ovl: fix deadlock in splice write

There's possibility of an ABBA deadlock in case of a splice write to an
overlayfs file and a concurrent splice write to a corresponding real file.

The call chain for splice to an overlay file:

 -> do_splice                     [takes sb_writers on overlay file]
   -> do_splice_from
     -> iter_file_splice_write    [takes pipe->mutex]
       -> vfs_iter_write
         ...
         -> ovl_write_iter        [takes sb_writers on real file]

And the call chain for splice to a real file:

 -> do_splice                     [takes sb_writers on real file]
   -> do_splice_from
     -> iter_file_splice_write    [takes pipe->mutex]

Syzbot successfully bisected this to commit 82a763e6 ("ovl: simplify
file splice").

Fix by reverting the write part of the above commit and by adding missing
bits from ovl_write_iter() into ovl_splice_write().

Fixes: 82a763e6 ("ovl: simplify file splice")
Reported-and-tested-by: syzbot+579885d1a9a833336209@syzkaller.appspotmail.com
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/file.c

@@ -392,6 +392,51 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	return ret;
 }
 
+/*
+ * Calling iter_file_splice_write() directly from overlay's f_op may deadlock
+ * due to lock order inversion between pipe->mutex in iter_file_splice_write()
+ * and file_start_write(real.file) in ovl_write_iter().
+ *
+ * So do everything ovl_write_iter() does and call iter_file_splice_write() on
+ * the real file.
+ */
+static ssize_t ovl_splice_write(struct pipe_inode_info *pipe, struct file *out,
+				loff_t *ppos, size_t len, unsigned int flags)
+{
+	struct fd real;
+	const struct cred *old_cred;
+	struct inode *inode = file_inode(out);
+	struct inode *realinode = ovl_inode_real(inode);
+	ssize_t ret;
+
+	inode_lock(inode);
+	/* Update mode */
+	ovl_copyattr(realinode, inode);
+	ret = file_remove_privs(out);
+	if (ret)
+		goto out_unlock;
+
+	ret = ovl_real_fdget(out, &real);
+	if (ret)
+		goto out_unlock;
+
+	old_cred = ovl_override_creds(inode->i_sb);
+	file_start_write(real.file);
+
+	ret = iter_file_splice_write(pipe, real.file, ppos, len, flags);
+
+	file_end_write(real.file);
+	/* Update size */
+	ovl_copyattr(realinode, inode);
+	revert_creds(old_cred);
+	fdput(real);
+
+out_unlock:
+	inode_unlock(inode);
+
+	return ret;
+}
+
 static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
 {
 	struct fd real;
@@ -603,7 +648,7 @@ const struct file_operations ovl_file_operations = {
 	.fadvise	= ovl_fadvise,
 	.flush		= ovl_flush,
 	.splice_read    = generic_file_splice_read,
-	.splice_write   = iter_file_splice_write,
+	.splice_write   = ovl_splice_write,
 
 	.copy_file_range	= ovl_copy_file_range,
 	.remap_file_range	= ovl_remap_file_range,