x86/ibt: Handle FineIBT in handle_cfi_failure()

Sami reminded me that FineIBT failure does not hook into the regular
CFI failure case, and as such CFI_PERMISSIVE does not work.

Reported-by: Sami Tolvanen <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Sami Tolvanen <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/include/asm/cfi.h

···
 
 extern u32 cfi_get_func_hash(void *func);
 
+#ifdef CONFIG_FINEIBT
+extern bool decode_fineibt_insn(struct pt_regs *regs, unsigned long *target, u32 *type);
+#else
+static inline bool
+decode_fineibt_insn(struct pt_regs *regs, unsigned long *target, u32 *type)
+{
+	return false;
+}
+
+#endif
+
 #else
 static inline enum bug_trap_type handle_cfi_failure(struct pt_regs *regs)
 {

arch/x86/kernel/alternative.c

···
 	"	endbr64				\n"
 	"	subl	$0x12345678, %r10d	\n"
 	"	je	fineibt_preamble_end	\n"
+	"fineibt_preamble_ud2:			\n"
 	"	ud2				\n"
 	"	nop				\n"
 	"fineibt_preamble_end:			\n"
···
 );
 
 extern u8 fineibt_preamble_start[];
+extern u8 fineibt_preamble_ud2[];
 extern u8 fineibt_preamble_end[];
 
 #define fineibt_preamble_size (fineibt_preamble_end - fineibt_preamble_start)
+#define fineibt_preamble_ud2  (fineibt_preamble_ud2 - fineibt_preamble_start)
 #define fineibt_preamble_hash 7
 
 asm(	".pushsection .rodata			\n"
···
 	default:
 		break;
 	}
+}
+
+/*
+ * regs->ip points to a UD2 instruction, return true and fill out target and
+ * type when this UD2 is from a FineIBT preamble.
+ *
+ * We check the preamble by checking for the ENDBR instruction relative to the
+ * UD2 instruction.
+ */
+bool decode_fineibt_insn(struct pt_regs *regs, unsigned long *target, u32 *type)
+{
+	unsigned long addr = regs->ip - fineibt_preamble_ud2;
+	u32 endbr, hash;
+
+	__get_kernel_nofault(&endbr, addr, u32, Efault);
+	if (endbr != gen_endbr())
+		return false;
+
+	*target = addr + fineibt_preamble_size;
+
+	__get_kernel_nofault(&hash, addr + fineibt_preamble_hash, u32, Efault);
+	*type = (u32)regs->r10 + hash;
+
+	return true;
+
+Efault:
+	return false;
 }
 
 #else

arch/x86/kernel/cfi.c

···
 	unsigned long target;
 	u32 type;
 
-	if (!is_cfi_trap(regs->ip))
-		return BUG_TRAP_TYPE_NONE;
+	switch (cfi_mode) {
+	case CFI_KCFI:
+		if (!is_cfi_trap(regs->ip))
+			return BUG_TRAP_TYPE_NONE;
 
-	if (!decode_cfi_insn(regs, &target, &type))
-		return report_cfi_failure_noaddr(regs, regs->ip);
+		if (!decode_cfi_insn(regs, &target, &type))
+			return report_cfi_failure_noaddr(regs, regs->ip);
+
+		break;
+
+	case CFI_FINEIBT:
+		if (!decode_fineibt_insn(regs, &target, &type))
+			return BUG_TRAP_TYPE_NONE;
+
+		break;
+
+	default:
+		return BUG_TRAP_TYPE_NONE;
+	}
 
 	return report_cfi_failure(regs, regs->ip, &target, type);
 }