From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5F7BFC47423 for ; Tue, 29 Sep 2020 12:20:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 163C320759 for ; Tue, 29 Sep 2020 12:20:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1601382022; bh=UmE6HDmKHh1tjp+4Mo2RuBKkzu03dbR/zl8DtI3X64A=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=eIlEFqeKtYO8Pe7DdbYiwF3qF1K1kik1SuZgJbs8br4jYAaLxCy/9+GK4paZITiIU VwNG4Z218mQ56VCeRDFErO9oM24ReNFN5bZ1T/FMNoamJKWMdlRu3oKt2y8AB2nY+n B/CFiWt323UY003phcN+uQYZ2LfgLVGCIVkc9xl8= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732484AbgI2MUV (ORCPT ); Tue, 29 Sep 2020 08:20:21 -0400 Received: from mail.kernel.org ([198.145.29.99]:49136 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729876AbgI2Lfg (ORCPT ); Tue, 29 Sep 2020 07:35:36 -0400 Received: from localhost (83-86-74-64.cable.dynamic.v4.ziggo.nl [83.86.74.64]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 04B472376E; Tue, 29 Sep 2020 11:21:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1601378496; bh=UmE6HDmKHh1tjp+4Mo2RuBKkzu03dbR/zl8DtI3X64A=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=MwPtmxh0P2V1aUiyLIt8yc2s2hiqCfFo2FQuFDgD5jRjpp/dRlIokZxPMFtiDBICb vCnqcqnPzoxd73mlQRtr5X7xGam2KOrrgXWWEwa3BIcVss0IFnb2Kqfu7fL19OUvAY 5ifVOC64Xb/3/RUN0lhmJlkufFA4VNCuANrTQqmY= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Al Viro , Sasha Levin Subject: [PATCH 4.19 034/245] fix dget_parent() fastpath race Date: Tue, 29 Sep 2020 12:58:05 +0200 Message-Id: <20200929105948.667514643@linuxfoundation.org> X-Mailer: git-send-email 2.28.0 In-Reply-To: <20200929105946.978650816@linuxfoundation.org> References: <20200929105946.978650816@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Al Viro [ Upstream commit e84009336711d2bba885fc9cea66348ddfce3758 ] We are overoptimistic about taking the fast path there; seeing the same value in ->d_parent after having grabbed a reference to that parent does *not* mean that it has remained our parent all along. That wouldn't be a big deal (in the end it is our parent and we have grabbed the reference we are about to return), but... the situation with barriers is messed up. We might have hit the following sequence: d is a dentry of /tmp/a/b CPU1: CPU2: parent = d->d_parent (i.e. dentry of /tmp/a) rename /tmp/a/b to /tmp/b rmdir /tmp/a, making its dentry negative grab reference to parent, end up with cached parent->d_inode (NULL) mkdir /tmp/a, rename /tmp/b to /tmp/a/b recheck d->d_parent, which is back to original decide that everything's fine and return the reference we'd got. The trouble is, caller (on CPU1) will observe dget_parent() returning an apparently negative dentry. It actually is positive, but CPU1 has stale ->d_inode cached. Use d->d_seq to see if it has been moved instead of rechecking ->d_parent. NOTE: we are *NOT* going to retry on any kind of ->d_seq mismatch; we just go into the slow path in such case. We don't wait for ->d_seq to become even either - again, if we are racing with renames, we can bloody well go to slow path anyway. Signed-off-by: Al Viro Signed-off-by: Sasha Levin --- fs/dcache.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/fs/dcache.c b/fs/dcache.c index 6e0022326afe3..20370a0997bf9 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -864,17 +864,19 @@ struct dentry *dget_parent(struct dentry *dentry) { int gotref; struct dentry *ret; + unsigned seq; /* * Do optimistic parent lookup without any * locking. */ rcu_read_lock(); + seq = raw_seqcount_begin(&dentry->d_seq); ret = READ_ONCE(dentry->d_parent); gotref = lockref_get_not_zero(&ret->d_lockref); rcu_read_unlock(); if (likely(gotref)) { - if (likely(ret == READ_ONCE(dentry->d_parent))) + if (!read_seqcount_retry(&dentry->d_seq, seq)) return ret; dput(ret); } -- 2.25.1