arm64: entry: Allow handling of undefined instructions from EL1

Rather than panic() when taking an undefined instruction exception from
EL1, allow a hook to be registered in case we want to emulate the
instruction, like we will for the SSBS PSTATE manipulation instructions.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/entry.S

@@ -589,7 +589,7 @@ el1_undef:
 	inherit_daif	pstate=x23, tmp=x2
 	mov	x0, sp
 	bl	do_undefinstr
-	ASM_BUG()
+	kernel_exit 1
 el1_dbg:
 	/*
 	 * Debug exception handling

arch/arm64/kernel/traps.c

@@ -310,10 +310,12 @@ static int call_undef_hook(struct pt_regs *regs)
 	int (*fn)(struct pt_regs *regs, u32 instr) = NULL;
 	void __user *pc = (void __user *)instruction_pointer(regs);
 
-	if (!user_mode(regs))
-		return 1;
-
-	if (compat_thumb_mode(regs)) {
+	if (!user_mode(regs)) {
+		__le32 instr_le;
+		if (probe_kernel_address((__force __le32 *)pc, instr_le))
+			goto exit;
+		instr = le32_to_cpu(instr_le);
+	} else if (compat_thumb_mode(regs)) {
 		/* 16-bit Thumb instruction */
 		__le16 instr_le;
 		if (get_user(instr_le, (__le16 __user *)pc))
@@ -407,6 +409,7 @@ asmlinkage void __exception do_undefinstr(struct pt_regs *regs)
 		return;
 
 	force_signal_inject(SIGILL, ILL_ILLOPC, regs->pc);
+	BUG_ON(!user_mode(regs));
 }
 
 void cpu_enable_cache_maint_trap(const struct arm64_cpu_capabilities *__unused)