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=-7.4 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,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 1E013C433E0 for ; Mon, 18 Jan 2021 08:00:13 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id C5ABF225AB for ; Mon, 18 Jan 2021 08:00:12 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C5ABF225AB Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.69416.124195 (Exim 4.92) (envelope-from ) id 1l1PS8-0003PY-SW; Mon, 18 Jan 2021 07:59:56 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 69416.124195; Mon, 18 Jan 2021 07:59:56 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1l1PS8-0003PR-Oo; Mon, 18 Jan 2021 07:59:56 +0000 Received: by outflank-mailman (input) for mailman id 69416; Mon, 18 Jan 2021 07:59:55 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1l1PS7-0003PM-Gq for xen-devel@lists.xenproject.org; Mon, 18 Jan 2021 07:59:55 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id e3d3f8c6-81a2-4275-9eca-c932dad413c9; Mon, 18 Jan 2021 07:59:54 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 73060AB7A; Mon, 18 Jan 2021 07:59:53 +0000 (UTC) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: e3d3f8c6-81a2-4275-9eca-c932dad413c9 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1610956793; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Tdtds8PQS/OMj5K5aVjnufqaH2P5oVL/zKE02diqvkE=; b=qsRbJcoGGtyxS+v/7Lh1M9Q91A76ACI2eSKw0LoXSQhwPkOAdtFhYKV3FQ28FvKWApDTFb rIzCP10iwgtqCi0vqhkJfu6PIexc0x7sPzhJUg7f503WoXsPiQ77KweWsiH6fo1+G/I17C 5vctGpchTj8IQ53tWoLe1W3Slj6TL7Y= Subject: Re: [PATCH v3 5/8] xen/hypfs: add support for id-based dynamic directories To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Cc: Andrew Cooper , George Dunlap , Ian Jackson , Julien Grall , Stefano Stabellini , Wei Liu , xen-devel@lists.xenproject.org References: <20201209160956.32456-1-jgross@suse.com> <20201209160956.32456-6-jgross@suse.com> <2894a231-9150-7c09-cc5c-7ef52087acf5@suse.com> <5e0ac85e-ecba-86ad-b350-ff30e3a40a68@suse.com> <18e4b437-6d25-cc3d-5521-11857f461beb@suse.com> From: Jan Beulich Message-ID: Date: Mon, 18 Jan 2021 08:59:52 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.1 MIME-Version: 1.0 In-Reply-To: <18e4b437-6d25-cc3d-5521-11857f461beb@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit On 18.01.2021 08:25, Jürgen Groß wrote: > On 18.12.20 10:09, Jan Beulich wrote: >> On 18.12.2020 09:57, Jürgen Groß wrote: >>> On 17.12.20 13:14, Jan Beulich wrote: >>>> On 17.12.2020 12:32, Jürgen Groß wrote: >>>>> On 17.12.20 12:28, Jan Beulich wrote: >>>>>> On 09.12.2020 17:09, Juergen Gross wrote: >>>>>>> +static const struct hypfs_entry *hypfs_dyndir_enter( >>>>>>> + const struct hypfs_entry *entry) >>>>>>> +{ >>>>>>> + const struct hypfs_dyndir_id *data; >>>>>>> + >>>>>>> + data = hypfs_get_dyndata(); >>>>>>> + >>>>>>> + /* Use template with original enter function. */ >>>>>>> + return data->template->e.funcs->enter(&data->template->e); >>>>>>> +} >>>>>> >>>>>> At the example of this (applies to other uses as well): I realize >>>>>> hypfs_get_dyndata() asserts that the pointer is non-NULL, but >>>>>> according to the bottom of ./CODING_STYLE this may not be enough >>>>>> when considering the implications of a NULL deref in the context >>>>>> of a PV guest. Even this living behind a sysctl doesn't really >>>>>> help, both because via XSM not fully privileged domains can be >>>>>> granted access, and because speculation may still occur all the >>>>>> way into here. (I'll send a patch to address the latter aspect in >>>>>> a few minutes.) While likely we have numerous existing examples >>>>>> with similar problems, I guess in new code we'd better be as >>>>>> defensive as possible. >>>>> >>>>> What do you suggest? BUG_ON()? >>>> >>>> Well, BUG_ON() would be a step in the right direction, converting >>>> privilege escalation to DoS. The question is if we can't do better >>>> here, gracefully failing in such a case (the usual pair of >>>> ASSERT_UNREACHABLE() plus return/break/goto approach doesn't fit >>>> here, at least not directly). >>>> >>>>> You are aware that this is nothing a user can influence, so it would >>>>> be a clear coding error in the hypervisor? >>>> >>>> A user (or guest) can't arrange for there to be a NULL pointer, >>>> but if there is one that can be run into here, this would still >>>> require an XSA afaict. >>> >>> I still don't see how this could happen without a major coding bug, >>> which IMO wouldn't go unnoticed during a really brief test (this is >>> the reason for ASSERT() in hypfs_get_dyndata() after all). >> >> True. Yet the NULL derefs wouldn't go unnoticed either. >> >>> Its not as if the control flow would allow many different ways to reach >>> any of the hypfs_get_dyndata() calls. >> >> I'm not convinced of this - this is a non-static function, and the >> call patch 8 adds (just to take an example) is not very obvious to >> have a guarantee that allocation did happen and was checked for >> success. Yes, in principle cpupool_gran_write() isn't supposed to >> be called in such a case, but it's the nature of bugs assumptions >> get broken. >> >>> I can add security checks at the appropriate places, but I think this >>> would be just dead code. OTOH if you are feeling strong here lets go >>> with it. >> >> Going with it isn't the only possible route. The other is to drop >> the ASSERT()s altogether. It simply seems to me that their addition >> is a half-hearted attempt when considering what was added to >> ./CODING_STYLE not all that long ago. > > IMO The ASSERT()s are serving two purposes here: catching errors > (which, as you stated already, might be catched later in any case), > and documenting for the code reader that the condition they are > testing should always be true and it a violation of it ought not to > happen. > > I can drop the ASSERT() calls, but I think they should be kept due > to their documentation aspect. Well, okay, I can see your point. Keep them in then, despite us coming nowhere close to having similar NULL related asserts in all similar places, I believe. Jan