KVM: arm64: nv: Respect stage-2 write permssion when setting stage-1 AF

Naturally, updating the Access Flag in a stage-1 descriptor requires
write permission at stage-2, although this isn't actually enforced in
KVM's software PTW.

Generate a stage-2 permission fault if the stage-1 walk attempts to
update the descriptor and its corresponding stage-2 translation lacks
write permission.

Fixes: bff8aa213dee ("KVM: arm64: Implement HW access flag management in stage-1 SW PTW")
Reviewed-by: Marc Zyngier <[e-mail]>
Link: https://msgid.link/[e-mail]
Signed-off-by: Oliver Upton <[e-mail]>

arch/arm64/kvm/at.c

···
 		   struct s1_walk_result *wr, u64 va)
 {
 	u64 va_top, va_bottom, baddr, desc, new_desc, ipa;
+	struct kvm_s2_trans s2_trans = {};
 	int level, stride, ret;
 
 	level = wi->sl;
···
 		ipa = baddr | index;
 
 		if (wi->s2) {
-			struct kvm_s2_trans s2_trans = {};
-
 			ret = kvm_walk_nested_s2(vcpu, ipa, &s2_trans);
 			if (ret) {
 				fail_s1_walk(wr,
···
 		new_desc |= PTE_AF;
 
 	if (new_desc != desc) {
+		if (wi->s2 && !kvm_s2_trans_writable(&s2_trans)) {
+			fail_s1_walk(wr, ESR_ELx_FSC_PERM_L(level), true);
+			return -EPERM;
+		}
+
 		ret = kvm_swap_s1_desc(vcpu, ipa, desc, new_desc, wi);
 		if (ret)
 			return ret;