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

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Date:   Fri, 2 Nov 2018 10:53:45 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        lkml <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH] xen: remove size limit of privcmd-buf mapping
 interface

On 02/11/2018 08:26, Jan Beulich wrote:
>>>> 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.

Turns out that is already done: the restrict_all callback of libxencall
will associate /dev/null with the file descriptor of /dev/xen/hypercall.


Juergen