[RFC PATCH 1/8] kvm: Allow reading/writing gmem using kvm_{read,write}_guest

[Patrick Roy <roypat@amazon.co.uk>]

If KVM can access guest-private memory without causing a host-kernel
panic (e.g. currently only if the vm type is KVM_SW_PROTECTED_VM), allow
`kvm_{read,write}_guest` to access gfns that are set to "private".  If
KVM cannot access guest-private memory (say, because it is running a TDX
VM), prepare a KVM_EXIT_MEMORY_FAULT (if possible) and return -EFAULT.

KVM can only prepare the memory fault exit inside the
`kvm_vcpu_{read,write}_guest` variant, as it needs a vcpu reference to
assign the exit reason to.

KVM accesses guest-private memory via kernel virtual addresses/the
direct map. In the special case of guest_memfd, it does not have to
worry about gfn->pfn mappings being invalidated, since guest_memfd pages
are immovable.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
---
 include/linux/kvm_host.h |  5 +++
 virt/kvm/kvm_main.c      | 85 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 90 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 2a6679b46427..8f980aafd5ca 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2407,6 +2407,11 @@ static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
 		vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE;
 }
 
+static inline bool kvm_can_access_gmem(struct kvm *kvm)
+{
+	return kvm->arch.vm_type == KVM_X86_SW_PROTECTED_VM;
+}
+
 #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
 static inline unsigned long kvm_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
 {
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 8c7cbc9ec9ee..b3b3de70a4df 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3286,11 +3286,51 @@ static int __kvm_read_guest_page(struct kvm_memory_slot *slot, gfn_t gfn,
 	return 0;
 }
 
+static int __kvm_read_guest_private_page(struct kvm *kvm,
+					 struct kvm_memory_slot *memslot, gfn_t gfn,
+					 void *data, int offset, int len)
+{
+	kvm_pfn_t pfn;
+	int r;
+	struct page *page;
+	void *kaddr;
+
+	if (!kvm_can_access_gmem(kvm))
+		return -EFAULT;
+
+	r = kvm_gmem_get_pfn(kvm, memslot, gfn, &pfn, NULL);
+
+	if (r < 0)
+		return -EFAULT;
+
+	page = pfn_to_page(pfn);
+	lock_page(page);
+	kaddr = page_address(page) + offset;
+	memcpy(data, kaddr, len);
+	unlock_page(page);
+	put_page(page);
+	return 0;
+}
+
+static int __kvm_vcpu_read_guest_private_page(struct kvm_vcpu *vcpu,
+					       struct kvm_memory_slot *memslot, gfn_t gfn,
+					       void *data, int offset, int len)
+{
+	if (!kvm_can_access_gmem(vcpu->kvm)) {
+		kvm_prepare_memory_fault_exit(vcpu, gfn + offset, len, false,
+					      false, true);
+		return -EFAULT;
+	}
+	return __kvm_read_guest_private_page(vcpu->kvm, memslot, gfn, data, offset, len);
+}
+
 int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
 			int len)
 {
 	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
 
+	if (kvm_mem_is_private(kvm, gfn))
+		return __kvm_read_guest_private_page(kvm, slot, gfn, data, offset, len);
 	return __kvm_read_guest_page(slot, gfn, data, offset, len);
 }
 EXPORT_SYMBOL_GPL(kvm_read_guest_page);
@@ -3300,6 +3340,8 @@ int kvm_vcpu_read_guest_page(struct kvm_vcpu *vcpu, gfn_t gfn, void *data,
 {
 	struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
 
+	if (kvm_mem_is_private(vcpu->kvm, gfn))
+		return __kvm_vcpu_read_guest_private_page(vcpu, slot, gfn, data, offset, len);
 	return __kvm_read_guest_page(slot, gfn, data, offset, len);
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_read_guest_page);
@@ -3390,11 +3432,52 @@ static int __kvm_write_guest_page(struct kvm *kvm,
 	return 0;
 }
 
+static int __kvm_write_guest_private_page(struct kvm *kvm,
+					  struct kvm_memory_slot *memslot, gfn_t gfn,
+					  const void *data, int offset, int len)
+{
+	kvm_pfn_t pfn;
+	int r;
+	struct page *page;
+	void *kaddr;
+
+	if (!kvm_can_access_gmem(kvm))
+		return -EFAULT;
+
+	r = kvm_gmem_get_pfn(kvm, memslot, gfn, &pfn, NULL);
+
+	if (r < 0)
+		return -EFAULT;
+
+	page = pfn_to_page(pfn);
+	lock_page(page);
+	kaddr = page_address(page) + offset;
+	memcpy(kaddr, data, len);
+	unlock_page(page);
+	put_page(page);
+
+	return 0;
+}
+
+static int __kvm_vcpu_write_guest_private_page(struct kvm_vcpu *vcpu,
+					       struct kvm_memory_slot *memslot, gfn_t gfn,
+					       const void *data, int offset, int len)
+{
+	if (!kvm_can_access_gmem(vcpu->kvm)) {
+		kvm_prepare_memory_fault_exit(vcpu, gfn + offset, len, true,
+					      false, true);
+		return -EFAULT;
+	}
+	return __kvm_write_guest_private_page(vcpu->kvm, memslot, gfn, data, offset, len);
+}
+
 int kvm_write_guest_page(struct kvm *kvm, gfn_t gfn,
 			 const void *data, int offset, int len)
 {
 	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
 
+	if (kvm_mem_is_private(kvm, gfn))
+		return __kvm_write_guest_private_page(kvm, slot, gfn, data, offset, len);
 	return __kvm_write_guest_page(kvm, slot, gfn, data, offset, len);
 }
 EXPORT_SYMBOL_GPL(kvm_write_guest_page);
@@ -3404,6 +3487,8 @@ int kvm_vcpu_write_guest_page(struct kvm_vcpu *vcpu, gfn_t gfn,
 {
 	struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
 
+	if (kvm_mem_is_private(vcpu->kvm, gfn))
+		return __kvm_vcpu_write_guest_private_page(vcpu, slot, gfn, data, offset, len);
 	return __kvm_write_guest_page(vcpu->kvm, slot, gfn, data, offset, len);
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_write_guest_page);

base-commit: 771df9ffadb8204e61d3e98f36c5067102aab78f
-- 
2.45.2