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[209.132.180.67]) by mx.google.com with ESMTP id r29si1445815pga.477.2018.11.01.07.28.35; Thu, 01 Nov 2018 07:28:50 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728239AbeKAXbE (ORCPT + 99 others); Thu, 1 Nov 2018 19:31:04 -0400 Received: from smtp.eu.citrix.com ([185.25.65.24]:52768 "EHLO SMTP.EU.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728089AbeKAXbE (ORCPT ); Thu, 1 Nov 2018 19:31:04 -0400 X-IronPort-AV: E=Sophos;i="5.54,452,1534809600"; d="scan'208";a="81274962" Subject: Re: [Xen-devel] [PATCH] xen: remove size limit of privcmd-buf mapping interface To: Jan Beulich , Juergen Gross CC: , , , , References: <20181101123307.8424-1-jgross@suse.com> <5BDB0B240200007800142507@prv1-mh.provo.novell.com> From: Andrew Cooper Openpgp: preference=signencrypt Autocrypt: addr=andrew.cooper3@citrix.com; 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Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <5BDB0B240200007800142507@prv1-mh.provo.novell.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Content-Language: en-GB X-ClientProxiedBy: AMSPEX02CAS02.citrite.net (10.69.22.113) To AMSPEX02CL02.citrite.net (10.69.22.126) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/11/18 14:18, Jan Beulich wrote: >>>> Juergen Gross 11/01/18 1:34 PM >>> >> Currently the size of hypercall buffers allocated via >> /dev/xen/hypercall is limited to a default of 64 memory pages. For live >> migration of guests this might be too small as the page dirty bitmask >> needs to be sized according to the size of the guest. This means >> migrating a 8GB sized guest is already exhausting the default buffer >> size for the dirty bitmap. >> >> There is no sensible way to set a sane limit, so just remove it >> completely. The device node's usage is limited to root anyway, so there >> is no additional DOS scenario added by allowing unlimited buffers. > But is this setting of permissions what we want long term? What about a > de-privileged qemu, which still needs to be able to issue at least dm-op > hypercalls? dm-op very deliberately doesn't have hypercall bounce buffers like this. The kernel is responsible for ensuring that the buffer list passed in the ioctl is safe memory, whether this is bouncing the buffers itself, or simply ensuring that the underlying userspace memory won't disappear across the hypercall. ~Andrew