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

BugLink: https://bugs.launchpad.net/bugs/1851549

commit b24e7598 upstream.

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 <gscrivan@redhat.com>
Tested-by: Giuseppe Scrivano <gscrivan@redhat.com>
Fixes: 4d99ff8f ("fuse: Turn writeback cache on")
Cc: <stable@vger.kernel.org> # v3.15+
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

fs/fuse/dir.c

@@ -1630,6 +1630,19 @@ int fuse_do_setattr(struct inode *inode, struct iattr *attr,
 	if (attr->ia_valid & ATTR_SIZE)
 		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);