[PATCH gmem 6/6] KVM: guest_memfd: Add interface for populating gmem pages with user data

[Michael Roth <michael.roth@amd.com>]

During guest run-time, kvm_arch_gmem_prepare() is issued as needed to
prepare newly-allocated gmem pages prior to mapping them into the guest.
In the case of SEV-SNP, this mainly involves setting the pages to
private in the RMP table.

However, for the GPA ranges comprising the initial guest payload, which
are encrypted/measured prior to starting the guest, the gmem pages need
to be accessed prior to setting them to private in the RMP table so they
can be initialized with the userspace-provided data. Additionally, an
SNP firmware call is needed afterward to encrypt them in-place and
measure the contents into the guest's launch digest.

While it is possible to bypass the kvm_arch_gmem_prepare() hooks so that
this handling can be done in an open-coded/vendor-specific manner, this
may expose more gmem-internal state/dependencies to external callers
than necessary. Try to avoid this by implementing an interface that
tries to handle as much of the common functionality inside gmem as
possible, while also making it generic enough to potentially be
usable/extensible for use-cases beyond just SEV-SNP.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 include/linux/kvm_host.h | 40 ++++++++++++++++++++++++++++++++++++++++
 virt/kvm/guest_memfd.c   | 40 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 80 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 5b8308b5e4af..8a75787090f3 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2473,4 +2473,44 @@ bool kvm_arch_gmem_prepare_needed(struct kvm *kvm);
 void kvm_arch_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end);
 #endif
 
+/**
+ * kvm_gmem_populate_args - kvm_gmem_populate() argument structure
+ *
+ * @gfn: starting GFN to be populated
+ * @src: userspace-provided buffer containing data to copy into GFN range
+ * @npages: number of pages to copy from userspace-buffer
+ * @do_memcpy: whether to do a direct memcpy of the data prior to issuing
+ *             the post-populate callback
+ * @post_populate: callback to issue for each gmem page that backs the GPA
+ *                 range (which will be filled with corresponding contents from
+ *                 @src if @do_memcpy was set)
+ * @opaque: opaque data to pass to @post_populate callback
+ */
+struct kvm_gmem_populate_args {
+	gfn_t gfn;
+	void __user *src;
+	int npages;
+	bool do_memcpy;
+	int (*post_populate)(struct kvm *kvm, struct kvm_memory_slot *slot,
+			     gfn_t gfn, kvm_pfn_t pfn, void __user *src, int order,
+			     void *opaque);
+	void *opaque;
+};
+
+/**
+ * kvm_gmem_populate() - Populate/prepare a GPA range with guest data
+ *
+ * @kvm: KVM instance
+ * @slot: slot containing the GPA range being prepared
+ * @args: argument structure
+ *
+ * This is primarily intended for cases where a gmem-backed GPA range needs
+ * to be initialized with userspace-provided data prior to being mapped into
+ * the guest as a private page. This should be called with the slots->lock
+ * held so that caller-enforced invariants regarding the expected memory
+ * attributes of the GPA range do not race with KVM_SET_MEMORY_ATTRIBUTES.
+ */
+int kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
+		      struct kvm_gmem_populate_args *args);
+
 #endif
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 3668a5f1d82b..3e3c4b7fff3b 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -643,3 +643,43 @@ int kvm_gmem_undo_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 	return r;
 }
 EXPORT_SYMBOL_GPL(kvm_gmem_undo_get_pfn);
+
+int kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
+		      struct kvm_gmem_populate_args *args)
+{
+	int ret, max_order, i;
+
+	for (i = 0; i < args->npages; i += (1 << max_order)) {
+		void __user *src = args->src + i * PAGE_SIZE;
+		gfn_t gfn = args->gfn + i;
+		kvm_pfn_t pfn;
+
+		ret = __kvm_gmem_get_pfn(kvm, slot, gfn, &pfn, &max_order, false);
+		if (ret)
+			break;
+
+		if (!IS_ALIGNED(gfn, (1 << max_order)) ||
+		    (args->npages - i) < (1 << max_order))
+			max_order = 0;
+
+		if (args->do_memcpy && args->src) {
+			ret = copy_from_user(pfn_to_kaddr(pfn), src, (1 << max_order) * PAGE_SIZE);
+			if (ret)
+				goto e_release;
+		}
+
+		if (args->post_populate) {
+			ret = args->post_populate(kvm, slot, gfn, pfn, src, max_order,
+						  args->opaque);
+			if (ret)
+				goto e_release;
+		}
+e_release:
+		put_page(pfn_to_page(pfn));
+		if (ret)
+			break;
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(kvm_gmem_populate);
-- 
2.25.1