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 17071C4332F for ; Thu, 3 Nov 2022 09:13:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230098AbiKCJNK (ORCPT ); Thu, 3 Nov 2022 05:13:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35214 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231272AbiKCJMp (ORCPT ); Thu, 3 Nov 2022 05:12:45 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 53580DF0C; Thu, 3 Nov 2022 02:12:34 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 9BF15B82680; Thu, 3 Nov 2022 09:12:33 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 785AAC43470; Thu, 3 Nov 2022 09:12:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1667466752; bh=h4rD3vVxeaaA5b2KcNeNlSt7tcbzEcaWs1xTxa/6b5o=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=m5HZpgVVrkXMFVeFGK6HhpyLpeErlrBKoSuJdp3WNBEG+cfALg4wbFMWBmG6jscQp K9oj+BXci0SiS39eJT/dG7iCeX9upXYItDbu4WWX1Oc6hGilV7A0Xbr5NJSwS8yOV9 eifhgnSKbCNPVjEfU2VnIWvQl0aFsRoMzDsUQLvhUiXwIeek5zLt5IIWisw3+VH7a1 nSl0S8iTfuWg8lx+ocOW1vEItnvphIZkfE6T3tFvOsK3FIyIjulkuS+5KwKull5Wy0 3yaUinLvNAPLrAWOh8dIkCLnQxdQVqMvugkQpK+nef91K9EaddA4RHzZ2Ja1DgSE4D YOfBYuiBUJtjA== Date: Thu, 3 Nov 2022 10:12:27 +0100 From: Christian Brauner To: Ondrej Mosnacek Cc: 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: <20221103091227.mm2nzjj35dzv4dex@wittgenstein> 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=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: selinux@vger.kernel.org On Thu, Nov 03, 2022 at 10:04:25AM +0100, Ondrej Mosnacek wrote: > On Wed, Nov 2, 2022 at 7:25 PM 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. > > Looks mostly good at first glance. I left comments for some minor > stuff I noticed. > > > 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. > > Technically it would be possible, for example with SELinux if the > audit daemon is dead. Not a likely situation, but I agree it's better > to be safe. > > > (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.) > > That would also run into the conundrum of holding a lock while > (potentially) calling into the LSM subsystem. And would it even fix > the information leak? Unless I'm missing something it would only > prevent a leak of the trusted xattr count, but not the presence of any > trusted xattr. No it wouldn't. I just meant this to illustrate that with your patch we could've made it so that capable() would've only been called once. > > > 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? > > Yes, I can't see any reason why that wouldn't be the best solution. > Why haven't I thought of that? :) > > I guess you will want to submit a patch for it along with your > rhashtable patch to avoid a conflict? Or would you like me to submit > it separately? I think you can send a patch for this separately as we don't need to massage the data structure for this. I think we can reasonably give this a Fixes: 38f38657444d ("xattr: extract simple_xattr code from tmpfs") # no backport But note the "# no backport" as imho it isn't worth backporting this to older kernels unless that's really desirable.