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=-18.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 2A6AEC433E6 for ; Mon, 4 Jan 2021 21:03:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 00B0A2225E for ; Mon, 4 Jan 2021 21:03:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726655AbhADVD3 (ORCPT ); Mon, 4 Jan 2021 16:03:29 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:38816 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725921AbhADVD2 (ORCPT ); Mon, 4 Jan 2021 16:03:28 -0500 X-Greylist: delayed 1415 seconds by postgrey-1.27 at vger.kernel.org; Mon, 04 Jan 2021 16:03:28 EST Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kwWYB-006sAQ-KF; Mon, 04 Jan 2021 20:33:59 +0000 Date: Mon, 4 Jan 2021 20:33:59 +0000 From: Al Viro To: Sargun Dhillon Cc: Christoph Hellwig , linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, Kyle Anderson , Manas Alekar Subject: Re: [PATCH] fs: Validate flags and capabilities before looking up path in ksys_umount Message-ID: <20210104203359.GO3579531@ZenIV.linux.org.uk> References: <20201223102604.2078-1-sargun@sargun.me> <20210104195127.GN3579531@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210104195127.GN3579531@ZenIV.linux.org.uk> Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, Jan 04, 2021 at 07:51:27PM +0000, Al Viro wrote: > On Wed, Dec 23, 2020 at 02:26:04AM -0800, Sargun Dhillon wrote: > > ksys_umount was refactored to into split into another function > > (path_umount) to enable sharing code. This changed the order that flags and > > permissions are validated in, and made it so that user_path_at was called > > before validating flags and capabilities. > > > > Unfortunately, libfuse2[1] and libmount[2] rely on the old flag validation > > behaviour to determine whether or not the kernel supports UMOUNT_NOFOLLOW. > > The other path that this validation is being checked on is > > init_umount->path_umount->can_umount. That's all internal to the kernel. > > > > [1]: https://github.com/libfuse/libfuse/blob/9bfbeb576c5901b62a171d35510f0d1a922020b7/util/fusermount.c#L403 > > [2]: https://github.com/karelzak/util-linux/blob/7ed579523b556b1270f28dbdb7ee07dee310f157/libmount/src/context_umount.c#L813 > > Sorry, I don't like that solution. If nothing else, it turns path_umount() into > a landmine for the future. Yes, we have a regression, yes, we need to do something > about it, but that's not a good way to do that. > > FWIW, I would rather separate the check of flags validity from can_umount() > and lift _that_ into ksys_umount(), with "path_umount() relies upon the > flags being minimally sane" comment slapped at path_umount() definition. > The rest of can_umount() really shouldn't be taken out of there. I mean something like the following; unlike your variant, may_mount() is left where it is. commit a0a6df9afcaf439a6b4c88a3b522e3d05fdef46f Author: Al Viro Date: Mon Jan 4 15:25:34 2021 -0500 umount(2): move the flag validity checks first Unfortunately, there's userland code that used to rely upon these checks being done before anything else to check for UMOUNT_NOFOLLOW support. That broke in 41525f56e256 ("fs: refactor ksys_umount"). Separate those from the rest of checks and move them to ksys_umount(); unlike everything else in there, this can be sanely done there. Reported-by: Sargun Dhillon Fixes: 41525f56e256 ("fs: refactor ksys_umount") Signed-off-by: Al Viro diff --git a/fs/namespace.c b/fs/namespace.c index d2db7dfe232b..9d33909d0f9e 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1713,8 +1713,6 @@ static int can_umount(const struct path *path, int flags) { struct mount *mnt = real_mount(path->mnt); - if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW)) - return -EINVAL; if (!may_mount()) return -EPERM; if (path->dentry != path->mnt->mnt_root) @@ -1728,6 +1726,7 @@ static int can_umount(const struct path *path, int flags) return 0; } +// caller is responsible for flags being sane int path_umount(struct path *path, int flags) { struct mount *mnt = real_mount(path->mnt); @@ -1749,6 +1748,10 @@ static int ksys_umount(char __user *name, int flags) struct path path; int ret; + // basic validity checks done first + if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW)) + return -EINVAL; + if (!(flags & UMOUNT_NOFOLLOW)) lookup_flags |= LOOKUP_FOLLOW; ret = user_path_at(AT_FDCWD, name, lookup_flags, &path);