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 6D2C8C433FE for ; Thu, 3 Nov 2022 10:53:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230489AbiKCKxL (ORCPT ); Thu, 3 Nov 2022 06:53:11 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57364 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229493AbiKCKxI (ORCPT ); Thu, 3 Nov 2022 06:53:08 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 679F610FC2 for ; Thu, 3 Nov 2022 03:52:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1667472730; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=kIYRkx7Y9W9jKAKjm+1jgkpGJFLvVwfrjVD1dASHnIM=; b=E4EWZz70wbLCSkEcNhgP2x2zmehYqAALcC4yJvPcSLPzcgKGcezNq1U4GtueXH+ZHAy3oU ZphdQWt1BQTyvyPELd4jklGjf4Pukq+tmP8iLEs9gG6zFnNavyjmFPf8HfjhgqdDO8Wk7p FxXrewoq6CvEVYYTq13pIe1tFLLqgbg= Received: from mail-pj1-f69.google.com (mail-pj1-f69.google.com [209.85.216.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_128_GCM_SHA256) id us-mta-627-D6p5c_RAOp6FBGdBJetSWg-1; Thu, 03 Nov 2022 06:52:09 -0400 X-MC-Unique: D6p5c_RAOp6FBGdBJetSWg-1 Received: by mail-pj1-f69.google.com with SMTP id m2-20020a17090a730200b0021020cce6adso3396678pjk.3 for ; Thu, 03 Nov 2022 03:52:09 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=kIYRkx7Y9W9jKAKjm+1jgkpGJFLvVwfrjVD1dASHnIM=; b=cG1Iv8djocG1ETwUAfEKufJt10lS0k+UcsMNAcEPJLAO3DrSHDp/seIm0LwucBMxm+ ODydYoB0XDYRLU3UKkVBWYyRRsoWr/jtepjLMYdXy+oS7slt+56hREFp1fC+uEqeZU2g ThO7RKf0trprauPZA7d7V5KUAQLUrc5R0oKZx1nkmWKk8fjLIXS5eMPH4OJr7u5dpc9J 5wTtkA/suSaaUs/6DwHTBPBHTg0+lUMNvcFyE2ldp2H9K+5SvuWGJLWxWj49Mgc/caNW dgm7h3lmFnTL7ZImOGkrp9YBmRya8HDzjnhCruX+PD2JoYK6/PuO+9e7uw9Q9lrecStc 5axw== X-Gm-Message-State: ACrzQf1qhJpeQ5i4A5sp+Eg9n7nQJxHeWHA1+4FLdHkl7azAb1RpMkgs q1s+k7FMYY9cpapr3tkMhDzqmblsc1xsZRTBUhnkz5WWNw/YpCi89W4FsUA4ePtXREI9WRMhEjW xh90CPZt1Av2Yfa7ijd8IW8UUSYZAPr2pud7ZVyskeg== X-Received: by 2002:a17:902:ab45:b0:186:7b95:f767 with SMTP id ij5-20020a170902ab4500b001867b95f767mr30293344plb.107.1667472728481; Thu, 03 Nov 2022 03:52:08 -0700 (PDT) X-Google-Smtp-Source: AMsMyM7s7znX4hyAsgjTBM4wVFx4O9ZyTZG5tNktnMvuG2E5V91rnNMU833toUePw5i7OC9qbOH0dlZoLSD+/0g/U08= X-Received: by 2002:a17:902:ab45:b0:186:7b95:f767 with SMTP id ij5-20020a170902ab4500b001867b95f767mr30293310plb.107.1667472728194; Thu, 03 Nov 2022 03:52:08 -0700 (PDT) MIME-Version: 1.0 References: <20220901152632.970018-1-omosnace@redhat.com> <20220905090811.ocnnc53y2bow7m3i@wittgenstein> <20220905153036.zzcovknz7ntgcn5f@wittgenstein> <20221102182451.aoos5udhf6rbb6us@wittgenstein> <20221103091227.mm2nzjj35dzv4dex@wittgenstein> In-Reply-To: <20221103091227.mm2nzjj35dzv4dex@wittgenstein> From: Ondrej Mosnacek Date: Thu, 3 Nov 2022 11:51:56 +0100 Message-ID: Subject: Re: [PATCH 0/2] fs: fix capable() call in simple_xattr_list() To: Christian Brauner Cc: Vasily Averin , Alexander Viro , Linux FS Devel , Linux Security Module list , SElinux list , rcu@vger.kernel.org, Martin Pitt Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Nov 3, 2022 at 10:12 AM Christian Brauner wrote: > 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. Ok, will do. > 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. Actually, it would be valuable to have it backported to linux-stable at least, since we have users encountering this on Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=2122888 In the end it's up to the backporter to assess each commit, but at least I wouldn't want to outright discourage the backport in the commit message. -- Ondrej Mosnacek Senior Software Engineer, Linux Security - SELinux kernel Red Hat, Inc.