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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 A6195C34026 for ; Tue, 18 Feb 2020 18:18:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 85A84208C4 for ; Tue, 18 Feb 2020 18:18:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726291AbgBRSSx (ORCPT ); Tue, 18 Feb 2020 13:18:53 -0500 Received: from orbyte.nwl.cc ([151.80.46.58]:50156 "EHLO orbyte.nwl.cc" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726239AbgBRSSx (ORCPT ); Tue, 18 Feb 2020 13:18:53 -0500 Received: from n0-1 by orbyte.nwl.cc with local (Exim 4.91) (envelope-from ) id 1j47SN-0000Y3-Dy; Tue, 18 Feb 2020 19:18:51 +0100 Date: Tue, 18 Feb 2020 19:18:51 +0100 From: Phil Sutter To: Pablo Neira Ayuso Cc: netfilter-devel@vger.kernel.org Subject: Re: [libnftnl PATCH] src: Fix nftnl_assert() on data_len Message-ID: <20200218181851.GC20005@orbyte.nwl.cc> Mail-Followup-To: Phil Sutter , Pablo Neira Ayuso , netfilter-devel@vger.kernel.org References: <20200214172417.11217-1-phil@nwl.cc> <20200214173247.2wbrvcqilqfmcqq5@salvia> <20200214173450.GR20005@orbyte.nwl.cc> <20200214174200.4xrvnlb72qebtvnb@salvia> <20200215004311.GS20005@orbyte.nwl.cc> <20200215131713.5gwn4ayk2udjff33@salvia> <20200215225855.GU20005@orbyte.nwl.cc> <20200218134227.yndixbtxjzq3jznk@salvia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200218134227.yndixbtxjzq3jznk@salvia> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: netfilter-devel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netfilter-devel@vger.kernel.org Hi Pablo, On Tue, Feb 18, 2020 at 02:42:27PM +0100, Pablo Neira Ayuso wrote: > On Sat, Feb 15, 2020 at 11:58:55PM +0100, Phil Sutter wrote: > > On Sat, Feb 15, 2020 at 02:17:13PM +0100, Pablo Neira Ayuso wrote: > > > On Sat, Feb 15, 2020 at 01:43:11AM +0100, Phil Sutter wrote: > > > > On Fri, Feb 14, 2020 at 06:42:00PM +0100, Pablo Neira Ayuso wrote: > > > > > On Fri, Feb 14, 2020 at 06:34:50PM +0100, Phil Sutter wrote: > > > > > > On Fri, Feb 14, 2020 at 06:32:47PM +0100, Pablo Neira Ayuso wrote: > > > > > > > On Fri, Feb 14, 2020 at 06:24:17PM +0100, Phil Sutter wrote: > > > > > > > > Typical idiom for *_get_u*() getters is to call *_get_data() and make > > > > > > > > sure data_len matches what each of them is returning. Yet they shouldn't > > > > > > > > trust *_get_data() to write into passed pointer to data_len since for > > > > > > > > chains and NFTNL_CHAIN_DEVICES attribute, it does not. Make sure these > > > > > > > > assert() calls trigger in those cases. > > > > > > > > > > > > > > The intention to catch for unset attributes through the assertion, > > > > > > > right? > > > > > > > > > > > > No, this is about making sure that no wrong getter is called, e.g. > > > > > > nftnl_chain_get_u64() with e.g. NFTNL_CHAIN_HOOKNUM attribute which is > > > > > > only 32bits. > > > > > > > > > > I think it will also catch the case I'm asking. If attribute is unset, > > > > > then nftnl_chain_get_data() returns NULL and the assertion checks > > > > > data_len, which has not been properly initialized. > > > > > > > > With nftnl_assert() being (shortened): > > > > > > > > | #define nftnl_assert(val, attr, expr) \ > > > > | ((!val || expr) ? \ > > > > | (void)0 : __nftnl_assert_fail(attr, __FILE__, __LINE__)) > > > > > > > > Check for 'expr' (which is passed as 'data_len == sizeof()') > > > > will only happen if 'val' is not NULL. Callers then return like so: > > > > > > > > | return val ? *val : 0; > > > > > > > > This means that if you pass an unset attribute to the getter, it will > > > > simply return 0. > > > > > > Thanks for explaining, Phil. If the problem is just > > > NFTNL_CHAIN_DEVICES and NFTNL_FLOWTABLE_DEVICES, probably this is just > > > fine? So zero data-length is reversed for arrays and update > > > nftnl_assert() to skip data_len == 0, ie. > > > > > > > | #define nftnl_assert(val, attr, expr) \ > > > > | ((!val || data_len == 0 || expr) ? \ > > > > | (void)0 : __nftnl_assert_fail(attr, __FILE__, __LINE__)) > > > > Your proposed patch would allow to call e.g.: > > > > | nftnl_chain_get_u32(c, NFTNL_CHAIN_DEVICES) > > > > This would return (uint32_t)*(&c->dev_array[0]), I highly doubt we > > should allow this. Unless I miss something, it is certainly a > > programming error if someone calls any of the nftnl_chain_get_{u,s}* > > getters on NFTNL_CHAIN_DEVICES attribute. So aborting with error message > > in nftnl_assert() is not only OK but actually helpful, no? > > Indeed, good point. > > I don't think nftnl_flowtable_set_data() is good for these two device > array. Well, right now it serves as a backend for all attribute setters, and your patch continues in that tradition. So while it may be a bit "rustic", I'd say it's good enough for its purpose. :) > I just sent a patch, I forgot to finish the _set_array() and > _get_array() helpers for the flowtable, the definition in the header > file prooves this. > > Can we introduce these new interfaces? Then, update nftables to use it. > Then, at some point, set *data_len = 0 for these array datatypes. Yes, > it's a bit longer term, but better fix this interface. But setting all > these data_len to zero when in most cases it is going to be thereafter > properly set to the datatype length is... > > Would this work for you? I know it is not so short term. While I think your patch is the right way to providing a sanitized access to the array attributes, I don't think it's really related to what my original patch was fixing, which is: Right now we are preventing users from passing wrong attribute types to getters by checking the attribute length. This does not work for NFTNL_CHAIN_DEVICES or NFTNL_FLOWTABLE_DEVICES because they don't set data_len. Hence the expression in nftnl_asser() call: | nftnl_assert(val, attr, data_len == sizeof()); Will lead to comparing with garbage from stack. This may in most cases fail as expected, but there's no guarantee. Your patch allows to use "a better" getter/setter for those problematic attributes, but it doesn't prevent the above from happening. My first approach was to make nftnl_chain_get_data() and nftnl_flowtable_get_data() set: | *data_len = 0; for the problematic attributes, but the value is not really correct - a "more correct" value, e.g.: | *data_len = c->dev_array_len * sizeof(char *); Could lead to a pass in getter sanitizing by accident although e.g. nftnl_chain_get_u64() is completely unfit even if c->dev_array_len was 1. So I decided to go the safe way and initialize data_len variables to zero instead which has the benefit of catching new attributes added later as well. If you don't like the approach of initializing all data_len variables, I would rather suggest to go with setting '*data_len = 0' in _get_data() routines as described above. This has the same effect but it's just a two lines change. What do you think? Cheers, Phil