[PATCH v4 06/10] x86/traps: Decode LOCK Jcc.d8 #UD

Peter Zijlstra
2025-02-24T12:46:00Z

Because overlapping code sequences are all the rage.

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

--- 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

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -97,6 +97,7 @@ __always_inline int is_valid_bugaddr(uns
* If it's a UD1, further decode to determine its use:
*
* FineIBT: ea (bad)
+ * FineIBT: 0f 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(uns
__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
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
fallthrough;

case BUG_EA:
+ case BUG_LOCK:
if (handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
handled = true;
break;