Merge tag 'x86-core-2024-09-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 core update from Thomas Gleixner:
 "Enable UBSAN traps for x86, which provides better reporting through
  metadata encodeded into UD1"

* tag 'x86-core-2024-09-17' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/traps: Enable UBSAN traps on x86

arch/x86/include/asm/bug.h

@@ -13,6 +13,18 @@
 #define INSN_UD2	0x0b0f
 #define LEN_UD2		2
 
+/*
+ * In clang we have UD1s reporting UBSAN failures on X86, 64 and 32bit.
+ */
+#define INSN_ASOP		0x67
+#define OPCODE_ESCAPE		0x0f
+#define SECOND_BYTE_OPCODE_UD1	0xb9
+#define SECOND_BYTE_OPCODE_UD2	0x0b
+
+#define BUG_NONE		0xffff
+#define BUG_UD1			0xfffe
+#define BUG_UD2			0xfffd
+
 #ifdef CONFIG_GENERIC_BUG
 
 #ifdef CONFIG_X86_32

arch/x86/kernel/traps.c

@@ -42,6 +42,7 @@
 #include <linux/hardirq.h>
 #include <linux/atomic.h>
 #include <linux/iommu.h>
+#include <linux/ubsan.h>
 
 #include <asm/stacktrace.h>
 #include <asm/processor.h>
@@ -91,6 +92,47 @@ __always_inline int is_valid_bugaddr(unsigned long addr)
 	return *(unsigned short *)addr == INSN_UD2;
 }
 
+/*
+ * 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.
+ */
+__always_inline int decode_bug(unsigned long addr, u32 *imm)
+{
+	u8 v;
+
+	if (addr < TASK_SIZE_MAX)
+		return BUG_NONE;
+
+	v = *(u8 *)(addr++);
+	if (v == INSN_ASOP)
+		v = *(u8 *)(addr++);
+	if (v != OPCODE_ESCAPE)
+		return BUG_NONE;
+
+	v = *(u8 *)(addr++);
+	if (v == SECOND_BYTE_OPCODE_UD2)
+		return BUG_UD2;
+
+	if (!IS_ENABLED(CONFIG_UBSAN_TRAP) || 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));
+
+	return BUG_UD1;
+}
+
+
 static nokprobe_inline int
 do_trap_no_signal(struct task_struct *tsk, int trapnr, const char *str,
 		  struct pt_regs *regs,	long error_code)
@@ -216,6 +258,8 @@ 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;
 
 	/*
 	 * Normally @regs are unpoisoned by irqentry_enter(), but handle_bug()
@@ -223,7 +267,8 @@ static noinstr bool handle_bug(struct pt_regs *regs)
 	 * irqentry_enter().
 	 */
 	kmsan_unpoison_entry_regs(regs);
-	if (!is_valid_bugaddr(regs->ip))
+	ud_type = decode_bug(regs->ip, &imm);
+	if (ud_type == BUG_NONE)
 		return handled;
 
 	/*
@@ -236,10 +281,14 @@ static noinstr bool handle_bug(struct pt_regs *regs)
 	 */
 	if (regs->flags & X86_EFLAGS_IF)
 		raw_local_irq_enable();
-	if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
-	    handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
-		regs->ip += LEN_UD2;
-		handled = true;
+	if (ud_type == BUG_UD2) {
+		if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN ||
+		    handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
+			regs->ip += LEN_UD2;
+			handled = true;
+		}
+	} else if (IS_ENABLED(CONFIG_UBSAN_TRAP)) {
+		pr_crit("%s at %pS\n", report_ubsan_failure(regs, imm), (void *)regs->ip);
 	}
 	if (regs->flags & X86_EFLAGS_IF)
 		raw_local_irq_disable();

include/linux/ubsan.h

@@ -4,6 +4,11 @@
 
 #ifdef CONFIG_UBSAN_TRAP
 const char *report_ubsan_failure(struct pt_regs *regs, u32 check_type);
+#else
+static inline const char *report_ubsan_failure(struct pt_regs *regs, u32 check_type)
+{
+	return NULL;
+}
 #endif
 
 #endif

lib/Kconfig.ubsan

@@ -29,8 +29,8 @@ config UBSAN_TRAP
 
 	  Also note that selecting Y will cause your kernel to Oops
 	  with an "illegal instruction" error with no further details
-	  when a UBSAN violation occurs. (Except on arm64, which will
-	  report which Sanitizer failed.) This may make it hard to
+	  when a UBSAN violation occurs. (Except on arm64 and x86, which
+	  will report which Sanitizer failed.) This may make it hard to
 	  determine whether an Oops was caused by UBSAN or to figure
 	  out the details of a UBSAN violation. It makes the kernel log
 	  output less useful for bug reports.