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

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Date:   Fri, 02 Nov 2018 01:26:12 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Juergen Gross" <jgross@...e.com>
Cc:     "Stefano Stabellini" <sstabellini@...nel.org>,
        "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <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

>>> On 01.11.18 at 17:27, <jgross@...e.com> wrote:
> On 01/11/2018 16:50, Jan Beulich wrote:
>>>>> Juergen Gross <jgross@...e.com> 11/01/18 3:23 PM >>>
>>> 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?
>> 
>> Possibly, but how does this help? As soon as it's unprivileged it must not
>> be able to hog resources anymore.
>> 
>> Anyway, with Andrew's reply my point may be irrelevant, but I have to
>> admit I'm not entirely sure.
> 
> I guess we want Xen tools to close /dev/xen/hypercall (or more precise:
> don't dup2() it) when qemu is de-privileging itself. This will make it
> very clear that it can't hog memory via mmap().
> 
> When you are fine with that I'll send a Xen patch for this.

If that doesn't prevent the process from making the hypercalls it
is permitted to do (I have to admit I don't recall if there are any
still needed besides the dmop ones), sure.

Jan