[tip: x86/core] x86/traps: Decode 0xEA instructions as #UD

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

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

Commit-ID:     2e044911be75ce3321c5b3d10205ac0b54f8cb92
Gitweb:        https://git.kernel.org/tip/2e044911be75ce3321c5b3d10205ac0b54f8cb92
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 24 Feb 2025 13:37:06 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Wed, 26 Feb 2025 12:22:10 +01:00

x86/traps: Decode 0xEA instructions as #UD

FineIBT will start using 0xEA as #UD. Normally '0xEA' is a 'bad',
invalid instruction for the CPU.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Kees Cook <kees@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20250224124200.166774696@xxxxxxxxxxxxx
---
 arch/x86/include/asm/bug.h |  1 +
 arch/x86/kernel/traps.c    | 20 +++++++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index 1a5e4b3..bc8a2ca 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -25,6 +25,7 @@
 #define BUG_UD2			0xfffe
 #define BUG_UD1			0xfffd
 #define BUG_UD1_UBSAN		0xfffc
+#define BUG_EA			0xffea
 
 #ifdef CONFIG_GENERIC_BUG
 
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 05b86c0..a02a51b 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -96,6 +96,7 @@ __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, further decode to determine its use:
  *
+ * FineIBT:      ea                      (bad)
  * 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
@@ -113,6 +114,10 @@ __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) {
+		*len = addr - start;
+		return BUG_EA;
+	}
 	if (v != OPCODE_ESCAPE)
 		return BUG_NONE;
 
@@ -309,10 +314,16 @@ static noinstr bool handle_bug(struct pt_regs *regs)
 
 	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 += ud_len;
+		if (report_bug(regs->ip, regs) == BUG_TRAP_TYPE_WARN) {
+			handled = true;
+			break;
+		}
+		fallthrough;
+
+	case BUG_EA:
+		if (handle_cfi_failure(regs) == BUG_TRAP_TYPE_WARN) {
 			handled = true;
+			break;
 		}
 		break;
 
@@ -328,6 +339,9 @@ static noinstr bool handle_bug(struct pt_regs *regs)
 		break;
 	}
 
+	if (handled)
+		regs->ip += ud_len;
+
 	if (regs->flags & X86_EFLAGS_IF)
 		raw_local_irq_disable();
 	instrumentation_end();