fs/fuse: fix ioctl type confusion

fuse_dev_ioctl() performed fuse_get_dev() on a user-supplied fd,
leading to a type confusion issue. Fix it by checking file->f_op.
Signed-off-by: Jann Horn <jann@thejh.net>
Acked-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fuse/dev.c

@@ -2246,7 +2246,15 @@ static long fuse_dev_ioctl(struct file *file, unsigned int cmd,
 
 			err = -EINVAL;
 			if (old) {
-				struct fuse_dev *fud = fuse_get_dev(old);
+				struct fuse_dev *fud = NULL;
+
+				/*
+				 * Check against file->f_op because CUSE
+				 * uses the same ioctl handler.
+				 */
+				if (old->f_op == file->f_op &&
+				    old->f_cred->user_ns == file->f_cred->user_ns)
+					fud = fuse_get_dev(old);
 
 				if (fud) {
 					mutex_lock(&fuse_mutex);