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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 8CC0AC32771 for ; Mon, 26 Sep 2022 11:09:47 +0000 (UTC) Received: from localhost ([::1]:40912 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1oclze-0002oA-HG for qemu-devel@archiver.kernel.org; Mon, 26 Sep 2022 07:09:46 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:53436) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1oclaG-0007DF-4k for qemu-devel@nongnu.org; Mon, 26 Sep 2022 06:43:32 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:53353) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1oclaC-0002d9-U9 for qemu-devel@nongnu.org; Mon, 26 Sep 2022 06:43:30 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1664189008; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=HzpRtq6W35i/c0TQsBCIa2MyePXvMDcKXWVUEtAzrUA=; b=g3CaQxx7cvfzn0WPGXPU2eNZF7vcLE+u+O1I5BtCHvCymKn4NaOhUCz69C/cAL2fLLtMGN a6maeI6wSe9N5kdmGV2uF0UtntCjO8Ih72OoO1huoLxK60UZS5mH4R4YtyG9BEquw43E1f XtWMteyUWZcNSNXP+COi25BGW6OKs/0= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-178-1R2U_vk9Nb-ddR4GINhIHg-1; Mon, 26 Sep 2022 06:43:25 -0400 X-MC-Unique: 1R2U_vk9Nb-ddR4GINhIHg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id EDDAD862FDF; Mon, 26 Sep 2022 10:43:24 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.163]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C9690140EBF4; Mon, 26 Sep 2022 10:43:24 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 9617A21E691D; Mon, 26 Sep 2022 12:43:23 +0200 (CEST) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: John Snow , Eric Blake , qemu-devel , Peter Maydell Subject: Re: Maximum QMP reply size References: <87v8pzwgbm.fsf@pond.sub.org> Date: Mon, 26 Sep 2022 12:43:23 +0200 In-Reply-To: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Mon, 26 Sep 2022 09:08:03 +0100") Message-ID: <87zgem8lh0.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Daniel P. Berrang=C3=A9 writes: > On Wed, Sep 07, 2022 at 01:54:05PM +0200, Markus Armbruster wrote: >> John Snow writes: >>=20 >> > Hi, I suspect I have asked this before, but I didn't write it down in >> > a comment, so I forget my justification... >> > >> > In the QMP lib, we need to set a buffering limit for how big a QMP >> > message can be -- In practice, I found that the largest possible >> > response was the QAPI schema reply, and I set the code to this: >> > >> > # Maximum allowable size of read buffer >> > _limit =3D (64 * 1024) >> > >> > However, I didn't document if this was a reasonable limit or just a >> > "worksforme" one. I assume that there's no hard limit for the protocol >> > or the implementation thereof in QEMU. Is there any kind of value here >> > that would be more sensible than another? >> > >> > I'm worried that if replies get bigger in the future (possibly in some >> > degenerate case I am presently unaware of) that the library default >> > will become nonsensical. >> > >> > Any pointers/tips? >>=20 >> Peter and Daniel already provided some. I can add a bit of insight into >> how QMP output works in QEMU, which may or may not help you. >>=20 >> QEMU executes one command after the other. A command's response >> (success or failure) is a QDict. Which is then formatted as JSON and >> appended to the monitor's output buffer. >>=20 >> Events work similarly. >>=20 >> The conversion to JSON does not limit the resulting string's size. If >> it runs out of memory, QEMU dies. >>=20 >> The output buffer is also unbounded. It drains into the monitor's >> character device. >>=20 >> If the QMP client sends enough commands without reading their responses, >> QEMU can run out of memory and die. >>=20 >> Now I'm ready to go back to your question, which is about a *single* >> message (QMP command response or event): nothing in QEMU limits the size >> of the QMP output message text. >>=20 >> Weak consolation: I guess QEMU is somewhat likely to run out of memory >> and die before your client software does. That's because QDict is a >> pig: an empty one eats 4120 Bytes on my system. Compares unfavourable >> to its text representation "{}". > > A malicious QEMU that's trying to attack the mgmt software client > wouldn't need to use QDict, so that's only consolation against > accidents. An evil QEMU would just write JSON directly onto the > monitor chardev. It wouldn't even have to be well formed JSON, > as it could just start a string and never end it. > > {"blah..repeated for 1 TB for data...." Yes, a malicious QEMU should be able to flood the QMP socket with constant memory and negligible CPU overhead.