[tip: x86/core] x86/traps: Decode LOCK Jcc.d8 #UD

["tip-bot2 for Peter Zijlstra" <tip-bot2@xxxxxxxxxxxxx>]

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     e874854d308841f2383ee3d04f0438f56ddddaaf
Gitweb:        https://git.kernel.org/tip/e874854d308841f2383ee3d04f0438f56ddddaaf
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 24 Feb 2025 13:37:09 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 26 Feb 2025 11:41:55 +01:00

x86/traps: Decode LOCK Jcc.d8 #UD

Because overlapping code sequences are all the rage.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Kees Cook <kees@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20250224124200.486463917@xxxxxxxxxxxxx
---
 arch/x86/include/asm/bug.h |  2 ++
 arch/x86/kernel/traps.c    | 26 +++++++++++++++++++++++---
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index bc8a2ca..f0e9acf 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -17,6 +17,7 @@
  * In clang we have UD1s reporting UBSAN failures on X86, 64 and 32bit.
  */
 #define INSN_ASOP		0x67
+#define INSN_LOCK		0xf0
 #define OPCODE_ESCAPE		0x0f
 #define SECOND_BYTE_OPCODE_UD1	0xb9
 #define SECOND_BYTE_OPCODE_UD2	0x0b
@@ -26,6 +27,7 @@
 #define BUG_UD1			0xfffd
 #define BUG_UD1_UBSAN		0xfffc
 #define BUG_EA			0xffea
+#define BUG_LOCK		0xfff0
 
 #ifdef CONFIG_GENERIC_BUG
 
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index c169f3b..f4263cb 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -97,6 +97,7 @@ __always_inline int is_valid_bugaddr(unsigned long addr)
  * If it's a UD1, further decode to determine its use:
  *
  * FineIBT:      ea                      (bad)
+ * FineIBT:      f0 75 f9                lock jne . - 6
  * UBSan{0}:     67 0f b9 00             ud1    (%eax),%eax
  * UBSan{10}:    67 0f b9 40 10          ud1    0x10(%eax),%eax
  * static_call:  0f b9 cc                ud1    %esp,%ecx
@@ -106,6 +107,7 @@ __always_inline int is_valid_bugaddr(unsigned long addr)
 __always_inline int decode_bug(unsigned long addr, s32 *imm, int *len)
 {
 	unsigned long start = addr;
+	bool lock = false;
 	u8 v;
 
 	if (addr < TASK_SIZE_MAX)
@@ -114,12 +116,29 @@ __always_inline int decode_bug(unsigned long addr, s32 *imm, int *len)
 	v = *(u8 *)(addr++);
 	if (v == INSN_ASOP)
 		v = *(u8 *)(addr++);
-	if (v == 0xea) {
+
+	if (v == INSN_LOCK) {
+		lock = true;
+		v = *(u8 *)(addr++);
+	}
+
+	switch (v) {
+	case 0x70 ... 0x7f: /* Jcc.d8 */
+		addr += 1; /* d8 */
+		*len = addr - start;
+		WARN_ON_ONCE(!lock);
+		return BUG_LOCK;
+
+	case 0xea:
 		*len = addr - start;
 		return BUG_EA;
-	}
-	if (v != OPCODE_ESCAPE)
+
+	case OPCODE_ESCAPE:
+		break;
+
+	default:
 		return BUG_NONE;
+	}
 
 	v = *(u8 *)(addr++);
 	if (v == SECOND_BYTE_OPCODE_UD2) {
@@ -322,6 +341,7 @@ static noinstr bool handle_bug(struct pt_regs *regs)
 		fallthrough;
 
 	case BUG_EA:
+	case BUG_LOCK:
 		if (handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
 			handled = true;
 			break;