KVM: arm64: Implement do_unshare() helper for unsharing memory

Tearing down a previously shared memory region results in the borrower
losing access to the underlying pages and returning them to the "owned"
state in the owner.

Implement a do_unshare() helper, along the same lines as do_share(), to
provide this functionality for the host-to-hyp case.
Reviewed-by: Andrew Walbran <qwandor@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211215161232.1480836-13-qperret@google.com

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

@@ -485,6 +485,16 @@ static int host_request_owned_transition(u64 *completer_addr,
 	return __host_check_page_state_range(addr, size, PKVM_PAGE_OWNED);
 }
 
+static int host_request_unshare(u64 *completer_addr,
+				const struct pkvm_mem_transition *tx)
+{
+	u64 size = tx->nr_pages * PAGE_SIZE;
+	u64 addr = tx->initiator.addr;
+
+	*completer_addr = tx->initiator.host.completer_addr;
+	return __host_check_page_state_range(addr, size, PKVM_PAGE_SHARED_OWNED);
+}
+
 static int host_initiate_share(u64 *completer_addr,
 			       const struct pkvm_mem_transition *tx)
 {
@@ -495,6 +505,16 @@ static int host_initiate_share(u64 *completer_addr,
 	return __host_set_page_state_range(addr, size, PKVM_PAGE_SHARED_OWNED);
 }
 
+static int host_initiate_unshare(u64 *completer_addr,
+				 const struct pkvm_mem_transition *tx)
+{
+	u64 size = tx->nr_pages * PAGE_SIZE;
+	u64 addr = tx->initiator.addr;
+
+	*completer_addr = tx->initiator.host.completer_addr;
+	return __host_set_page_state_range(addr, size, PKVM_PAGE_OWNED);
+}
+
 static enum pkvm_page_state hyp_get_page_state(kvm_pte_t pte)
 {
 	if (!kvm_pte_valid(pte))
@@ -535,6 +555,17 @@ static int hyp_ack_share(u64 addr, const struct pkvm_mem_transition *tx,
 	return __hyp_check_page_state_range(addr, size, PKVM_NOPAGE);
 }
 
+static int hyp_ack_unshare(u64 addr, const struct pkvm_mem_transition *tx)
+{
+	u64 size = tx->nr_pages * PAGE_SIZE;
+
+	if (__hyp_ack_skip_pgtable_check(tx))
+		return 0;
+
+	return __hyp_check_page_state_range(addr, size,
+					    PKVM_PAGE_SHARED_BORROWED);
+}
+
 static int hyp_complete_share(u64 addr, const struct pkvm_mem_transition *tx,
 			      enum kvm_pgtable_prot perms)
 {
@@ -545,6 +576,14 @@ static int hyp_complete_share(u64 addr, const struct pkvm_mem_transition *tx,
 	return pkvm_create_mappings_locked(start, end, prot);
 }
 
+static int hyp_complete_unshare(u64 addr, const struct pkvm_mem_transition *tx)
+{
+	u64 size = tx->nr_pages * PAGE_SIZE;
+	int ret = kvm_pgtable_hyp_unmap(&pkvm_pgtable, addr, size);
+
+	return (ret != size) ? -EFAULT : 0;
+}
+
 static int check_share(struct pkvm_mem_share *share)
 {
 	const struct pkvm_mem_transition *tx = &share->tx;
@@ -621,6 +660,82 @@ static int do_share(struct pkvm_mem_share *share)
 	return WARN_ON(__do_share(share));
 }
 
+static int check_unshare(struct pkvm_mem_share *share)
+{
+	const struct pkvm_mem_transition *tx = &share->tx;
+	u64 completer_addr;
+	int ret;
+
+	switch (tx->initiator.id) {
+	case PKVM_ID_HOST:
+		ret = host_request_unshare(&completer_addr, tx);
+		break;
+	default:
+		ret = -EINVAL;
+	}
+
+	if (ret)
+		return ret;
+
+	switch (tx->completer.id) {
+	case PKVM_ID_HYP:
+		ret = hyp_ack_unshare(completer_addr, tx);
+		break;
+	default:
+		ret = -EINVAL;
+	}
+
+	return ret;
+}
+
+static int __do_unshare(struct pkvm_mem_share *share)
+{
+	const struct pkvm_mem_transition *tx = &share->tx;
+	u64 completer_addr;
+	int ret;
+
+	switch (tx->initiator.id) {
+	case PKVM_ID_HOST:
+		ret = host_initiate_unshare(&completer_addr, tx);
+		break;
+	default:
+		ret = -EINVAL;
+	}
+
+	if (ret)
+		return ret;
+
+	switch (tx->completer.id) {
+	case PKVM_ID_HYP:
+		ret = hyp_complete_unshare(completer_addr, tx);
+		break;
+	default:
+		ret = -EINVAL;
+	}
+
+	return ret;
+}
+
+/*
+ * do_unshare():
+ *
+ * The page owner revokes access from another component for a range of
+ * pages which were previously shared using do_share().
+ *
+ * Initiator: SHARED_OWNED	=> OWNED
+ * Completer: SHARED_BORROWED	=> NOPAGE
+ */
+static int do_unshare(struct pkvm_mem_share *share)
+{
+	int ret;
+
+	ret = check_unshare(share);
+	if (ret)
+		return ret;
+
+	return WARN_ON(__do_unshare(share));
+}
+
 int __pkvm_host_share_hyp(u64 pfn)
 {
 	int ret;