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=-2.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no 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 E6BA6C433ED for ; Sun, 25 Apr 2021 18:15:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BBEBB61139 for ; Sun, 25 Apr 2021 18:15:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231184AbhDYSQc (ORCPT ); Sun, 25 Apr 2021 14:16:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34192 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230459AbhDYSQc (ORCPT ); Sun, 25 Apr 2021 14:16:32 -0400 Received: from mail-ej1-x635.google.com (mail-ej1-x635.google.com [IPv6:2a00:1450:4864:20::635]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8500EC061574; Sun, 25 Apr 2021 11:15:50 -0700 (PDT) Received: by mail-ej1-x635.google.com with SMTP id ja3so5154953ejc.9; Sun, 25 Apr 2021 11:15:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=a/ZtSBDvqTlG56qptq7qwAgMe8JoTOcZRN0BpM31Y6Q=; b=EHdMCZHHaGasg4SfOoxLwM9k24VLi/DajXoJH3DJacWrmFpGK1n20uze8m5xXuz4qJ wCtvgwwLEwZg3aaKzDue6me8AiCojRKfUNbKT6rALXKBPGgEni4LBVz0L8698DoxWiD2 daYWMSU7pDuDypncA71Zltq7ZRZHSaa/IQq5v4PN7YRUaNnN7NFRHzNKE5xPEcxT0sXs Kr3EBsLeBG6lGmqjtWuM6wfz1IuJCFLKgCsdiFlxTLfizWIQK0Tn9P8pOmMwFD/whjtE VURZpMT/EpLcOdQQgWeH1iNeLbKHOC7/NMJCx+9q9hlcu7/J6Md1xKiUYzOGeca8KxT9 4Esg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=a/ZtSBDvqTlG56qptq7qwAgMe8JoTOcZRN0BpM31Y6Q=; b=XfTewP3PrQG5bBK/USpeBZKYeLw5qOn1QBGxV6twf5Nzm0GDPmkk17PcYmLsxFHj23 DDvXVzTX88vWPnU0Py7914lKtCX2R9V0REbSMXcNKAdQcHPlq6r6eO2CbC8h8Pt3GX35 WVWePuzJXP67aeLGriIiJJbhILNTQu5PgGA9Tu2ucpQHSyeU6FwLf4Kcka8xo0+fdTAG NAwuaA+yr/6kOEFn/HfyonBUAqwfBX9BamPZgb4Vz78hd6ynUJnbkpGnsECq1jVxSXuF 8p8hTlhOaF2XBlFxl/zVPuwkP5xZU2thzSzOopskvAYJPTFgo0F+T5Wrb2XuyW8L6oPp AXqA== X-Gm-Message-State: AOAM532Uu1NUBDS3Z+hIbp99WzphMo4dVY8trIIw5hHxkE41A2EkKXd/ 1JRfK2R0nw9CllPVZOT05oMDovc0//WufhimJPBPWjtcjb7fRQ== X-Google-Smtp-Source: ABdhPJw10TCE534B8I5UX7zjBr5VS6kO6hjL4hJHk2mAAq0UjE7nEP6Kr+BYyFbO7ztXJ/hHR3FeQIjLJab45n2kxUM= X-Received: by 2002:a17:906:1c8f:: with SMTP id g15mr14445082ejh.20.1619374549025; Sun, 25 Apr 2021 11:15:49 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: haosdent Date: Mon, 26 Apr 2021 02:15:37 +0800 Message-ID: Subject: Re: NULL pointer dereference when access /proc/net To: Al Viro Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, zhengyu.duan@shopee.com, Haosong Huang Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org > in RCU mode we really, really should not assume ->d_inode stable. Got it, but looks like the ->d_inode is NULL after out of RCU. In `lookup_fast` and `walk_component` ``` dentry = __d_lookup_rcu(parent, &nd->last, &seq); ... *inode = d_backing_inode(dentry); ``` ``` static int walk_component(struct nameidata *nd, int flags) ... err = lookup_fast(nd, &path, &inode, &seq); if (unlikely(err <= 0)) { ... path.dentry = lookup_slow(&nd->last, nd->path.dentry, nd->flags); ... seq = 0; /* we are already out of RCU mode */ inode = d_backing_inode(path.dentry); } ``` On Mon, Apr 26, 2021 at 2:00 AM haosdent wrote: > > > In the kernels of 4.8..4.18 period there it used to do > > so, but only in non-RCU mode (which is the reason for explicit rcu argument passed > > through that callchain). > > Yep, we saw the `inode` parameter pass to `__atime_needs_update` is already NULL > > ``` > bool __atime_needs_update(const struct path *path, struct inode *inode, > bool rcu) > { > struct vfsmount *mnt = path->mnt; > struct timespec now; > > if (inode->i_flags & S_NOATIME) <=== Oops at here because the params > inode is NULL > return false; > ``` > > ``` > [exception RIP: __atime_needs_update+5] > ... **RSI: 0000000000000000** <=== the second params of > __atime_needs_update "struct inode *inode" is NULL > ``` > > On Mon, Apr 26, 2021 at 1:22 AM Al Viro wrote: > > > > On Mon, Apr 26, 2021 at 01:04:46AM +0800, haosdent wrote: > > > Hi, Alexander, thanks a lot for your quick reply. > > > > > > > Not really - the crucial part is ->d_count == -128, i.e. it's already past > > > > __dentry_kill(). > > > > > > Thanks a lot for your information, we would check this. > > > > > > > Which tree is that? > > > > If you have some patches applied on top of that... > > > > > > We use Ubuntu Linux Kernel "4.15.0-42.45~16.04.1" from launchpad directly > > > without any modification, the mapping Linux Kernel should be > > > "4.15.18" according > > > to https://people.canonical.com/~kernel/info/kernel-version-map.html > > > > Umm... OK, I don't have it Ubuntu source at hand, but the thing to look into > > would be > > * nd->flags contains LOOKUP_RCU > > * in the mainline from that period (i.e. back when __atime_needs_update() > > used to exist) we had atime_needs_update_rcu() called in get_link() under those > > conditions, with > > static inline bool atime_needs_update_rcu(const struct path *path, > > struct inode *inode) > > { > > return __atime_needs_update(path, inode, true); > > } > > and __atime_needs_update() passing its last argument (rcu:true in this case) to > > relatime_need_update() in > > if (!relatime_need_update(path, inode, now, rcu)) > > relatime_need_update() hitting > > update_ovl_inode_times(path->dentry, inode, rcu); > > and update_ovl_inode_times() starting with > > if (rcu || likely(!(dentry->d_flags & DCACHE_OP_REAL))) > > return; > > with subsequent accesses to ->d_inode. Those obviously are *NOT* supposed > > to be reached in rcu mode, due to that check. > > > > Your oops looks like something similar to that call chain had been involved and > > somehow had managed to get through to those ->d_inode uses. > > > > Again, in RCU mode we really, really should not assume ->d_inode stable. That's > > why atime_needs_update() gets inode as a separate argument and does *NOT* look > > at path->dentry at all. In the kernels of 4.8..4.18 period there it used to do > > so, but only in non-RCU mode (which is the reason for explicit rcu argument passed > > through that callchain). > > > > -- > Best Regards, > Haosdent Huang -- Best Regards, Haosdent Huang