[PATCH v5] x86/traps: Enable UBSAN traps on x86

[Gatlin Newhouse <gatlin.newhouse@gmail.com>]

Currently ARM architectures extract which specific sanitizer
has caused a trap via encoded data in the trap instruction.[1]
Clang on x86 currently encodes the same data in ud1 instructions
but the x86 handle_bug() and is_valid_bugaddr() functions
currently only look at ud2s.

Bring x86 to parity with arm64, similar to commit 25b84002afb9
("arm64: Support Clang UBSAN trap codes for better reporting").
Enable the reporting of UBSAN sanitizer detail on x86 architectures
compiled with clang when CONFIG_UBSAN_TRAP=y.

[1] Details are in llvm/lib/Target/X86/X86MCInstLower.cpp. See:
https://github.com/llvm/llvm-project/commit/c5978f42ec8e9#diff-bb68d7cd885f41cfc35843998b0f9f534adb60b415f647109e597ce448e92d9f

EmitAndCountInstruction() uses the UD1Lm template, which uses a
OpSize32. See:
https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/X86/X86InstrSystem.td#L27

Signed-off-by: Gatlin Newhouse <gatlin.newhouse@gmail.com>
---
Changes in v5:
  - Added references to the LLVM commits in the commit message from
    Kees and Marco's feedback
  - Renamed incorrect defines, and removed handle_ubsan_failure()'s
    duplicated work per Peter's feedback

Changes in v4:
  - Implement Peter's suggestions for decode_bug(), and fix
    inconsistent capitalization in hex values.

Changes in v3:
  - Address Thomas's remarks about: change log structure,
    get_ud_type() instead of is_valid_bugaddr(), handle_bug()
    changes, and handle_ubsan_failure().

Changes in v2:
  - Name the new constants 'LEN_ASOP' and 'INSN_ASOP' instead of
    'LEN_REX' and 'INSN_REX'
  - Change handle_ubsan_failure() from enum bug_trap_type to void
    function

v1: https://lore.kernel.org/linux-hardening/20240529022043.3661757-1-gatlin.newhouse@gmail.com/
v2: https://lore.kernel.org/linux-hardening/20240601031019.3708758-1-gatlin.newhouse@gmail.com/
v3: https://lore.kernel.org/linux-hardening/20240625032509.4155839-1-gatlin.newhouse@gmail.com/
v4: https://lore.kernel.org/linux-hardening/20240710203250.238782-1-gatlin.newhouse@gmail.com/
---
 MAINTAINERS                  |  2 ++
 arch/x86/include/asm/bug.h   | 12 ++++++++
 arch/x86/include/asm/ubsan.h | 18 ++++++++++++
 arch/x86/kernel/Makefile     |  1 +
 arch/x86/kernel/traps.c      | 57 ++++++++++++++++++++++++++++++++----
 arch/x86/kernel/ubsan.c      | 19 ++++++++++++
 6 files changed, 104 insertions(+), 5 deletions(-)
 create mode 100644 arch/x86/include/asm/ubsan.h
 create mode 100644 arch/x86/kernel/ubsan.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 28e20975c26f..b8512887ffb1 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -22635,6 +22635,8 @@ L:	kasan-dev@googlegroups.com
 L:	linux-hardening@vger.kernel.org
 S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git for-next/hardening
+F:	arch/x86/include/asm/ubsan.h
+F:	arch/x86/kernel/ubsan.c
 F:	Documentation/dev-tools/ubsan.rst
 F:	include/linux/ubsan.h
 F:	lib/Kconfig.ubsan
diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index a3ec87d198ac..751e45ea27ca 100644
--- a/arch/x86/include/asm/bug.h
+++ b/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
diff --git a/arch/x86/include/asm/ubsan.h b/arch/x86/include/asm/ubsan.h
new file mode 100644
index 000000000000..1d7c2b4129de
--- /dev/null
+++ b/arch/x86/include/asm/ubsan.h
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_UBSAN_H
+#define _ASM_X86_UBSAN_H
+
+/*
+ * Clang Undefined Behavior Sanitizer trap mode support.
+ */
+#include <linux/bug.h>
+#include <linux/ubsan.h>
+#include <asm/ptrace.h>
+
+#ifdef CONFIG_UBSAN_TRAP
+void handle_ubsan_failure(struct pt_regs *regs, u32 type);
+#else
+static inline void handle_ubsan_failure(struct pt_regs *regs, u32 type) { return; }
+#endif /* CONFIG_UBSAN_TRAP */
+
+#endif /* _ASM_X86_UBSAN_H */
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 74077694da7d..fe1d9db27500 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -145,6 +145,7 @@ obj-$(CONFIG_UNWINDER_GUESS)		+= unwind_guess.o
 obj-$(CONFIG_AMD_MEM_ENCRYPT)		+= sev.o
 
 obj-$(CONFIG_CFI_CLANG)			+= cfi.o
+obj-$(CONFIG_UBSAN_TRAP)		+= ubsan.o
 
 obj-$(CONFIG_CALL_THUNKS)		+= callthunks.o
 
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 4fa0b17e5043..6350d00a6555 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -67,6 +67,7 @@
 #include <asm/vdso.h>
 #include <asm/tdx.h>
 #include <asm/cfi.h>
+#include <asm/ubsan.h>
 
 #ifdef CONFIG_X86_64
 #include <asm/x86_init.h>
@@ -91,6 +92,45 @@ __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 (v != SECOND_BYTE_OPCODE_UD1)
+		return BUG_NONE;
+
+	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 +256,8 @@ 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 +265,8 @@ 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 +279,14 @@ 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 {
+		handle_ubsan_failure(regs, imm);
 	}
 	if (regs->flags & X86_EFLAGS_IF)
 		raw_local_irq_disable();
diff --git a/arch/x86/kernel/ubsan.c b/arch/x86/kernel/ubsan.c
new file mode 100644
index 000000000000..63f819928820
--- /dev/null
+++ b/arch/x86/kernel/ubsan.c
@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Clang Undefined Behavior Sanitizer trap mode support.
+ */
+#include <linux/bug.h>
+#include <linux/string.h>
+#include <linux/printk.h>
+#include <linux/ubsan.h>
+#include <asm/ptrace.h>
+#include <asm/ubsan.h>
+
+/*
+ * Checks for the information embedded in the UD1 trap instruction
+ * for the UB Sanitizer in order to pass along debugging output.
+ */
+void handle_ubsan_failure(struct pt_regs *regs, u32 type)
+{
+	pr_crit("%s at %pS\n", report_ubsan_failure(regs, type), (void *)regs->ip);
+}
-- 
2.25.1