KVM: arm64: Defer EL2 stage-1 mapping on share

We currently blindly map into EL2 stage-1 *any* page passed to the
__pkvm_host_share_hyp() HVC. This is less than ideal from a security
perspective as it makes exploitation of potential hypervisor gadgets
easier than it should be. But interestingly, pKVM should never need to
access SHARED_BORROWED pages that it hasn't previously pinned, so there
is no need to map the page before that.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20250416152648.2982950-7-qperret@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Quentin Perret

arch/arm64/kvm/hyp/nvhe/mem_protect.c

@@ -693,7 +693,6 @@ static int __guest_check_page_state_range(struct pkvm_hyp_vcpu *vcpu, u64 addr,
 int __pkvm_host_share_hyp(u64 pfn)
 {
 	u64 phys = hyp_pfn_to_phys(pfn);
-	void *virt = __hyp_va(phys);
 	u64 size = PAGE_SIZE;
 	int ret;
 
@@ -710,7 +709,6 @@ int __pkvm_host_share_hyp(u64 pfn)
 	}
 
 	__hyp_set_page_state_range(phys, size, PKVM_PAGE_SHARED_BORROWED);
-	WARN_ON(pkvm_create_mappings_locked(virt, virt + size, PAGE_HYP));
 	WARN_ON(__host_set_page_state_range(phys, size, PKVM_PAGE_SHARED_OWNED));
 
 unlock:
@@ -742,7 +740,6 @@ int __pkvm_host_unshare_hyp(u64 pfn)
 	}
 
 	__hyp_set_page_state_range(phys, size, PKVM_NOPAGE);
-	WARN_ON(kvm_pgtable_hyp_unmap(&pkvm_pgtable, virt, size) != size);
 	WARN_ON(__host_set_page_state_range(phys, size, PKVM_PAGE_OWNED));
 
 unlock:
@@ -818,6 +815,7 @@ int hyp_pin_shared_mem(void *from, void *to)
 	u64 end = PAGE_ALIGN((u64)to);
 	u64 phys = __hyp_pa(start);
 	u64 size = end - start;
+	struct hyp_page *p;
 	int ret;
 
 	host_lock_component();
@@ -831,8 +829,14 @@ int hyp_pin_shared_mem(void *from, void *to)
 	if (ret)
 		goto unlock;
 
-	for (cur = start; cur < end; cur += PAGE_SIZE)
-		hyp_page_ref_inc(hyp_virt_to_page(cur));
+	for (cur = start; cur < end; cur += PAGE_SIZE) {
+		p = hyp_virt_to_page(cur);
+		hyp_page_ref_inc(p);
+		if (p->refcount == 1)
+			WARN_ON(pkvm_create_mappings_locked((void *)cur,
+							    (void *)cur + PAGE_SIZE,
+							    PAGE_HYP));
+	}
 
 unlock:
 	hyp_unlock_component();
@@ -845,12 +849,17 @@ void hyp_unpin_shared_mem(void *from, void *to)
 {
 	u64 cur, start = ALIGN_DOWN((u64)from, PAGE_SIZE);
 	u64 end = PAGE_ALIGN((u64)to);
+	struct hyp_page *p;
 
 	host_lock_component();
 	hyp_lock_component();
 
-	for (cur = start; cur < end; cur += PAGE_SIZE)
-		hyp_page_ref_dec(hyp_virt_to_page(cur));
+	for (cur = start; cur < end; cur += PAGE_SIZE) {
+		p = hyp_virt_to_page(cur);
+		if (p->refcount == 1)
+			WARN_ON(kvm_pgtable_hyp_unmap(&pkvm_pgtable, cur, PAGE_SIZE) != PAGE_SIZE);
+		hyp_page_ref_dec(p);
+	}
 
 	hyp_unlock_component();
 	host_unlock_component();