KVM: arm64: Prevent userspace from disabling AArch64 support at any virtualisable EL

A sorry excuse for a selftest is trying to disable AArch64 support.
And yes, this goes as well as you can imagine.

Let's forbid this sort of things. Normal userspace shouldn't get
caught doing that.

Signed-off-by: Marc Zyngier <[email protected]>
Reviewed-by: Ganapatrao Kulkarni <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Oliver Upton <[email protected]>

Author: Marc Zyngier

arch/arm64/kvm/sys_regs.c

@@ -1945,6 +1945,12 @@ static int set_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
 	if ((hw_val & mpam_mask) == (user_val & mpam_mask))
 		user_val &= ~ID_AA64PFR0_EL1_MPAM_MASK;
 
+	/* Fail the guest's request to disable the AA64 ISA at EL{0,1,2} */
+	if (!FIELD_GET(ID_AA64PFR0_EL1_EL0, user_val) ||
+	    !FIELD_GET(ID_AA64PFR0_EL1_EL1, user_val) ||
+	    (vcpu_has_nv(vcpu) && !FIELD_GET(ID_AA64PFR0_EL1_EL2, user_val)))
+		return -EINVAL;
+
 	return set_id_reg(vcpu, rd, user_val);
 }