Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759432AbcDJS6R (ORCPT ); Sun, 10 Apr 2016 14:58:17 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:52966 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758236AbcDJS6O (ORCPT ); Sun, 10 Apr 2016 14:58:14 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Tejun Heo , Tahsin Erdogan , Jens Axboe Subject: [PATCH 4.5 220/238] writeback, cgroup: fix premature wb_put() in locked_inode_to_wb_and_lock_list() Date: Sun, 10 Apr 2016 11:36:37 -0700 Message-Id: <20160410183508.142679403@linuxfoundation.org> X-Mailer: git-send-email 2.8.0 In-Reply-To: <20160410183456.398741366@linuxfoundation.org> References: <20160410183456.398741366@linuxfoundation.org> User-Agent: quilt/0.64 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1899 Lines: 56 4.5-stable review patch. If anyone has any objections, please let me know. ------------------ From: Tejun Heo commit 614a4e3773148a31f58dc174bbf578ceb63510c2 upstream. locked_inode_to_wb_and_lock_list() wb_get()'s the wb associated with the target inode, unlocks inode, locks the wb's list_lock and verifies that the inode is still associated with the wb. To prevent the wb going away between dropping inode lock and acquiring list_lock, the wb is pinned while inode lock is held. The wb reference is put right after acquiring list_lock citing that the wb won't be dereferenced anymore. This isn't true. If the inode is still associated with the wb, the inode has reference and it's safe to return the wb; however, if inode has been switched, the wb still needs to be unlocked which is a dereference and can lead to use-after-free if it it races with wb destruction. Fix it by putting the reference after releasing list_lock. Signed-off-by: Tejun Heo Fixes: 87e1d789bf55 ("writeback: implement [locked_]inode_to_wb_and_lock_list()") Tested-by: Tahsin Erdogan Signed-off-by: Jens Axboe Signed-off-by: Greg Kroah-Hartman --- fs/fs-writeback.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -281,13 +281,15 @@ locked_inode_to_wb_and_lock_list(struct wb_get(wb); spin_unlock(&inode->i_lock); spin_lock(&wb->list_lock); - wb_put(wb); /* not gonna deref it anymore */ /* i_wb may have changed inbetween, can't use inode_to_wb() */ - if (likely(wb == inode->i_wb)) - return wb; /* @inode already has ref */ + if (likely(wb == inode->i_wb)) { + wb_put(wb); /* @inode already has ref */ + return wb; + } spin_unlock(&wb->list_lock); + wb_put(wb); cpu_relax(); spin_lock(&inode->i_lock); }