[PATCH v11 08/18] KVM: guest_memfd: Allow host to map guest_memfd pages

[Fuad Tabba <tabba@google.com>]

This patch enables support for shared memory in guest_memfd, including
mapping that memory from host userspace.

This functionality is gated by the KVM_GMEM_SHARED_MEM Kconfig option,
and enabled for a given instance by the GUEST_MEMFD_FLAG_SUPPORT_SHARED
flag at creation time.

Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 include/linux/kvm_host.h | 13 +++++++
 include/uapi/linux/kvm.h |  1 +
 virt/kvm/Kconfig         |  4 +++
 virt/kvm/guest_memfd.c   | 76 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 94 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 80371475818f..640ce714cfb2 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -729,6 +729,19 @@ static inline bool kvm_arch_supports_gmem(struct kvm *kvm)
 }
 #endif
 
+/*
+ * Returns true if this VM supports shared mem in guest_memfd.
+ *
+ * Arch code must define kvm_arch_supports_gmem_shared_mem if support for
+ * guest_memfd is enabled.
+ */
+#if !defined(kvm_arch_supports_gmem_shared_mem)
+static inline bool kvm_arch_supports_gmem_shared_mem(struct kvm *kvm)
+{
+	return false;
+}
+#endif
+
 #ifndef kvm_arch_has_readonly_mem
 static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
 {
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index b6ae8ad8934b..c2714c9d1a0e 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1566,6 +1566,7 @@ struct kvm_memory_attributes {
 #define KVM_MEMORY_ATTRIBUTE_PRIVATE           (1ULL << 3)
 
 #define KVM_CREATE_GUEST_MEMFD	_IOWR(KVMIO,  0xd4, struct kvm_create_guest_memfd)
+#define GUEST_MEMFD_FLAG_SUPPORT_SHARED	(1ULL << 0)
 
 struct kvm_create_guest_memfd {
 	__u64 size;
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index 559c93ad90be..e90884f74404 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -128,3 +128,7 @@ config HAVE_KVM_ARCH_GMEM_PREPARE
 config HAVE_KVM_ARCH_GMEM_INVALIDATE
        bool
        depends on KVM_GMEM
+
+config KVM_GMEM_SHARED_MEM
+       select KVM_GMEM
+       bool
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 6db515833f61..7a158789d1df 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -312,7 +312,79 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
 	return gfn - slot->base_gfn + slot->gmem.pgoff;
 }
 
+static bool kvm_gmem_supports_shared(struct inode *inode)
+{
+	u64 flags;
+
+	if (!IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM))
+		return false;
+
+	flags = (u64)inode->i_private;
+
+	return flags & GUEST_MEMFD_FLAG_SUPPORT_SHARED;
+}
+
+static vm_fault_t kvm_gmem_fault_shared(struct vm_fault *vmf)
+{
+	struct inode *inode = file_inode(vmf->vma->vm_file);
+	struct folio *folio;
+	vm_fault_t ret = VM_FAULT_LOCKED;
+
+	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
+		return VM_FAULT_SIGBUS;
+
+	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
+	if (IS_ERR(folio)) {
+		int err = PTR_ERR(folio);
+
+		if (err == -EAGAIN)
+			return VM_FAULT_RETRY;
+
+		return vmf_error(err);
+	}
+
+	if (WARN_ON_ONCE(folio_test_large(folio))) {
+		ret = VM_FAULT_SIGBUS;
+		goto out_folio;
+	}
+
+	if (!folio_test_uptodate(folio)) {
+		clear_highpage(folio_page(folio, 0));
+		kvm_gmem_mark_prepared(folio);
+	}
+
+	vmf->page = folio_file_page(folio, vmf->pgoff);
+
+out_folio:
+	if (ret != VM_FAULT_LOCKED) {
+		folio_unlock(folio);
+		folio_put(folio);
+	}
+
+	return ret;
+}
+
+static const struct vm_operations_struct kvm_gmem_vm_ops = {
+	.fault = kvm_gmem_fault_shared,
+};
+
+static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	if (!kvm_gmem_supports_shared(file_inode(file)))
+		return -ENODEV;
+
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
+	    (VM_SHARED | VM_MAYSHARE)) {
+		return -EINVAL;
+	}
+
+	vma->vm_ops = &kvm_gmem_vm_ops;
+
+	return 0;
+}
+
 static struct file_operations kvm_gmem_fops = {
+	.mmap		= kvm_gmem_mmap,
 	.open		= generic_file_open,
 	.release	= kvm_gmem_release,
 	.fallocate	= kvm_gmem_fallocate,
@@ -428,6 +500,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
 	}
 
 	file->f_flags |= O_LARGEFILE;
+	allow_write_access(file);
 
 	inode = file->f_inode;
 	WARN_ON(file->f_mapping != inode->i_mapping);
@@ -463,6 +536,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
 	u64 flags = args->flags;
 	u64 valid_flags = 0;
 
+	if (kvm_arch_supports_gmem_shared_mem(kvm))
+		valid_flags |= GUEST_MEMFD_FLAG_SUPPORT_SHARED;
+
 	if (flags & ~valid_flags)
 		return -EINVAL;
 
-- 
2.49.0.1266.g31b7d2e469-goog