KVM: Validate hva in kvm_gpc_activate_hva() to fix __kvm_gpc_refresh() WARN

Check that the virtual address is "ok" when activating a gfn_to_pfn_cache
with a host VA to ensure that KVM never attempts to use a bad address.

This fixes a bug where KVM fails to check the incoming address when
handling KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO_HVA in kvm_xen_vcpu_set_attr().

Reported-by: syzbot+fd555292a1da3180fc82@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=fd555292a1da3180fc82
Tested-by: syzbot+fd555292a1da3180fc82@syzkaller.appspotmail.com
Signed-off-by: Pei Li <peili.dev@gmail.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/r/20240627-bug5-v2-1-2c63f7ee6739@gmail.com
[sean: rewrite changelog with --verbose]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: peilii

arch/x86/kvm/xen.c

@@ -741,7 +741,7 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 		} else {
 			void __user * hva = u64_to_user_ptr(data->u.shared_info.hva);
 
-			if (!PAGE_ALIGNED(hva) || !access_ok(hva, PAGE_SIZE)) {
+			if (!PAGE_ALIGNED(hva)) {
 				r = -EINVAL;
 			} else if (!hva) {
 				kvm_gpc_deactivate(&kvm->arch.xen.shinfo_cache);

virt/kvm/pfncache.c

@@ -430,6 +430,9 @@ int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
 
 int kvm_gpc_activate_hva(struct gfn_to_pfn_cache *gpc, unsigned long uhva, unsigned long len)
 {
+	if (!access_ok((void __user *)uhva, len))
+		return -EINVAL;
+
 	return __kvm_gpc_activate(gpc, INVALID_GPA, uhva, len);
 }