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=-12.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 84D3AC433B4 for ; Mon, 17 May 2021 15:35:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 60C0C6113C for ; Mon, 17 May 2021 15:35:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242765AbhEQPhJ (ORCPT ); Mon, 17 May 2021 11:37:09 -0400 Received: from mx2.suse.de ([195.135.220.15]:57638 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239215AbhEQPfC (ORCPT ); Mon, 17 May 2021 11:35:02 -0400 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=1621265625; 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=P6+oiXgUBbiaQ/XIdIi+J5lKOSw2bw8dPTfk4MM0uUE=; b=u0zDqmEJFdoLYXoTYowO0v/J27aISHzHszyWtgmZuKH+yd+ZSrOxGCOf0HZARqivSnMxJS NNX8CgyZW4gE/h3faxtUuyR6KAZVvVY+yYcSZ735UPBNopr+2siAxh+7xs/1wfEaQAM5nW iGm5n19sIIzLG48c6IV2RHhLcNXVzNw= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 4AF1DB27A; Mon, 17 May 2021 15:33:44 +0000 (UTC) Subject: Re: [PATCH 4/8] xen/blkfront: don't trust the backend response data blindly To: Juergen Gross Cc: Konrad Rzeszutek Wilk , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Boris Ostrovsky , Stefano Stabellini , Jens Axboe , xen-devel@lists.xenproject.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <20210513100302.22027-1-jgross@suse.com> <20210513100302.22027-5-jgross@suse.com> <315ad8b9-8a98-8d3e-f66c-ab32af2731a8@suse.com> <6095c4b9-a9bb-8a38-fb6c-a5483105b802@suse.com> <030ef85e-b5af-f46e-c8dc-88b8d195c4e1@suse.com> From: Jan Beulich Message-ID: <477f01cd-8793-705c-10f9-cf0c0cd6ed84@suse.com> Date: Mon, 17 May 2021 17:33:42 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: <030ef85e-b5af-f46e-c8dc-88b8d195c4e1@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 17.05.2021 17:22, Juergen Gross wrote: > On 17.05.21 17:12, Jan Beulich wrote: >> On 17.05.2021 16:23, Juergen Gross wrote: >>> On 17.05.21 16:11, Jan Beulich wrote: >>>> On 13.05.2021 12:02, Juergen Gross wrote: >>>>> @@ -1574,10 +1580,16 @@ static irqreturn_t blkif_interrupt(int irq, void *dev_id) >>>>> spin_lock_irqsave(&rinfo->ring_lock, flags); >>>>> again: >>>>> rp = rinfo->ring.sring->rsp_prod; >>>>> + if (RING_RESPONSE_PROD_OVERFLOW(&rinfo->ring, rp)) { >>>>> + pr_alert("%s: illegal number of responses %u\n", >>>>> + info->gd->disk_name, rp - rinfo->ring.rsp_cons); >>>>> + goto err; >>>>> + } >>>>> rmb(); /* Ensure we see queued responses up to 'rp'. */ >>>> >>>> I think you want to insert after the barrier. >>> >>> Why? The relevant variable which is checked is "rp". The result of the >>> check is in no way depending on the responses themselves. And any change >>> of rsp_cons is protected by ring_lock, so there is no possibility of >>> reading an old value here. >> >> But this is a standard double read situation: You might check a value >> and then (via a separate read) use a different one past the barrier. > > Yes and no. > > rsp_cons should never be written by the other side, and additionally > it would be read multiple times anyway. But I'm talking about rsp_prod, as that's what rp gets loaded from. Jan > So if the other side is writing it, the write could always happen after > the test and before the loop is started. This is no real issue here as > the frontend would very soon stumble over an illegal response (either > no request pending, or some other inconsistency). The test is meant to > have a more detailed error message in case it hits. > > In the end it doesn't really matter, so I can change it. I just wanted > to point out that IMO both variants are equally valid. > > > Juergen >