linux-kernel - Re: [Xen-devel] [PATCH] xen: remove size limit of privcmd-buf mapping interface

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Message-Id: <5BDB0B240200007800142507@prv1-mh.provo.novell.com>
Date:   Thu, 01 Nov 2018 08:18:12 -0600
From:   "Jan Beulich" <jbeulich@...e.com>
To:     "Juergen Gross" <jgross@...e.com>
Cc:     <sstabellini@...nel.org>, <xen-devel@...ts.xenproject.org>,
        <boris.ostrovsky@...cle.com>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen: remove size limit of privcmd-buf
 mapping interface

>>> Juergen Gross <jgross@...e.com> 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?

Jan