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;
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