From nobody Mon Sep 29 21:07:47 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6917FC25B0D for ; Tue, 16 Aug 2022 00:58:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244942AbiHPAzt (ORCPT ); Mon, 15 Aug 2022 20:55:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36192 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1344545AbiHPAtF (ORCPT ); Mon, 15 Aug 2022 20:49:05 -0400 Received: from sin.source.kernel.org (sin.source.kernel.org [IPv6:2604:1380:40e1:4800::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A5C8E169C75; Mon, 15 Aug 2022 13:45:54 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by sin.source.kernel.org (Postfix) with ESMTPS id E0478CE1187; Mon, 15 Aug 2022 20:45:52 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B020AC433C1; Mon, 15 Aug 2022 20:45:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1660596351; bh=+qpLPaKGzazoYN5NC/HJnxora1lgK1xSLQ58YvKLZK8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=EPQR4Y5wpukCCaGP04UA0mwLGOaEV5i2c6FXhoXfT6cW/Q+ZJcf9s7ao2uSuXh3BN 76ZP6bXtgM8DQ+RQtwMWr+TUShok5CyQb4Y2EpUkHYAI0uftifFUY/OjvzWoRK1tLN dxJc2mj98ld6LsHifpm53y8BhNYb5MKQof0gx9OY= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, stable@kernel.org, Al Viro Subject: [PATCH 5.19 1053/1157] __follow_mount_rcu(): verify that mount_lock remains unchanged Date: Mon, 15 Aug 2022 20:06:49 +0200 Message-Id: <20220815180522.175089622@linuxfoundation.org> X-Mailer: git-send-email 2.37.2 In-Reply-To: <20220815180439.416659447@linuxfoundation.org> References: <20220815180439.416659447@linuxfoundation.org> User-Agent: quilt/0.67 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" From: Al Viro commit 20aac6c60981f5bfacd66661d090d907bf1482f0 upstream. Validate mount_lock seqcount as soon as we cross into mount in RCU mode. Sure, ->mnt_root is pinned and will remain so until we do rcu_read_unlock() anyway, and we will eventually fail to unlazy if the mount_lock had been touched, but we might run into a hard error (e.g. -ENOENT) before trying to unlazy. And it's possible to end up with RCU pathwalk racing with rename() and umount() in a way that would fail with -ENOENT while non-RCU pathwalk would've succeeded with any timings. Once upon a time we hadn't needed that, but analysis had been subtle, brittle and went out of window as soon as RENAME_EXCHANGE had been added. It's narrow, hard to hit and won't get you anything other than stray -ENOENT that could be arranged in much easier way with the same priveleges, but it's a bug all the same. Cc: stable@kernel.org X-sky-is-falling: unlikely Fixes: da1ce0670c14 "vfs: add cross-rename" Signed-off-by: Al Viro Signed-off-by: Greg Kroah-Hartman --- fs/namei.c | 2 ++ 1 file changed, 2 insertions(+) --- a/fs/namei.c +++ b/fs/namei.c @@ -1505,6 +1505,8 @@ static bool __follow_mount_rcu(struct na * becoming unpinned. */ flags =3D dentry->d_flags; + if (read_seqretry(&mount_lock, nd->m_seq)) + return false; continue; } if (read_seqretry(&mount_lock, nd->m_seq))