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Subject[PATCH v4 10/14] KVM: arm64: Implement do_share() helper for sharing memory
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From: Will Deacon <will@kernel.org>

By default, protected KVM isolates memory pages so that they are
accessible only to their owner: be it the host kernel, the hypervisor
at EL2 or (in future) the guest. Establishing shared-memory regions
between these components therefore involves a transition for each page
so that the owner can share memory with a borrower under a certain set
of permissions.

Introduce a do_share() helper for safely sharing a memory region between
two components. Currently, only host-to-hyp sharing is implemented, but
the code is easily extended to handle other combinations and the
permission checks for each component are reusable.

Reviewed-by: Andrew Walbran <qwandor@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
---
arch/arm64/kvm/hyp/nvhe/mem_protect.c | 237 ++++++++++++++++++++++++++
1 file changed, 237 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 907d3cbf1809..666278632fed 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -472,3 +472,240 @@ void handle_host_mem_abort(struct kvm_cpu_context *host_ctxt)
ret = host_stage2_idmap(addr);
BUG_ON(ret && ret != -EAGAIN);
}
+
+/* This corresponds to locking order */
+enum pkvm_component_id {
+ PKVM_ID_HOST,
+ PKVM_ID_HYP,
+};
+
+struct pkvm_mem_transition {
+ u64 nr_pages;
+
+ struct {
+ enum pkvm_component_id id;
+ /* Address in the initiator's address space */
+ u64 addr;
+
+ union {
+ struct {
+ /* Address in the completer's address space */
+ u64 completer_addr;
+ } host;
+ };
+ } initiator;
+
+ struct {
+ enum pkvm_component_id id;
+ } completer;
+};
+
+struct pkvm_mem_share {
+ const struct pkvm_mem_transition tx;
+ const enum kvm_pgtable_prot completer_prot;
+};
+
+struct check_walk_data {
+ enum pkvm_page_state desired;
+ enum pkvm_page_state (*get_page_state)(kvm_pte_t pte);
+};
+
+static int __check_page_state_visitor(u64 addr, u64 end, u32 level,
+ kvm_pte_t *ptep,
+ enum kvm_pgtable_walk_flags flag,
+ void * const arg)
+{
+ struct check_walk_data *d = arg;
+ kvm_pte_t pte = *ptep;
+
+ if (kvm_pte_valid(pte) && !addr_is_memory(kvm_pte_to_phys(pte)))
+ return -EINVAL;
+
+ return d->get_page_state(pte) == d->desired ? 0 : -EPERM;
+}
+
+static int check_page_state_range(struct kvm_pgtable *pgt, u64 addr, u64 size,
+ struct check_walk_data *data)
+{
+ struct kvm_pgtable_walker walker = {
+ .cb = __check_page_state_visitor,
+ .arg = data,
+ .flags = KVM_PGTABLE_WALK_LEAF,
+ };
+
+ return kvm_pgtable_walk(pgt, addr, size, &walker);
+}
+
+static enum pkvm_page_state host_get_page_state(kvm_pte_t pte)
+{
+ if (!kvm_pte_valid(pte) && pte)
+ return PKVM_NOPAGE;
+
+ return pkvm_getstate(kvm_pgtable_stage2_pte_prot(pte));
+}
+
+static int __host_check_page_state_range(u64 addr, u64 size,
+ enum pkvm_page_state state)
+{
+ struct check_walk_data d = {
+ .desired = state,
+ .get_page_state = host_get_page_state,
+ };
+
+ hyp_assert_lock_held(&host_kvm.lock);
+ return check_page_state_range(&host_kvm.pgt, addr, size, &d);
+}
+
+static int __host_set_page_state_range(u64 addr, u64 size,
+ enum pkvm_page_state state)
+{
+ enum kvm_pgtable_prot prot = pkvm_mkstate(PKVM_HOST_MEM_PROT, state);
+
+ return host_stage2_idmap_locked(addr, size, prot);
+}
+
+static int host_request_owned_transition(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_OWNED);
+}
+
+static int host_initiate_share(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_SHARED_OWNED);
+}
+
+static enum pkvm_page_state hyp_get_page_state(kvm_pte_t pte)
+{
+ if (!kvm_pte_valid(pte))
+ return PKVM_NOPAGE;
+
+ return pkvm_getstate(kvm_pgtable_stage2_pte_prot(pte));
+}
+
+static int __hyp_check_page_state_range(u64 addr, u64 size,
+ enum pkvm_page_state state)
+{
+ struct check_walk_data d = {
+ .desired = state,
+ .get_page_state = hyp_get_page_state,
+ };
+
+ hyp_assert_lock_held(&pkvm_pgd_lock);
+ return check_page_state_range(&pkvm_pgtable, addr, size, &d);
+}
+
+static bool __hyp_ack_skip_pgtable_check(const struct pkvm_mem_transition *tx)
+{
+ return !(IS_ENABLED(CONFIG_NVHE_EL2_DEBUG) ||
+ tx->initiator.id != PKVM_ID_HOST);
+}
+
+static int hyp_ack_share(u64 addr, const struct pkvm_mem_transition *tx,
+ enum kvm_pgtable_prot perms)
+{
+ u64 size = tx->nr_pages * PAGE_SIZE;
+
+ if (perms != PAGE_HYP)
+ return -EPERM;
+
+ if (__hyp_ack_skip_pgtable_check(tx))
+ return 0;
+
+ return __hyp_check_page_state_range(addr, size, PKVM_NOPAGE);
+}
+
+static int hyp_complete_share(u64 addr, const struct pkvm_mem_transition *tx,
+ enum kvm_pgtable_prot perms)
+{
+ void *start = (void *)addr, *end = start + (tx->nr_pages * PAGE_SIZE);
+ enum kvm_pgtable_prot prot;
+
+ prot = pkvm_mkstate(perms, PKVM_PAGE_SHARED_BORROWED);
+ return pkvm_create_mappings_locked(start, end, prot);
+}
+
+static int check_share(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_owned_transition(&completer_addr, tx);
+ break;
+ default:
+ ret = -EINVAL;
+ }
+
+ if (ret)
+ return ret;
+
+ switch (tx->completer.id) {
+ case PKVM_ID_HYP:
+ ret = hyp_ack_share(completer_addr, tx, share->completer_prot);
+ break;
+ default:
+ ret = -EINVAL;
+ }
+
+ return ret;
+}
+
+static int __do_share(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_share(&completer_addr, tx);
+ break;
+ default:
+ ret = -EINVAL;
+ }
+
+ if (ret)
+ return ret;
+
+ switch (tx->completer.id) {
+ case PKVM_ID_HYP:
+ ret = hyp_complete_share(completer_addr, tx, share->completer_prot);
+ break;
+ default:
+ ret = -EINVAL;
+ }
+
+ return ret;
+}
+
+/*
+ * do_share():
+ *
+ * The page owner grants access to another component with a given set
+ * of permissions.
+ *
+ * Initiator: OWNED => SHARED_OWNED
+ * Completer: NOPAGE => SHARED_BORROWED
+ */
+static int do_share(struct pkvm_mem_share *share)
+{
+ int ret;
+
+ ret = check_share(share);
+ if (ret)
+ return ret;
+
+ return WARN_ON(__do_share(share));
+}
--
2.34.1.173.g76aa8bc2d0-goog
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