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From: "Jan Beulich" <JBeulich@suse.com>
To: "Juergen Gross" <jgross@suse.com>
Cc: "Stefano Stabellini" <sstabellini@kernel.org>,
	"xen-devel" <xen-devel@lists.xenproject.org>,
	"Boris Ostrovsky" <boris.ostrovsky@oracle.com>,
	<linux-kernel@vger.kernel.org>, <stable@vger.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen: remove size limit of privcmd-buf mapping interface
Date: Fri, 02 Nov 2018 01:26:12 -0600	[thread overview]
Message-ID: <5BDBFC1402000078001F7101@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <12dd8625-2cad-f156-8bae-487e909af412@suse.com>

>>> On 01.11.18 at 17:27, <jgross@suse.com> wrote:
> On 01/11/2018 16:50, Jan Beulich wrote:
>>>>> Juergen Gross <jgross@suse.com> 11/01/18 3:23 PM >>>
>>> On 01/11/2018 15:18, Jan Beulich wrote:
>>>>>>> Juergen Gross <jgross@suse.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



  reply	other threads:[~2018-11-02  7:27 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-01 12:33 Juergen Gross
2018-11-01 14:18 ` [Xen-devel] " Jan Beulich
2018-11-01 14:23   ` Juergen Gross
2018-11-01 15:50     ` Jan Beulich
     [not found]     ` <5BDB20AB020000780014251B@suse.com>
2018-11-01 16:27       ` Juergen Gross
2018-11-02  7:26         ` Jan Beulich [this message]
2018-11-01 14:23   ` Andrew Cooper
2018-11-09  7:03 ` Juergen Gross
2018-11-09 14:23   ` Boris Ostrovsky
2018-11-02  9:53 [Xen-devel] " Juergen Gross

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