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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8A65DC433FE for ; Thu, 3 Nov 2022 01:59:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230372AbiKCB7n (ORCPT ); Wed, 2 Nov 2022 21:59:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49412 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230193AbiKCB7m (ORCPT ); Wed, 2 Nov 2022 21:59:42 -0400 Received: from mail.hallyn.com (mail.hallyn.com [178.63.66.53]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6795D5F69; Wed, 2 Nov 2022 18:59:40 -0700 (PDT) Received: by mail.hallyn.com (Postfix, from userid 1001) id A30B8AB3; Wed, 2 Nov 2022 20:59:38 -0500 (CDT) Date: Wed, 2 Nov 2022 20:59:38 -0500 From: "Serge E. Hallyn" To: Christian Brauner Cc: Ondrej Mosnacek , Vasily Averin , Alexander Viro , Linux FS Devel , Linux Security Module list , SElinux list , rcu@vger.kernel.org, Martin Pitt Subject: Re: [PATCH 0/2] fs: fix capable() call in simple_xattr_list() Message-ID: <20221103015938.GA27053@mail.hallyn.com> References: <20220901152632.970018-1-omosnace@redhat.com> <20220905090811.ocnnc53y2bow7m3i@wittgenstein> <20220905153036.zzcovknz7ntgcn5f@wittgenstein> <20221102182451.aoos5udhf6rbb6us@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221102182451.aoos5udhf6rbb6us@wittgenstein> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: selinux@vger.kernel.org On Wed, Nov 02, 2022 at 07:24:51PM +0100, Christian Brauner wrote: > On Mon, Sep 05, 2022 at 05:30:36PM +0200, Christian Brauner wrote: > > On Mon, Sep 05, 2022 at 12:15:01PM +0200, Ondrej Mosnacek wrote: > > > On Mon, Sep 5, 2022 at 11:08 AM Christian Brauner wrote: > > > > On Thu, Sep 01, 2022 at 05:26:30PM +0200, Ondrej Mosnacek wrote: > > > > > The goal of these patches is to avoid calling capable() unconditionally > > > > > in simple_xattr_list(), which causes issues under SELinux (see > > > > > explanation in the second patch). > > > > > > > > > > The first patch tries to make this change safer by converting > > > > > simple_xattrs to use the RCU mechanism, so that capable() is not called > > > > > while the xattrs->lock is held. I didn't find evidence that this is an > > > > > issue in the current code, but it can't hurt to make that change > > > > > either way (and it was quite straightforward). > > > > > > > > Hey Ondrey, > > > > > > > > There's another patchset I'd like to see first which switches from a > > > > linked list to an rbtree to get rid of performance issues in this code > > > > that can be used to dos tmpfs in containers: > > > > > > > > https://lore.kernel.org/lkml/d73bd478-e373-f759-2acb-2777f6bba06f@openvz.org > > > > > > > > I don't think Vasily has time to continue with this so I'll just pick it > > > > up hopefully this or the week after LPC. > > > > > > Hm... does rbtree support lockless traversal? Because if not, that > > > > The rfc that Vasily sent didn't allow for that at least. > > > > > would make it impossible to fix the issue without calling capable() > > > inside the critical section (or doing something complicated), AFAICT. > > > Would rhashtable be a workable alternative to rbtree for this use > > > case? Skimming it seems to support both lockless > > > lookup and traversal using RCU. And according to its manpage, > > > *listxattr(2) doesn't guarantee that the returned names are sorted. > > > > I've never used the rhashtable infrastructure in any meaningful way. All > > I can say from looking at current users that it looks like it could work > > well for us here: > > > > struct simple_xattr { > > struct rhlist_head rhlist_head; > > char *name; > > size_t size; > > char value[]; > > }; > > > > static const struct rhashtable_params simple_xattr_rhashtable = { > > .head_offset = offsetof(struct simple_xattr, rhlist_head), > > .key_offset = offsetof(struct simple_xattr, name), > > > > or sm like this. > > I have a patch in rough shape that converts struct simple_xattr to use > an rhashtable: > > https://gitlab.com/brauner/linux/-/commits/fs.xattr.simple.rework/ > > Light testing, not a lot useful comments and no meaningful commit > message as of yet but I'll get to that. > > Even though your issue is orthogonal to the performance issues I'm > trying to fix I went back to your patch, Ondrej to apply it on top. > But I think it has one problem. > > Afaict, by moving the capable() call from the top of the function into > the actual traversal portion an unprivileged user can potentially learn > whether a file has trusted.* xattrs set. At least if dmesg isn't > restricted on the kernel. That may very well be the reason why the > capable() call is on top. > (Because the straightforward fix for this would be to just call > capable() a single time if at least one trusted xattr is encountered and > store the result. That's pretty easy to do by making turning the trusted > variable into an int, setting it to -1, and only if it's -1 and a > trusted xattr has been found call capable() and store the result.) > > One option to fix all of that is to switch simple_xattr_list() to use > > ns_capable_noaudit(&init_user_ns, CAP_SYS_ADMIN) > > which doesn't generate an audit event. > > I think this is even the correct thing to do as listing xattrs isn't a > targeted operation. IOW, if the the user had used getxattr() to request > a trusted.* xattr then logging a denial makes sense as the user > explicitly wanted to retrieve a trusted.* xattr. But if the user just > requested to list all xattrs then silently skipping trusted without > logging an explicit denial xattrs makes sense. > > Does that sound acceptable? Agreed, auditing that seems like unwanted noise.