KVM: arm64: Inject UNDEF when accessing MTE sysregs with MTE disabled

When MTE hardware is present but disabled via software (`arm64.nomte` or
`CONFIG_ARM64_MTE=n`), the kernel clears `HCR_EL2.ATA` and sets
`HCR_EL2.TID5`, to prevent the use of MTE instructions.

Additionally, accesses to certain MTE system registers trap to EL2 with
exception class ESR_ELx_EC_SYS64. To emulate hardware without MTE (where
such accesses would cause an Undefined Instruction exception), inject
UNDEF into the host.

Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260122112218.531948-4-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Fuad Tabba

arch/arm64/kvm/hyp/nvhe/hyp-main.c

@@ -687,6 +687,69 @@ static void handle_host_smc(struct kvm_cpu_context *host_ctxt)
 	kvm_skip_host_instr();
 }
 
+/*
+ * Inject an Undefined Instruction exception into the host.
+ *
+ * This is open-coded to allow control over PSTATE construction without
+ * complicating the generic exception entry helpers.
+ */
+static void inject_undef64(void)
+{
+	u64 spsr_mask, vbar, sctlr, old_spsr, new_spsr, esr, offset;
+
+	spsr_mask = PSR_N_BIT | PSR_Z_BIT | PSR_C_BIT | PSR_V_BIT | PSR_DIT_BIT | PSR_PAN_BIT;
+
+	vbar = read_sysreg_el1(SYS_VBAR);
+	sctlr = read_sysreg_el1(SYS_SCTLR);
+	old_spsr = read_sysreg_el2(SYS_SPSR);
+
+	new_spsr = old_spsr & spsr_mask;
+	new_spsr |= PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT;
+	new_spsr |= PSR_MODE_EL1h;
+
+	if (!(sctlr & SCTLR_EL1_SPAN))
+		new_spsr |= PSR_PAN_BIT;
+
+	if (sctlr & SCTLR_ELx_DSSBS)
+		new_spsr |= PSR_SSBS_BIT;
+
+	if (system_supports_mte())
+		new_spsr |= PSR_TCO_BIT;
+
+	esr = (ESR_ELx_EC_UNKNOWN << ESR_ELx_EC_SHIFT) | ESR_ELx_IL;
+	offset = CURRENT_EL_SP_ELx_VECTOR + except_type_sync;
+
+	write_sysreg_el1(esr, SYS_ESR);
+	write_sysreg_el1(read_sysreg_el2(SYS_ELR), SYS_ELR);
+	write_sysreg_el1(old_spsr, SYS_SPSR);
+	write_sysreg_el2(vbar + offset, SYS_ELR);
+	write_sysreg_el2(new_spsr, SYS_SPSR);
+}
+
+static bool handle_host_mte(u64 esr)
+{
+	switch (esr_sys64_to_sysreg(esr)) {
+	case SYS_RGSR_EL1:
+	case SYS_GCR_EL1:
+	case SYS_TFSR_EL1:
+	case SYS_TFSRE0_EL1:
+		/* If we're here for any reason other than MTE, it's a bug. */
+		if (read_sysreg(HCR_EL2) & HCR_ATA)
+			return false;
+		break;
+	case SYS_GMID_EL1:
+		/* If we're here for any reason other than MTE, it's a bug. */
+		if (!(read_sysreg(HCR_EL2) & HCR_TID5))
+			return false;
+		break;
+	default:
+		return false;
+	}
+
+	inject_undef64();
+	return true;
+}
+
 void handle_trap(struct kvm_cpu_context *host_ctxt)
 {
 	u64 esr = read_sysreg_el2(SYS_ESR);
@@ -702,6 +765,10 @@ void handle_trap(struct kvm_cpu_context *host_ctxt)
 	case ESR_ELx_EC_DABT_LOW:
 		handle_host_mem_abort(host_ctxt);
 		break;
+	case ESR_ELx_EC_SYS64:
+		if (handle_host_mte(esr))
+			break;
+		fallthrough;
 	default:
 		BUG();
 	}