[RFC PATCH 27/39] KVM: guest_memfd: Allow mmapping guest_memfd files

[Ackerley Tng]

guest_memfd files can always be mmap()ed to userspace, but
faultability is controlled by an attribute on the inode.

Co-developed-by: Fuad Tabba <ta...@google.com>
Signed-off-by: Fuad Tabba <ta...@google.com>
Co-developed-by: Ackerley Tng <ackerley...@google.com>
Signed-off-by: Ackerley Tng <ackerley...@google.com>

---
 virt/kvm/guest_memfd.c | 46 ++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index b603518f7b62..fc2483e35876 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -781,7 +781,8 @@ static long kvm_gmem_punch_hole(struct inode *inode, loff_t 
offset, loff_t len)
 {
        struct list_head *gmem_list = &inode->i_mapping->i_private_list;
        pgoff_t start = offset >> PAGE_SHIFT;
-       pgoff_t end = (offset + len) >> PAGE_SHIFT;
+       pgoff_t nr = len >> PAGE_SHIFT;
+       pgoff_t end = start + nr;
        struct kvm_gmem *gmem;
 
        /*
@@ -790,6 +791,9 @@ static long kvm_gmem_punch_hole(struct inode *inode, loff_t 
offset, loff_t len)
         */
        filemap_invalidate_lock(inode->i_mapping);
 
+       /* TODO: Check if even_cows should be 0 or 1 */
+       unmap_mapping_range(inode->i_mapping, start, len, 0);
+
        list_for_each_entry(gmem, gmem_list, entry)
                kvm_gmem_invalidate_begin(gmem, start, end);
 
@@ -946,6 +950,9 @@ static void kvm_gmem_hugetlb_teardown(struct inode *inode)
 {
        struct kvm_gmem_hugetlb *hgmem;
 
+       /* TODO: Check if even_cows should be 0 or 1 */
+       unmap_mapping_range(inode->i_mapping, 0, LLONG_MAX, 0);
+
        truncate_inode_pages_final_prepare(inode->i_mapping);
        kvm_gmem_hugetlb_truncate_folios_range(inode, 0, LLONG_MAX);
 
@@ -1003,11 +1010,46 @@ static void kvm_gmem_init_mount(void)
        kvm_gmem_mnt = kern_mount(&kvm_gmem_fs);
        BUG_ON(IS_ERR(kvm_gmem_mnt));
 
-       /* For giggles. Userspace can never map this anyways. */
        kvm_gmem_mnt->mnt_flags |= MNT_NOEXEC;
 }
 
+static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
+{
+       struct inode *inode;
+       struct folio *folio;
+
+       inode = file_inode(vmf->vma->vm_file);
+       if (!kvm_gmem_is_faultable(inode, vmf->pgoff))
+               return VM_FAULT_SIGBUS;
+
+       folio = kvm_gmem_get_folio(inode, vmf->pgoff);
+       if (!folio)
+               return VM_FAULT_SIGBUS;
+
+       vmf->page = folio_file_page(folio, vmf->pgoff);
+       return VM_FAULT_LOCKED;
+}
+
+static const struct vm_operations_struct kvm_gmem_vm_ops = {
+       .fault = kvm_gmem_fault,
+};
+
+static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+       if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) !=
+           (VM_SHARED | VM_MAYSHARE)) {
+               return -EINVAL;
+       }
+
+       file_accessed(file);
+       vm_flags_set(vma, VM_DONTDUMP);
+       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,
-- 
2.46.0.598.g6f2099f65c-goog