f2fs: fix freezing filesystem during resize

Using FREEZE_HOLDER_USERSPACE has two consequences:

(1) If userspace freezes the filesystem after mnt_drop_write_file() but
    before freeze_super() was called filesystem resizing will fail
    because the freeze isn't marked as nestable.

(2) If the kernel has successfully frozen the filesystem via
    FREEZE_HOLDER_USERSPACE userspace can simply undo it by using the
    FITHAW ioctl.

Fix both issues by using FREEZE_HOLDER_KERNEL. It will nest with
FREEZE_HOLDER_USERSPACE and cannot be undone by userspace.

And it is the correct thing to do because the kernel temporarily freezes
the filesystem.

Signed-off-by: Christian Brauner <[email protected]>

Author: brauner

fs/f2fs/gc.c

@@ -2271,12 +2271,12 @@ int f2fs_resize_fs(struct file *filp, __u64 block_count)
 	if (err)
 		return err;
 
-	err = freeze_super(sbi->sb, FREEZE_HOLDER_USERSPACE, NULL);
+	err = freeze_super(sbi->sb, FREEZE_HOLDER_KERNEL, NULL);
 	if (err)
 		return err;
 
 	if (f2fs_readonly(sbi->sb)) {
-		err = thaw_super(sbi->sb, FREEZE_HOLDER_USERSPACE, NULL);
+		err = thaw_super(sbi->sb, FREEZE_HOLDER_KERNEL, NULL);
 		if (err)
 			return err;
 		return -EROFS;
@@ -2333,6 +2333,6 @@ int f2fs_resize_fs(struct file *filp, __u64 block_count)
 out_err:
 	f2fs_up_write(&sbi->cp_global_sem);
 	f2fs_up_write(&sbi->gc_lock);
-	thaw_super(sbi->sb, FREEZE_HOLDER_USERSPACE, NULL);
+	thaw_super(sbi->sb, FREEZE_HOLDER_KERNEL, NULL);
 	return err;
 }