From patchwork Thu Nov 9 03:22:07 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: NeilBrown X-Patchwork-Id: 10050061 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 988DB601EB for ; Thu, 9 Nov 2017 03:25:44 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 914EE28F0E for ; Thu, 9 Nov 2017 03:25:44 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 85E0B2A95E; Thu, 9 Nov 2017 03:25:44 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 2411128F0E for ; Thu, 9 Nov 2017 03:25:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752405AbdKIDZe (ORCPT ); Wed, 8 Nov 2017 22:25:34 -0500 Received: from mx2.suse.de ([195.135.220.15]:56148 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752347AbdKIDZb (ORCPT ); Wed, 8 Nov 2017 22:25:31 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id A2633AC95; Thu, 9 Nov 2017 03:25:29 +0000 (UTC) From: NeilBrown To: Al Viro Date: Thu, 09 Nov 2017 14:22:07 +1100 Subject: [PATCH 3/3] VFS: close race between getcwd() and d_move() Cc: linux-fsdevel@vger.kernel.org, Linus Torvalds , linux-kernel@vger.kernel.org Message-ID: <151019772763.30101.16040338743875884111.stgit@noble> In-Reply-To: <151019756744.30101.3832608128627682973.stgit@noble> References: <151019756744.30101.3832608128627682973.stgit@noble> User-Agent: StGit/0.17.1-dirty MIME-Version: 1.0 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP d_move() will call __d_drop() and then __d_rehash() on the dentry being moved. This creates a small window when the dentry appears to be unhashed. Many tests of d_unhashed() are made under ->d_lock and so are safe from racing with this window, but some aren't. In particular, getcwd() calls d_unlinked() (which calls d_unhashed()) without d_lock protection, so it can race. This races has been seen in practice with lustre, which uses d_move() as part of name lookup. See: https://jira.hpdd.intel.com/browse/LU-9735 It could race with a regular rename(), and result in ENOENT instead of either the 'before' or 'after' name. The race can be demonstrated with a simple program which has two threads, one renaming a directory back and forth while another calls getcwd() within that directory: it should never fail, but does. See: https://patchwork.kernel.org/patch/9455345/ We could fix this race by taking d_lock and rechecking when d_unhashed() reports true. Alternately when can remove the window, which is the approach this patch takes. When __d_drop and __d_rehash are used to move a dentry, an extra flag is passed which causes d_hash.pprev to not be cleared, and to not be tested. Signed-off-by: NeilBrown --- fs/dcache.c | 31 ++++++++++++++++++++----------- 1 file changed, 20 insertions(+), 11 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index d5952306206b..3130d62f29c9 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -471,8 +471,11 @@ static void dentry_lru_add(struct dentry *dentry) * reason (NFS timeouts or autofs deletes). * * __d_drop requires dentry->d_lock. + * ___d_drop takes an extra @moving argument. + * If true, d_hash.pprev is not cleared, so there is no transient d_unhashed() + * state. */ -void __d_drop(struct dentry *dentry) +static void inline ___d_drop(struct dentry *dentry, bool moving) { if (!d_unhashed(dentry)) { struct hlist_bl_head *b; @@ -493,12 +496,18 @@ void __d_drop(struct dentry *dentry) } else hlist_bl_lock(b); __hlist_bl_del(&dentry->d_hash); - dentry->d_hash.pprev = NULL; + if (likely(!moving)) + dentry->d_hash.pprev = NULL; hlist_bl_unlock(b); /* After this call, in-progress rcu-walk path lookup will fail. */ write_seqcount_invalidate(&dentry->d_seq); } } + +void __d_drop(struct dentry *dentry) +{ + ___d_drop(dentry, false); +} EXPORT_SYMBOL(__d_drop); void d_drop(struct dentry *dentry) @@ -2387,10 +2396,10 @@ void d_delete(struct dentry * dentry) } EXPORT_SYMBOL(d_delete); -static void __d_rehash(struct dentry *entry) +static void __d_rehash(struct dentry *entry, bool moving) { struct hlist_bl_head *b = d_hash(entry->d_name.hash); - BUG_ON(!d_unhashed(entry)); + BUG_ON(!moving && !d_unhashed(entry)); hlist_bl_lock(b); hlist_bl_add_head_rcu(&entry->d_hash, b); hlist_bl_unlock(b); @@ -2406,7 +2415,7 @@ static void __d_rehash(struct dentry *entry) void d_rehash(struct dentry * entry) { spin_lock(&entry->d_lock); - __d_rehash(entry); + __d_rehash(entry, false); spin_unlock(&entry->d_lock); } EXPORT_SYMBOL(d_rehash); @@ -2580,7 +2589,7 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode) raw_write_seqcount_end(&dentry->d_seq); fsnotify_update_flags(dentry); } - __d_rehash(dentry); + __d_rehash(dentry, false); if (dir) end_dir_add(dir, n); spin_unlock(&dentry->d_lock); @@ -2642,7 +2651,7 @@ struct dentry *d_exact_alias(struct dentry *entry, struct inode *inode) alias = NULL; } else { __dget_dlock(alias); - __d_rehash(alias); + __d_rehash(alias, false); spin_unlock(&alias->d_lock); } spin_unlock(&inode->i_lock); @@ -2828,8 +2837,8 @@ static void __d_move(struct dentry *dentry, struct dentry *target, /* unhash both */ /* __d_drop does write_seqcount_barrier, but they're OK to nest. */ - __d_drop(dentry); - __d_drop(target); + ___d_drop(dentry, true); + ___d_drop(target, exchange); /* Switch the names.. */ if (exchange) @@ -2838,9 +2847,9 @@ static void __d_move(struct dentry *dentry, struct dentry *target, copy_name(dentry, target); /* rehash in new place(s) */ - __d_rehash(dentry); + __d_rehash(dentry, true); if (exchange) - __d_rehash(target); + __d_rehash(target, true); /* ... and switch them in the tree */ if (IS_ROOT(dentry)) {