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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS autolearn=ham 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 B9A1EC10F11 for ; Mon, 22 Apr 2019 15:03:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 911A920656 for ; Mon, 22 Apr 2019 15:03:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727987AbfDVPDQ (ORCPT ); Mon, 22 Apr 2019 11:03:16 -0400 Received: from szxga07-in.huawei.com ([45.249.212.35]:39158 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727410AbfDVPDP (ORCPT ); Mon, 22 Apr 2019 11:03:15 -0400 Received: from DGGEMS404-HUB.china.huawei.com (unknown [172.30.72.60]) by Forcepoint Email with ESMTP id 4B53AC50CA159CD0BCAF; Mon, 22 Apr 2019 23:02:36 +0800 (CST) Received: from [127.0.0.1] (10.177.31.14) by DGGEMS404-HUB.china.huawei.com (10.3.19.204) with Microsoft SMTP Server id 14.3.439.0; Mon, 22 Apr 2019 23:02:29 +0800 Subject: Re: [PATCH] dcache: ensure d_flags & d_inode are consistent in lookup_fast() To: , CC: , References: <20190419084810.63732-1-houtao1@huawei.com> From: Hou Tao Message-ID: <69981458-59dc-277d-c66d-2620cff7bb57@huawei.com> Date: Mon, 22 Apr 2019 23:02:28 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <20190419084810.63732-1-houtao1@huawei.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.177.31.14] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ping ? On 2019/4/19 16:48, Hou Tao wrote: > After extending the size of dentry from 192-bytes to 208-bytes > under aarch64, we got oops during the running of xfstests generic/429: > > Unable to handle kernel NULL pointer dereference at virtual address 0000000000000002 > CPU: 3 PID: 2725 Comm: t_encrypted_d_r Tainted: G D 5.1.0-rc4 > pc : inode_permission+0x28/0x160 > lr : link_path_walk.part.11+0x27c/0x528 > ...... > Call trace: > inode_permission+0x28/0x160 > link_path_walk.part.11+0x27c/0x528 > path_lookupat+0x64/0x208 > filename_lookup+0xa0/0x178 > user_path_at_empty+0x58/0x70 > vfs_statx+0x94/0x118 > __se_sys_newfstatat+0x58/0x98 > __arm64_sys_newfstatat+0x24/0x30 > el0_svc_common+0x7c/0x148 > el0_svc_handler+0x38/0x88 > el0_svc+0x8/0xc > > If we revert the size extension of dentry, the oops will be gone. > However if we just move the d_inode field from the begin of dentry > struct to the end of dentry struct (poorly simulate the way how > __randomize_layout works), the oops will reoccur. > > The following scenario illustrates the problem: > > precondition: > * dentry A has just been unlinked and becomes a negative dentry > * dentry A is encrypted, so it has d_revalidate hook: fscrypt_d_revalidate() > * lookup process is looking A/file, and creation process is creating A > > lookup process: creation process: > > lookup_fast > __d_lookup_rcu returns dentry A > > d_revalidate returns -ECHILD > > d_revalidate again succeed > __d_set_inode_and_type > dentry->d_inode = inode > WRITE_ONCE(dentry->d_flags, flags) > > d_is_negative(dentry) return false > follow_managed doesn't nothing > // inconsistent with d_flags > d_backing_inode() return NULL > nd->inode = NULL > > may_lookup() > // oops occurs > inode_permission(nd->inode > > The root cause is the inconsistency between d_flags & d_inode > during the REF-walk in lookup_fast(): d_is_negative(dentry) > returns false, but d_backing_inode() still returns a NULL pointer. > > The RCU-walk path in lookup_fast() uses d_seq to ensure d_flags & d_inode > are consistent, and lookup_slow() use inode lock to ensure that, so only > the REF-walk path in lookup_fast() is problematic. > > Fixing it by adding a paired smp_rmb/smp_wmb between the reading/writing > of d_inode & d_flags to ensure the consistency. > > Signed-off-by: Hou Tao > --- > fs/dcache.c | 2 ++ > fs/namei.c | 7 +++++++ > 2 files changed, 9 insertions(+) > > diff --git a/fs/dcache.c b/fs/dcache.c > index aac41adf4743..1eb85f9fcb0f 100644 > --- a/fs/dcache.c > +++ b/fs/dcache.c > @@ -316,6 +316,8 @@ static inline void __d_set_inode_and_type(struct dentry *dentry, > unsigned flags; > > dentry->d_inode = inode; > + /* paired with smp_rmb() in lookup_fast() */ > + smp_wmb(); > flags = READ_ONCE(dentry->d_flags); > flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU); > flags |= type_flags; > diff --git a/fs/namei.c b/fs/namei.c > index dede0147b3f6..833f760c70b2 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -1628,6 +1628,13 @@ static int lookup_fast(struct nameidata *nd, > return -ENOENT; > } > > + /* > + * Paired with smp_wmb() in __d_set_inode_and_type() to ensure > + * d_backing_inode is not NULL after the checking of d_flags > + * in d_is_negative() completes. > + */ > + smp_rmb(); > + > path->mnt = mnt; > path->dentry = dentry; > err = follow_managed(path, nd); >