[PATCH 5/6] KVM: arm64: Defer EL2 stage-1 mapping on share

[Quentin Perret <qperret@google.com>]

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 pages that it hasn't previously pinned, so there is no need to
map the page before that.

Signed-off-by: Quentin Perret <qperret@google.com>
---
 arch/arm64/kvm/hyp/nvhe/mem_protect.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 3ab8c81500c2..ae39d74be1f2 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -686,7 +686,6 @@ 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;
 
@@ -703,7 +702,6 @@ 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:
@@ -735,7 +733,6 @@ 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:
@@ -811,6 +808,7 @@ 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();
@@ -824,8 +822,14 @@ 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();
@@ -838,12 +842,17 @@ 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();
-- 
2.48.1.658.g4767266eb4-goog