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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 15A35C43331 for ; Fri, 27 Mar 2020 17:43:18 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id D59B0206F1 for ; Fri, 27 Mar 2020 17:43:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="OpS63rHy" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D59B0206F1 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:44970 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jHt0n-0001X6-0Z for qemu-devel@archiver.kernel.org; Fri, 27 Mar 2020 13:43:17 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:34080) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jHt01-00012K-58 for qemu-devel@nongnu.org; Fri, 27 Mar 2020 13:42:30 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jHszz-0008Ry-Dq for qemu-devel@nongnu.org; Fri, 27 Mar 2020 13:42:28 -0400 Received: from us-smtp-delivery-74.mimecast.com ([63.128.21.74]:41779) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jHszz-0008Q7-9z for qemu-devel@nongnu.org; Fri, 27 Mar 2020 13:42:27 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1585330946; 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=0vWKhsSddCOtgM8qeGHumtcIG5bNnoqUnnoDsnmwWgI=; b=OpS63rHyfkv1hNrLhl7NFlpboWvckXLOX1sBbrakHrKIQgW0tqgCb1HPZvx4L+ksDsOVZ4 aOXi/Qx/MjUM0c1W3H7k03anBV9XiSkxsTEuuQBhrlYno4ubv7iFDNi5Zj0WNM1hrnKbtL a8p1B8jScalkxq+d7gMe/RdeXuZIQis= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-270-0jLHd2WmMZigbDFfGjrDgA-1; Fri, 27 Mar 2020 13:42:23 -0400 X-MC-Unique: 0jLHd2WmMZigbDFfGjrDgA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id E7C23800D5E; Fri, 27 Mar 2020 17:42:22 +0000 (UTC) Received: from [10.3.113.103] (ovpn-113-103.phx2.redhat.com [10.3.113.103]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7A39060C18; Fri, 27 Mar 2020 17:42:22 +0000 (UTC) Subject: Re: [PATCH 3/3] nbd: Use shutdown(SHUT_WR) after last item sent To: =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <20200327161936.2225989-1-eblake@redhat.com> <20200327161936.2225989-4-eblake@redhat.com> <20200327163548.GP1619@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <4a56f56e-60b8-6b1f-f805-31a192eb6148@redhat.com> Date: Fri, 27 Mar 2020 12:42:21 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200327163548.GP1619@redhat.com> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 63.128.21.74 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , qemu-devel@nongnu.org, qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 3/27/20 11:35 AM, Daniel P. Berrang=C3=A9 wrote: > On Fri, Mar 27, 2020 at 11:19:36AM -0500, Eric Blake wrote: >> Although the remote end should always be tolerant of a socket being >> arbitrarily closed, there are situations where it is a lot easier if >> the remote end can be guaranteed to read EOF even before the socket >> has closed. In particular, when using gnutls, if we fail to inform >> the remote end about an impending teardown, the remote end cannot >> distinguish between our closing the socket as intended vs. a malicious >> intermediary interrupting things, and may result in spurious error >> messages. >=20 > Does this actually matter in the NBD case ? >=20 > It has an explicit NBD command for requesting shutdown, and once > that's processed, it is fine to just close the socket abruptly - I > don't see a benefit to a TLS shutdown sequence on top. You're right that the NBD protocol has ways for the client to advertise=20 it will be shutting down, AND documents that the server must be robust=20 to clients that just abruptly disconnect after that point. But we don't=20 have control over all such servers, and there may very well be a server=20 that logs an error on abrupt closure, where it would be silent if we did=20 a proper gnutls_bye. Which is more important: maximum speed in=20 disconnecting after we expressed intent, or maximum attempt at catering=20 to all sorts of remote implementations that might not be as tolerant as=20 qemu is of an abrupt termination? > AFAIK, the TLS level clean shutdown is only required if the > application protocol does not have any way to determine an > unexpected shutdown itself. 'man gnutls_bye' states: Note that not all implementations will properly terminate a TLS=20 connec=E2=80=90 tion. Some of them, usually for performance reasons, will=20 terminate only the underlying transport layer, and thus not=20 distinguishing between a malicious party prematurely terminating the=20 connection and normal termination. You're right that because the protocol has an explicit message, we can=20 reliably distinguish any early termination prior to=20 NBD_OPT_ABORT/NBD_CMD_DISC as being malicious, so the only case where it=20 matters is if we have a premature termination after we asked for clean=20 shutdown, at which point a malicious termination didn't lose any data.=20 So on that front, I guess you are right that not using gnutls_bye isn't=20 going to have much impact. >=20 > This is relevant for HTTP where the connection data stream may not > have a well defined end condition. >=20 > In the NBD case though, we have an explicit NBD_CMD_DISC to trigger > the disconnect. After processing that message, an EOF is acceptable > regardless of whether , > before processing that message, any EOF is a unexpected. >=20 >> Or, we can end up with a deadlock where both ends are stuck >> on a read() from the other end but neither gets an EOF. >=20 > If the socket has been closed abruptly why would it get stuck in > read() - it should see EOF surely ? That's what I'm trying to figure out: the nbdkit testsuite definitely=20 hung even though 'qemu-nbd --list' exited, but I haven't yet figured out=20 whether the bug lies in nbdkit proper or in libnbd, nor whether a=20 cleaner tls shutdown would have prevented the hang in a more reliable=20 manner. https://www.redhat.com/archives/libguestfs/2020-March/msg00191.html --=20 Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org