fuse: flush dirty data/metadata before non-truncate setattr

If writeback cache is enabled, then writes might get reordered with
chmod/chown/utimes.  The problem with this is that performing the write in
the fuse daemon might itself change some of these attributes.  In such case
the following sequence of operations will result in file ending up with the
wrong mode, for example:

  int fd = open ("suid", O_WRONLY|O_CREAT|O_EXCL);
  write (fd, "1", 1);
  fchown (fd, 0, 0);
  fchmod (fd, 04755);
  close (fd);

This patch fixes this by flushing pending writes before performing
chown/chmod/utimes.

Reported-by: Giuseppe Scrivano <[email protected]>
Tested-by: Giuseppe Scrivano <[email protected]>
Fixes: 4d99ff8 ("fuse: Turn writeback cache on")
Cc: <[email protected]> # v3.15+
Signed-off-by: Miklos Szeredi <[email protected]>

Author: Miklos Szeredi

fs/fuse/dir.c

@@ -1522,6 +1522,19 @@ int fuse_do_setattr(struct dentry *dentry, struct iattr *attr,
 		is_truncate = true;
 	}
 
+	/* Flush dirty data/metadata before non-truncate SETATTR */
+	if (is_wb && S_ISREG(inode->i_mode) &&
+	    attr->ia_valid &
+			(ATTR_MODE | ATTR_UID | ATTR_GID | ATTR_MTIME_SET |
+			 ATTR_TIMES_SET)) {
+		err = write_inode_now(inode, true);
+		if (err)
+			return err;
+
+		fuse_set_nowrite(inode);
+		fuse_release_nowrite(inode);
+	}
+
 	if (is_truncate) {
 		fuse_set_nowrite(inode);
 		set_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);