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

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Date:   Thu, 1 Nov 2018 17:27:18 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...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

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.


Juergen