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=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_GIT 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 DA0CDC282E1 for ; Sat, 20 Apr 2019 00:30:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AFCB020859 for ; Sat, 20 Apr 2019 00:30:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727732AbfDTAaT (ORCPT ); Fri, 19 Apr 2019 20:30:19 -0400 Received: from szxga04-in.huawei.com ([45.249.212.190]:6666 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725280AbfDTAaT (ORCPT ); Fri, 19 Apr 2019 20:30:19 -0400 Received: from DGGEMS404-HUB.china.huawei.com (unknown [172.30.72.59]) by Forcepoint Email with ESMTP id 793F74D7984DBF622F8A; Fri, 19 Apr 2019 16:43:48 +0800 (CST) Received: from huawei.com (10.90.53.225) by DGGEMS404-HUB.china.huawei.com (10.3.19.204) with Microsoft SMTP Server id 14.3.408.0; Fri, 19 Apr 2019 16:43:42 +0800 From: Hou Tao To: , CC: , , Subject: [PATCH] dcache: ensure d_flags & d_inode are consistent in lookup_fast() Date: Fri, 19 Apr 2019 16:48:10 +0800 Message-ID: <20190419084810.63732-1-houtao1@huawei.com> X-Mailer: git-send-email 2.16.2.dirty MIME-Version: 1.0 Content-Type: text/plain X-Originating-IP: [10.90.53.225] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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); -- 2.16.2.dirty