[PATCH 13/16] KVM: arm64: Move double-sharing logic into hyp-specific function

[Quentin Perret]

From: Will Deacon <will at kernel.org>

Strictly speaking, double-sharing a page is an invalid transition and
should be rejected, however we allow this in order to simplify the
book-keeping when KVM metadata (such as vcpu structures) co-exists in
the same page.

Given that double-sharing is only required for pages shared with the
hypervisor by the host, move the handling into a hyp-specific function
to check incoming shares, therefore preventing double-sharing outside
of this particular transition.

Signed-off-by: Will Deacon <will at kernel.org>
Signed-off-by: Quentin Perret <qperret at google.com>
---
 arch/arm64/kvm/hyp/nvhe/mem_protect.c | 57 +++++++++++++++++++--------
 1 file changed, 41 insertions(+), 16 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 909e60f71b06..3378117d010c 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -536,6 +536,33 @@ static int ack_share(struct pkvm_page_share_ack *ack,
 	}
 }
 
+static int hyp_check_incoming_share(struct pkvm_page_req *req,
+				    struct pkvm_page_share_ack *ack,
+				    enum pkvm_component_id initiator,
+				    enum kvm_pgtable_prot prot)
+{
+	/*
+	 * We allow the host to share the same page twice, but that means we
+	 * have to check that the states really do match exactly.
+	 */
+	if (initiator != PKVM_ID_HOST)
+		return -EPERM;
+
+	if (req->initiator.state != PKVM_PAGE_SHARED_OWNED)
+		return -EPERM;
+
+	if (ack->completer.state != PKVM_PAGE_SHARED_BORROWED)
+		return -EPERM;
+
+	if (ack->completer.phys != req->phys)
+		return -EPERM;
+
+	if (ack->completer.prot != prot)
+		return -EPERM;
+
+	return 0;
+}
+
 /*
  * Check that the page states in the initiator and the completer are compatible
  * for the requested page-sharing operation to go ahead.
@@ -544,6 +571,8 @@ static int check_share(struct pkvm_page_req *req,
 		       struct pkvm_page_share_ack *ack,
 		       struct pkvm_mem_share *share)
 {
+	struct pkvm_mem_transition *tx = &share->tx;
+
 	if (!addr_is_memory(req->phys))
 		return -EINVAL;
 
@@ -552,25 +581,22 @@ static int check_share(struct pkvm_page_req *req,
 		return 0;
 	}
 
-	if (req->initiator.state != PKVM_PAGE_SHARED_OWNED)
-		return -EPERM;
-
-	if (ack->completer.state != PKVM_PAGE_SHARED_BORROWED)
-		return -EPERM;
-
-	if (ack->completer.phys != req->phys)
-		return -EPERM;
-
-	if (ack->completer.prot != share->prot)
+	switch (tx->completer.id) {
+	case PKVM_ID_HYP:
+		return hyp_check_incoming_share(req, ack, tx->initiator.id,
+						share->prot);
+	default:
 		return -EPERM;
-
-	return 0;
+	}
 }
 
 static int host_initiate_share(struct pkvm_page_req *req)
 {
 	enum kvm_pgtable_prot prot;
 
+	if (req->initiator.state == PKVM_PAGE_SHARED_OWNED)
+		return 0;
+
 	prot = pkvm_mkstate(PKVM_HOST_MEM_PROT, PKVM_PAGE_SHARED_OWNED);
 	return host_stage2_idmap_locked(req->initiator.addr, PAGE_SIZE, prot);
 }
@@ -595,6 +621,9 @@ static int hyp_complete_share(struct pkvm_page_req *req,
 	void *start = (void *)req->completer.addr, *end = start + PAGE_SIZE;
 	enum kvm_pgtable_prot prot;
 
+	if (req->initiator.state == PKVM_PAGE_SHARED_OWNED)
+		return 0;
+
 	prot = pkvm_mkstate(perms, PKVM_PAGE_SHARED_BORROWED);
 	return pkvm_create_mappings_locked(start, end, prot);
 }
@@ -653,10 +682,6 @@ static int do_share(struct pkvm_mem_share *share)
 		if (ret)
 			break;
 
-		/* Allow double-sharing by skipping over the page */
-		if (req.initiator.state == PKVM_PAGE_SHARED_OWNED)
-			continue;
-
 		ret = initiate_share(&req, share);
 		if (ret)
 			break;
-- 
2.33.0.882.g93a45727a2-goog