From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S966877AbcJZMd7 (ORCPT ); Wed, 26 Oct 2016 08:33:59 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:55919 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966846AbcJZMd4 (ORCPT ); Wed, 26 Oct 2016 08:33:56 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Oleg Nesterov , Al Viro Subject: [PATCH 4.4 071/112] fs/super.c: fix race between freeze_super() and thaw_super() Date: Wed, 26 Oct 2016 14:22:54 +0200 Message-Id: <20161026122307.792894841@linuxfoundation.org> X-Mailer: git-send-email 2.10.1 In-Reply-To: <20161026122304.797016625@linuxfoundation.org> References: <20161026122304.797016625@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 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Oleg Nesterov commit 89f39af129382a40d7cd1f6914617282cfeee28e upstream. Change thaw_super() to check frozen != SB_FREEZE_COMPLETE rather than frozen == SB_UNFROZEN, otherwise it can race with freeze_super() which drops sb->s_umount after SB_FREEZE_WRITE to preserve the lock ordering. In this case thaw_super() will wrongly call s_op->unfreeze_fs() before it was actually frozen, and call sb_freeze_unlock() which leads to the unbalanced percpu_up_write(). Unfortunately lockdep can't detect this, so this triggers misc BUG_ON()'s in kernel/rcu/sync.c. Reported-and-tested-by: Nikolay Borisov Signed-off-by: Oleg Nesterov Signed-off-by: Al Viro Signed-off-by: Greg Kroah-Hartman --- fs/super.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) --- a/fs/super.c +++ b/fs/super.c @@ -1326,8 +1326,8 @@ int freeze_super(struct super_block *sb) } } /* - * This is just for debugging purposes so that fs can warn if it - * sees write activity when frozen is set to SB_FREEZE_COMPLETE. + * For debugging purposes so that fs can warn if it sees write activity + * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super(). */ sb->s_writers.frozen = SB_FREEZE_COMPLETE; up_write(&sb->s_umount); @@ -1346,7 +1346,7 @@ int thaw_super(struct super_block *sb) int error; down_write(&sb->s_umount); - if (sb->s_writers.frozen == SB_UNFROZEN) { + if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) { up_write(&sb->s_umount); return -EINVAL; }