fuse: do not allow mapping a non-regular backing file

We do not support passthrough operations other than read/write on
regular file, so allowing non-regular backing files makes no sense.

Fixes: efad715 ("fuse: allow O_PATH fd for FUSE_DEV_IOC_BACKING_OPEN")
Cc: stable@vger.kernel.org
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: amir73il

fs/fuse/passthrough.c

@@ -237,6 +237,11 @@ int fuse_backing_open(struct fuse_conn *fc, struct fuse_backing_map *map)
 	if (!file)
 		goto out;
 
+	/* read/write/splice/mmap passthrough only relevant for regular files */
+	res = d_is_dir(file->f_path.dentry) ? -EISDIR : -EINVAL;
+	if (!d_is_reg(file->f_path.dentry))
+		goto out_fput;
+
 	backing_sb = file_inode(file)->i_sb;
 	res = -ELOOP;
 	if (backing_sb->s_stack_depth >= fc->max_stack_depth)