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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 83747C43461 for ; Thu, 17 Sep 2020 00:28:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4CDB3206CA for ; Thu, 17 Sep 2020 00:28:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726152AbgIQA2C (ORCPT ); Wed, 16 Sep 2020 20:28:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41456 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726011AbgIQA2A (ORCPT ); Wed, 16 Sep 2020 20:28:00 -0400 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [IPv6:2002:c35c:fd02::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1B099C061756; Wed, 16 Sep 2020 17:22:48 -0700 (PDT) Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kIhh8-0004tj-2A; Thu, 17 Sep 2020 00:22:38 +0000 Date: Thu, 17 Sep 2020 01:22:38 +0100 From: Al Viro To: Eric Biggers Cc: Anant Thazhemadam , linux-kernel-mentees@lists.linuxfoundation.org, syzbot+4191a44ad556eacc1a7a@syzkaller.appspotmail.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] fs: fix KMSAN uninit-value bug by initializing nd in do_file_open_root Message-ID: <20200917002238.GO3421308@ZenIV.linux.org.uk> References: <20200916052657.18683-1-anant.thazhemadam@gmail.com> <20200916054157.GC825@sol.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200916054157.GC825@sol.localdomain> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 15, 2020 at 10:41:57PM -0700, Eric Biggers wrote: > Looking at the actual KMSAN report, it looks like it's nameidata::dir_mode or > nameidata::dir_uid that is uninitialized. You need to figure out the correct > solution, not just blindly initialize with zeroes -- that could hide a bug. > Is there a bug that is preventing these fields from being initialized to the > correct values, are these fields being used when they shouldn't be, etc... False positive, and this is the wrong place to shut it up. ->dir_uid and ->dir_mode are set when link_path_walk() resolves the pathname to directory + final component. They are used when deciding whether to reject a trailing symlink (on fs.protected_symlinks setups) and whether to allow creation in sticky directories (on fs.protected_regular and fs.protected_fifos setups). Both operations really need the results of successful link_path_walk(). I don't see how that could be not a false positive. If we hit the use in may_create_in_sticky(), we'd need the combination of * pathname that consists only of slashes (or it will be initialized) * LAST_NORM in nd->last_type, which is flat-out impossible, since we are left with LAST_ROOT for such pathnames. The same goes for may_follow_link() use - we need WALK_TRAILING in flags to hit it in the first place, which can come from two sources - return walk_component(nd, WALK_TRAILING); in lookup_last() (and walk_component() won't go anywhere near the call chain leading to may_follow_link() without LAST_NORM in nd->last_type) and res = step_into(nd, WALK_TRAILING, dentry, inode, seq); in open_last_lookups(), which also won't go anywhere near that line without LAST_NORM in the nd->last_type. IOW, unless we manage to call that without having called link_path_walk() at all or after link_path_walk() returning an error, we shouldn't hit that. And if we *do* go there without link_path_walk() or with an error from link_path_walk(), we have a much worse problem. I want to see the details of reproducer. If it's for real, we have a much more serious problem; if it's a false positive, the right place to deal with it would be elsewhere (perhaps on return from link_path_walk() with a slashes-only pathname), but in any case it should only be done after we manage to understand what's going on.