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 lists1p.gnu.org (lists1p.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 3994EC43458 for ; Mon, 29 Jun 2026 14:05:03 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1weCbV-0006SI-3F; Mon, 29 Jun 2026 10:04:53 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1weCbP-0006Pe-69 for qemu-devel@nongnu.org; Mon, 29 Jun 2026 10:04:48 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1weCbN-0001wM-6u for qemu-devel@nongnu.org; Mon, 29 Jun 2026 10:04:46 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1782741884; h=from:from:reply-to: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=JbsW93YYU3tw/Ze7G2E1IFppwyWu2kha8gQ/Nfnv4sw=; b=N73DSR8ilwYnF7GNjPjur6UUAMtdLM/nxA3kyBL3Y+JcBIVfBHev17+1ila7/Theoh6oHR P88UKo1Ew3fNl5+jr/uAdI3gjTOvfZJuJSC1pFkcIdGI07Lu23uaKMI6sz9p84KdYHpGMS 6wSViW2qX7BSgrHfi5CyPueqov1Ka2Y= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-257-nYt-jPWcM-uUMF-0sKt7Yw-1; Mon, 29 Jun 2026 10:04:41 -0400 X-MC-Unique: nYt-jPWcM-uUMF-0sKt7Yw-1 X-Mimecast-MFC-AGG-ID: nYt-jPWcM-uUMF-0sKt7Yw_1782741880 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 3E30D1800365; Mon, 29 Jun 2026 14:04:40 +0000 (UTC) Received: from redhat.com (unknown [10.44.50.121]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 657CB3000B4D; Mon, 29 Jun 2026 14:04:38 +0000 (UTC) Date: Mon, 29 Jun 2026 15:04:35 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Maydell Cc: qemu-devel@nongnu.org, Paolo Bonzini , "Michael S. Tsirkin" Subject: Re: [PATCH] hw/dma/i8257: Return zeroes for read_memory in verify mode Message-ID: References: <20260629140128.1900095-1-peter.maydell@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260629140128.1900095-1-peter.maydell@linaro.org> User-Agent: Mutt/2.3.2 (2026-04-26) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-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: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, Jun 29, 2026 at 03:01:28PM +0100, Peter Maydell wrote: > The i8257 DMA controller has a "verify" mode, which the datasheet > describes like this: > > > DMA verify, which does not actually involve the transfer of data. > > When an 8257 channel is in the DMA verify mode, it will respond the > > same as described for transfer operations, except that no memory or > > I/O read/write control signals will be generated. When an 8257 > > channel is in the DMA verify mode, it will respond the same as > > described for transfer operations, except that no memory or I/O read > > /write control signals will be generated, thus preventing the > > transfer of data. The 8257, however, will gain control of the system > > bus and will acknowledge the peripheral's DMA request for each DMA > > cycle. The perihperal can use these acknowledge signals to enable an > > internal access of each byte of a data block in order to execute some > > verification procedure, such as the accumulation of a CRC check word. > > In practice, for QEMU's purposes the only real user of this is the > floppy controller, which can be made to perform a "read data from > floppy disk and check the checksum" by telling the fdc to do a read > and the DMA controller to do a verify. This causes the fdc to do all > the usual read actions including the checksum, but the data is never > written to memory. However, it is possible for a guest doing > something silly to program the DMA controller to do a verify > operation for a device that wants to read from memory. Currently we > simply return early from i8257_dma_read_memory() without writing to > the buffer. None of the callers (the GUS, sb16 and cs4231a sound > cards, plus the fdc) expect this, so they will take the uninitialized > data as if it were from the guest. This can cause us to leak host > data off the stack into the guest. > > Make i8257_dma_read_memory() fill the buffer with zeroes rather > than leaving it untouched for a verify operation. > > Cc: qemu-stable@nongnu.org > Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3487 > Signed-off-by: Peter Maydell > --- > hw/dma/i8257.c | 13 +++++++++++++ > 1 file changed, 13 insertions(+) Reviewed-by: Daniel P. Berrangé With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|