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=-19.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,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 3E113C43331 for ; Wed, 30 Dec 2020 13:04:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0F21522A84 for ; Wed, 30 Dec 2020 13:04:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727054AbgL3NE1 (ORCPT ); Wed, 30 Dec 2020 08:04:27 -0500 Received: from mail.kernel.org ([198.145.29.99]:53388 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726749AbgL3NEY (ORCPT ); Wed, 30 Dec 2020 08:04:24 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 5C045223E8; Wed, 30 Dec 2020 13:03:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1609333416; bh=NqEGqp1novZlMhQPthLuK9lzcA9lKdPOjw64y0wArsg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=l+ptVpuEIKHpA85YGvJHshuFy4UrIxls5unzpq22404LLdCOh5ngVNAz5sgNUzkB7 jFEGKFUw9/evMXKenMZ1FhLJ2WUoE89TjAf8lXbU8yhD9tkDjRbcvEXsaEyY5cTni0 rdb3sVhgz1ob66rALqA+FvPElqvBBBvUYHSzyGj5nF4tg/BnqktAq0vLd0sIWQPAKL Y5gJEVyhNqS0WRJYO/8ZfzXA++7UL/drXhMtPEyaH01qDpkeArWXUaiIjx3jolebeV etiSzMRT91dWtJioxjRM4HRb3VzH4blcIAimMBPPmWpAUsEIFy/mV3/Fl49UJ3rx5+ imaLVC1bf7a7A== From: Sasha Levin To: linux-kernel@vger.kernel.org, stable@vger.kernel.org Cc: Eric Biggers , Miklos Szeredi , Al Viro , Sasha Levin , linux-fsdevel@vger.kernel.org Subject: [PATCH AUTOSEL 5.10 16/31] fs/namespace.c: WARN if mnt_count has become negative Date: Wed, 30 Dec 2020 08:02:58 -0500 Message-Id: <20201230130314.3636961-16-sashal@kernel.org> X-Mailer: git-send-email 2.27.0 In-Reply-To: <20201230130314.3636961-1-sashal@kernel.org> References: <20201230130314.3636961-1-sashal@kernel.org> MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Eric Biggers [ Upstream commit edf7ddbf1c5eb98b720b063b73e20e8a4a1ce673 ] Missing calls to mntget() (or equivalently, too many calls to mntput()) are hard to detect because mntput() delays freeing mounts using task_work_add(), then again using call_rcu(). As a result, mnt_count can often be decremented to -1 without getting a KASAN use-after-free report. Such cases are still bugs though, and they point to real use-after-frees being possible. For an example of this, see the bug fixed by commit 1b0b9cc8d379 ("vfs: fsmount: add missing mntget()"), discussed at https://lkml.kernel.org/linux-fsdevel/20190605135401.GB30925@xxxxxxxxxxxxxxxxxxxxxxxxx/T/#u. This bug *should* have been trivial to find. But actually, it wasn't found until syzkaller happened to use fchdir() to manipulate the reference count just right for the bug to be noticeable. Address this by making mntput_no_expire() issue a WARN if mnt_count has become negative. Suggested-by: Miklos Szeredi Signed-off-by: Eric Biggers Signed-off-by: Al Viro Signed-off-by: Sasha Levin --- fs/namespace.c | 9 ++++++--- fs/pnode.h | 2 +- 2 files changed, 7 insertions(+), 4 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index cebaa3e817940..93006abe7946a 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -156,10 +156,10 @@ static inline void mnt_add_count(struct mount *mnt, int n) /* * vfsmount lock must be held for write */ -unsigned int mnt_get_count(struct mount *mnt) +int mnt_get_count(struct mount *mnt) { #ifdef CONFIG_SMP - unsigned int count = 0; + int count = 0; int cpu; for_each_possible_cpu(cpu) { @@ -1139,6 +1139,7 @@ static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput); static void mntput_no_expire(struct mount *mnt) { LIST_HEAD(list); + int count; rcu_read_lock(); if (likely(READ_ONCE(mnt->mnt_ns))) { @@ -1162,7 +1163,9 @@ static void mntput_no_expire(struct mount *mnt) */ smp_mb(); mnt_add_count(mnt, -1); - if (mnt_get_count(mnt)) { + count = mnt_get_count(mnt); + if (count != 0) { + WARN_ON(count < 0); rcu_read_unlock(); unlock_mount_hash(); return; diff --git a/fs/pnode.h b/fs/pnode.h index 49a058c73e4c7..26f74e092bd98 100644 --- a/fs/pnode.h +++ b/fs/pnode.h @@ -44,7 +44,7 @@ int propagate_mount_busy(struct mount *, int); void propagate_mount_unlock(struct mount *); void mnt_release_group_id(struct mount *); int get_dominating_id(struct mount *mnt, const struct path *root); -unsigned int mnt_get_count(struct mount *mnt); +int mnt_get_count(struct mount *mnt); void mnt_set_mountpoint(struct mount *, struct mountpoint *, struct mount *); void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp, -- 2.27.0