[PATCH RFC 4/4] mm: guest_memfd: Add ability for mmap'ing pages

[Elliot Berman <quic_eberman@quicinc.com>]

Confidential/protected guest virtual machines want to share some memory
back with the host Linux. For example, virtqueues allow host and
protected guest to exchange data. In MMU-only isolation of protected
guest virtual machines, the transition between "shared" and "private"
can be done in-place without a trusted hypervisor copying pages.

Add support for this feature and allow Linux to mmap host-accessible
pages. When the owner provides an ->accessible() callback in the
struct guest_memfd_operations, guest_memfd allows folios to be mapped
when the ->accessible() callback returns 0.

To safely make inaccessible:

```
folio = guest_memfd_grab_folio(inode, index, flags);
r = guest_memfd_make_inaccessible(inode, folio);
if (r)
        goto err;

hypervisor_does_guest_mapping(folio);

folio_unlock(folio);
```

hypervisor_does_s2_mapping(folio) should make it so
ops->accessible(...) on those folios fails.

The folio lock ensures atomicity.

Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
---
 include/linux/guest_memfd.h |  7 ++++
 mm/guest_memfd.c            | 81 ++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 87 insertions(+), 1 deletion(-)

diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h
index f9e4a27aed67..edcb4ba60cb0 100644
--- a/include/linux/guest_memfd.h
+++ b/include/linux/guest_memfd.h
@@ -16,12 +16,18 @@
  * @invalidate_end: called after invalidate_begin returns success. Optional.
  * @prepare: called before a folio is mapped into the guest address space.
  *           Optional.
+ * @accessible: called after prepare returns success and before it's mapped
+ *              into the guest address space. Returns 0 if the folio can be
+ *              accessed.
+ *              Optional. If not present, assumes folios are never accessible.
  * @release: Called when releasing the guest_memfd file. Required.
  */
 struct guest_memfd_operations {
 	int (*invalidate_begin)(struct inode *inode, pgoff_t offset, unsigned long nr);
 	void (*invalidate_end)(struct inode *inode, pgoff_t offset, unsigned long nr);
 	int (*prepare)(struct inode *inode, pgoff_t offset, struct folio *folio);
+	int (*accessible)(struct inode *inode, struct folio *folio,
+			  pgoff_t offset, unsigned long nr);
 	int (*release)(struct inode *inode);
 };
 
@@ -48,5 +54,6 @@ struct file *guest_memfd_alloc(const char *name,
 			       const struct guest_memfd_operations *ops,
 			       loff_t size, unsigned long flags);
 bool is_guest_memfd(struct file *file, const struct guest_memfd_operations *ops);
+int guest_memfd_make_inaccessible(struct file *file, struct folio *folio);
 
 #endif
diff --git a/mm/guest_memfd.c b/mm/guest_memfd.c
index e9d8cab72b28..6b5609932ca5 100644
--- a/mm/guest_memfd.c
+++ b/mm/guest_memfd.c
@@ -9,6 +9,8 @@
 #include <linux/pagemap.h>
 #include <linux/set_memory.h>
 
+#include "internal.h"
+
 static inline int guest_memfd_folio_private(struct folio *folio)
 {
 	unsigned long nr_pages = folio_nr_pages(folio);
@@ -89,7 +91,7 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
 			goto out_err;
 	}
 
-	if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
+	if (!ops->accessible && (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)) {
 		r = guest_memfd_folio_private(folio);
 		if (r)
 			goto out_err;
@@ -107,6 +109,82 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
 }
 EXPORT_SYMBOL_GPL(guest_memfd_grab_folio);
 
+int guest_memfd_make_inaccessible(struct file *file, struct folio *folio)
+{
+	unsigned long gmem_flags = (unsigned long)file->private_data;
+	unsigned long i;
+	int r;
+
+	unmap_mapping_folio(folio);
+
+	/**
+	 * We can't use the refcount. It might be elevated due to
+	 * guest/vcpu trying to access same folio as another vcpu
+	 * or because userspace is trying to access folio for same reason
+	 *
+	 * folio_lock serializes the transitions between (in)accessible
+	 */
+	if (folio_maybe_dma_pinned(folio))
+		return -EBUSY;
+
+	if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
+		r = guest_memfd_folio_private(folio);
+		if (r)
+			return r;
+	}
+
+	return 0;
+}
+
+static vm_fault_t gmem_fault(struct vm_fault *vmf)
+{
+	struct file *file = vmf->vma->vm_file;
+	struct inode *inode = file_inode(file);
+	const struct guest_memfd_operations *ops = inode->i_private;
+	struct folio *folio;
+	pgoff_t off;
+	int r;
+
+	folio = guest_memfd_grab_folio(file, vmf->pgoff, GUEST_MEMFD_GRAB_UPTODATE);
+	if (!folio)
+		return VM_FAULT_SIGBUS;
+
+	off = vmf->pgoff & (folio_nr_pages(folio) - 1);
+	r = ops->accessible(inode, folio, off, 1);
+	if (r) {
+		folio_unlock(folio);
+		folio_put(folio);
+		return VM_FAULT_SIGBUS;
+	}
+
+	guest_memfd_folio_clear_private(folio);
+
+	vmf->page = folio_page(folio, off);
+
+	return VM_FAULT_LOCKED;
+}
+
+static const struct vm_operations_struct gmem_vm_ops = {
+	.fault = gmem_fault,
+};
+
+static int gmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	const struct guest_memfd_operations *ops = file_inode(file)->i_private;
+
+	if (!ops->accessible)
+		return -EPERM;
+
+	/* No support for private mappings to avoid COW.  */
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
+	    (VM_SHARED | VM_MAYSHARE))
+		return -EINVAL;
+
+	file_accessed(file);
+	vma->vm_ops = &gmem_vm_ops;
+	return 0;
+}
+
 static long gmem_punch_hole(struct file *file, loff_t offset, loff_t len)
 {
 	struct inode *inode = file_inode(file);
@@ -220,6 +298,7 @@ static int gmem_release(struct inode *inode, struct file *file)
 static struct file_operations gmem_fops = {
 	.open = generic_file_open,
 	.llseek = generic_file_llseek,
+	.mmap = gmem_mmap,
 	.release = gmem_release,
 	.fallocate = gmem_fallocate,
 	.owner = THIS_MODULE,

-- 
2.34.1