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=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 75D0AC43331 for ; Tue, 31 Mar 2020 02:10:21 +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 046E220774 for ; Tue, 31 Mar 2020 02:10:20 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 046E220774 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:59320 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jJ6M7-0002yA-UG for qemu-devel@archiver.kernel.org; Mon, 30 Mar 2020 22:10:19 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:51938) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jJ6LD-0002Y4-6s for qemu-devel@nongnu.org; Mon, 30 Mar 2020 22:09:24 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jJ6LA-0002e8-TR for qemu-devel@nongnu.org; Mon, 30 Mar 2020 22:09:22 -0400 Received: from mga17.intel.com ([192.55.52.151]:44666) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jJ6LA-0002ZU-J8 for qemu-devel@nongnu.org; Mon, 30 Mar 2020 22:09:20 -0400 IronPort-SDR: MGk+vndv5ZpthB5FhDR40B8nnGfcgPXzQDcu1FalF8PJtQYsPuCzqr1Do4LhZCDuGH63AijcAS TaINMxF3QniA== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by fmsmga107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Mar 2020 19:09:16 -0700 IronPort-SDR: SfD6hkA9creN0GAcIMNk+WRmik3MOlYOH/73Uw0PEqy6H8gUjFCtEILsV5h7ZDxJmiyyHZTMWt K7TKAYlHGFSw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.72,326,1580803200"; d="scan'208";a="272581793" Received: from joy-optiplex-7040.sh.intel.com (HELO joy-OptiPlex-7040) ([10.239.13.16]) by fmsmga004.fm.intel.com with ESMTP; 30 Mar 2020 19:09:14 -0700 Date: Mon, 30 Mar 2020 21:59:41 -0400 From: Yan Zhao To: Alex Williamson Subject: Re: [PATCH] hw/vfio: let readonly flag take effect for mmaped regions Message-ID: <20200331015941.GD6631@joy-OptiPlex-7040> References: <20200327111934.71066-1-yan.y.zhao@intel.com> <20200327112537.2efd65ac@w520.home> <20200330013527.GA30683@joy-OptiPlex-7040> <20200330063402.GE30683@joy-OptiPlex-7040> <20200330085923.19d7345f@w520.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200330085923.19d7345f@w520.home> User-Agent: Mutt/1.9.4 (2018-02-28) X-detected-operating-system: by eggs.gnu.org: FreeBSD 9.x [fuzzy] X-Received-From: 192.55.52.151 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: , Reply-To: Yan Zhao Cc: "pbonzini@redhat.com" , "Zeng, Xin" , "qemu-devel@nongnu.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Mar 30, 2020 at 10:59:23PM +0800, Alex Williamson wrote: > On Mon, 30 Mar 2020 02:34:02 -0400 > Yan Zhao wrote: > > > On Mon, Mar 30, 2020 at 09:35:27AM +0800, Yan Zhao wrote: > > > On Sat, Mar 28, 2020 at 01:25:37AM +0800, Alex Williamson wrote: > > > > On Fri, 27 Mar 2020 11:19:34 +0000 > > > > yan.y.zhao@intel.com wrote: > > > > > > > > > From: Yan Zhao > > > > > > > > > > currently, vfio regions without VFIO_REGION_INFO_FLAG_WRITE are only > > > > > read-only when VFIO_REGION_INFO_FLAG_MMAP is not set. > > > > > > > > > > regions with flag VFIO_REGION_INFO_FLAG_READ | VFIO_REGION_INFO_FLAG_MMAP > > > > > are only read-only in host page table for qemu. > > > > > > > > > > This patch sets corresponding ept page entries read-only for regions > > > > > with flag VFIO_REGION_INFO_FLAG_READ | VFIO_REGION_INFO_FLAG_MMAP. > > > > > > > > > > accordingly, it ignores guest write when guest writes to the read-only > > > > > regions are trapped. > > > > > > > > > > Signed-off-by: Yan Zhao > > > > > Signed-off-by: Xin Zeng > > > > > --- > > > > > > > > Currently we set the r/w protection on the mmap, do I understand > > > > correctly that the change in the vfio code below results in KVM exiting > > > > to QEMU to handle a write to a read-only region and therefore we need > > > > the memory.c change to drop the write? This prevents a SIGBUS or > > > > similar? > > > yes, correct. the change in memory.c is to prevent a SIGSEGV in host as > > > it's mmaped to read-only. we think it's better to just drop the writes > > > from guest rather than corrupt the qemu. > > > > > > > > > > > Meanwhile vfio_region_setup() uses the same vfio_region_ops for all > > > > regions and vfio_region_write() would still allow writes, so if the > > > > device were using x-no-mmap=on, I think we'd still get a write to this > > > > region and expect the vfio device to drop it. Should we prevent that > > > > write in QEMU as well? > > > yes, it expects vfio device to drop it right now. > > > As the driver sets the flag without VFIO_REGION_INFO_FLAG_WRITE, it should > > > handle it properly. > > > both dropping in qemu and dropping in vfio device are fine to us. > > > we wonder which one is your preference :) > > The kernel and device should always do the right thing, we cannot rely > on the user to honor the mapping, but it's also a reasonable response > from the kernel to kill the process with a SIGSEGV if the user ignores > the protections. So I don't think it's an either/or, the kernel needs > to do the right thing for itself and in this case QEMU should do the > right thing for itself, which is to drop writes for regions that don't > support it. So in general, I agree with your patch. > hi Alex so is there anything I need to do? do I need to add a write dropping in vfio_region_write() too? if yes, do I need to keep the trace_vfio_region_write() before dropping ? Thanks Yan > > > > Can you also identify what device and region requires this so that we > > > > can decide whether this is QEMU 5.0 or 5.1 material? PCI BARs are of > > > > course always R/W and the ROM uses different ops and doesn't support > > > > mmap, so this is a device specific region of some sort. Thanks, > > > > > > > It's a virtual mdev device for which we want to emulate a virtual > > > read-only MMIO BAR. > > > Is there any consideration that PCI BARs have to be R/W ? > > > we didn't find it out in PCI specification. > > What the device chooses to do with writes to a BAR is its own business, > the PCI spec shouldn't try to define that. There's also no PCI spec > mechanism to declare the access protections for an entire BAR, that's > device specific behavior. The current QEMU vfio-pci behavior is > therefore somewhat implicit in knowing this for a directly assigned > device. We can mmap the device and we expect writes to unwritable > registers within that mapping to be dropped. > > For an mdev device, we can't rely on the user honoring the access > protections, ie. the user shouldn't be able to exploit the kernel or > device by doing so, but I also agree that QEMU, as a friendly vfio > user, should avoid unsupported operations and protect itself from how > the kernel may handle the fault. > > Since this mdev device doesn't exist yet, I'm thinking this is QEMU > v5.1 material though. > > > looks MMIO regions in vfio platform are also possible to be read-only and > > mmaped. > > Yes. Thanks, > > Alex >