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

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Message-ID: <b64f7fa4-a553-bcb4-6051-c91080642b8e@suse.com>
Date:   Thu, 1 Nov 2018 15:23:09 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
        sstabellini@...nel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH] xen: remove size limit of privcmd-buf mapping
 interface

On 01/11/2018 15:18, Jan Beulich wrote:
>>>> 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?

Wouldn't that qemu have opened the node while still being privileged?


Juergen