x86/traps: Cleanup and robustify decode_bug()

Notably, don't attempt to decode an immediate when MOD == 3.

Additionally have it return the instruction length, such that WARN
like bugs can more reliably skip to the correct instruction.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20250207122546.721120726@infradead.org

3 files changed, 65 insertions, 26 deletions

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index e85ac0c7c039..1a5e4b372694 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -22,8 +22,9 @@
 #define SECOND_BYTE_OPCODE_UD2	0x0b
 
 #define BUG_NONE		0xffff
-#define BUG_UD1			0xfffe
-#define BUG_UD2			0xfffd
+#define BUG_UD2			0xfffe
+#define BUG_UD1			0xfffd
+#define BUG_UD1_UBSAN		0xfffc
 
 #ifdef CONFIG_GENERIC_BUG
 
diff --git a/arch/x86/include/asm/ibt.h b/arch/x86/include/asm/ibt.h
index d955e0d1cbf1..f0ca5c0b1a6b 100644
--- a/arch/x86/include/asm/ibt.h
+++ b/arch/x86/include/asm/ibt.h
@@ -41,7 +41,7 @@
 	_ASM_PTR fname "\n\t"				\
 	".popsection\n\t"
 
-static inline __attribute_const__ u32 gen_endbr(void)
+static __always_inline __attribute_const__ u32 gen_endbr(void)
 {
 	u32 endbr;
 
@@ -56,7 +56,7 @@ static inline __attribute_const__ u32 gen_endbr(void)
 	return endbr;
 }
 
-static inline __attribute_const__ u32 gen_endbr_poison(void)
+static __always_inline __attribute_const__ u32 gen_endbr_poison(void)
 {
 	/*
 	 * 4 byte NOP that isn't NOP4 (in fact it is OSP NOP3), such that it
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 2dbadf347b5f..05b86c05e446 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -94,10 +94,17 @@ __always_inline int is_valid_bugaddr(unsigned long addr)
 
 /*
  * Check for UD1 or UD2, accounting for Address Size Override Prefixes.
- * If it's a UD1, get the ModRM byte to pass along to UBSan.
+ * If it's a UD1, further decode to determine its use:
+ *
+ * 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
+ *
+ * Notably UBSAN uses EAX, static_call uses ECX.
  */
-__always_inline int decode_bug(unsigned long addr, u32 *imm)
+__always_inline int decode_bug(unsigned long addr, s32 *imm, int *len)
 {
+	unsigned long start = addr;
 	u8 v;
 
 	if (addr < TASK_SIZE_MAX)
@@ -110,24 +117,42 @@ __always_inline int decode_bug(unsigned long addr, u32 *imm)
 		return BUG_NONE;
 
 	v = *(u8 *)(addr++);
-	if (v == SECOND_BYTE_OPCODE_UD2)
+	if (v == SECOND_BYTE_OPCODE_UD2) {
+		*len = addr - start;
 		return BUG_UD2;
+	}
 
-	if (!IS_ENABLED(CONFIG_UBSAN_TRAP) || v != SECOND_BYTE_OPCODE_UD1)
+	if (v != SECOND_BYTE_OPCODE_UD1)
 		return BUG_NONE;
 
-	/* Retrieve the immediate (type value) for the UBSAN UD1 */
-	v = *(u8 *)(addr++);
-	if (X86_MODRM_RM(v) == 4)
-		addr++;
-
 	*imm = 0;
-	if (X86_MODRM_MOD(v) == 1)
-		*imm = *(u8 *)addr;
-	else if (X86_MODRM_MOD(v) == 2)
-		*imm = *(u32 *)addr;
-	else
-		WARN_ONCE(1, "Unexpected MODRM_MOD: %u\n", X86_MODRM_MOD(v));
+	v = *(u8 *)(addr++);		/* ModRM */
+
+	if (X86_MODRM_MOD(v) != 3 && X86_MODRM_RM(v) == 4)
+		addr++;			/* SIB */
+
+	/* Decode immediate, if present */
+	switch (X86_MODRM_MOD(v)) {
+	case 0: if (X86_MODRM_RM(v) == 5)
+			addr += 4; /* RIP + disp32 */
+		break;
+
+	case 1: *imm = *(s8 *)addr;
+		addr += 1;
+		break;
+
+	case 2: *imm = *(s32 *)addr;
+		addr += 4;
+		break;
+
+	case 3: break;
+	}
+
+	/* record instruction length */
+	*len = addr - start;
+
+	if (X86_MODRM_REG(v) == 0)	/* EAX */
+		return BUG_UD1_UBSAN;
 
 	return BUG_UD1;
 }
@@ -258,10 +283,10 @@ static inline void handle_invalid_op(struct pt_regs *regs)
 static noinstr bool handle_bug(struct pt_regs *regs)
 {
 	bool handled = false;
-	int ud_type;
-	u32 imm;
+	int ud_type, ud_len;
+	s32 ud_imm;
 
-	ud_type = decode_bug(regs->ip, &imm);
+	ud_type = decode_bug(regs->ip, &ud_imm, &ud_len);
 	if (ud_type == BUG_NONE)
 		return handled;
 
@@ -281,15 +306,28 @@ static noinstr bool handle_bug(struct pt_regs *regs)
 	 */
 	if (regs->flags & X86_EFLAGS_IF)
 		raw_local_irq_enable();
-	if (ud_type == BUG_UD2) {
+
+	switch (ud_type) {
+	case BUG_UD2:
 		if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
 		    handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
-			regs->ip += LEN_UD2;
+			regs->ip += ud_len;
 			handled = true;
 		}
-	} else if (IS_ENABLED(CONFIG_UBSAN_TRAP)) {
-		pr_crit("%s at %pS\n", report_ubsan_failure(regs, imm), (void *)regs->ip);
+		break;
+
+	case BUG_UD1_UBSAN:
+		if (IS_ENABLED(CONFIG_UBSAN_TRAP)) {
+			pr_crit("%s at %pS\n",
+				report_ubsan_failure(regs, ud_imm),
+				(void *)regs->ip);
+		}
+		break;
+
+	default:
+		break;
 	}
+
 	if (regs->flags & X86_EFLAGS_IF)
 		raw_local_irq_disable();
 	instrumentation_end();