Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it or drop it?

[HATAYAMA Daisuke]

From: Wen Congyang <address@hidden>
Subject: Re: [Qemu-devel] qmp: dump-guest-memory: -p option has issues, fix it 
or drop it?
Date: Wed, 19 Sep 2012 10:07:04 +0800

> At 09/19/2012 08:18 AM, Luiz Capitulino Wrote:
>> On Tue, 18 Sep 2012 16:13:30 -0500
>> Anthony Liguori <address@hidden> wrote:
>> 
>>> Markus Armbruster <address@hidden> writes:
>>>
>>>> Jan Kiszka <address@hidden> writes:
>>>>
>>>>>>>>>  * The issues discussed in this email plus the fact that the guest
>>>>>>>>>    memory may be corrupted, and the guest may be in real-mode even
>>>>>>>>>    when paging is enabled
>>>>>>>>>
>>>>>>>>
>>>>>>>> Yes, there are some limitations with this option. Jan said that he
>>>>>>>> always use gdb to deal with vmcore, so he needs such information.
>>>>>>>
>>>>>>> The point is to overcome the focus on Linux-only dump processing tools.
>>>>>>
>>>>>> While I don't care for supporting alternate dump processing tools
>>>>>> myself, I certainly don't mind supporting them, as long as the code
>>>>>> satisfies basic safety and reliability requirements.
>>>>>>
>>>>>> This code doesn't, as far as I can tell.
>>>>>
>>>>> It works, thought not under all circumstances.
>>>>
>>>> I don't doubt it works often enough to be useful to somebody.  But basic
>>>> safety and reliability requirements are a bit more than that.  They
>>>> include "don't explode in ways a reasonable user can't be expected to
>>>> foresee".  I don't think a reasonable user can be expected to see that
>>>> -p is safe only for trusted guests.
>>>
>>> We shipped the API, we're not removing it.  Our compatibility isn't
>>> "whatever libvirt is currently using".
>>>
>>> It's perfectly reasonable to ask to document the behavior of the
>>> method.  It's also a trivial patch to qapi-schema.json.
>> 
>> I feel that documenting it is not enough. It would be fine to do that
>> if the worst case was a bad dump file, but the worst case as the
>> code stands right now will affect the host.
>> 
>>> It's unreasonable to ask for an interface to be removed just because it
>>> could be misused when it has a legimitate use-case.
>> 
>> The point is not that it can be misused. The issue we're concerned about
>> is that a malicious guest could cause qemu to allocate dozens of
>> gigabytes of RAM.
> 
> Hmm, I guess the malicious guest's page table provides wrong information.
> We allocate memory to store memory mapping. Each memory mapping needs
> less than 40 bytes memory. The num of memory mapping is less than
> (2^48) / (2^12) = 2^36. And 2^36 * 40 = 64G * 40, too many memory....
> 
> What about this:
> 1. if the num of memory mapping > 100000, we only store 100000 memory
>    mappings.
> 
> 2. The memory mapping which has smaller virtual address will be dropped?
> 
> In this case, the memory we need is less than 10MB. So we will not allocate
> too many memory.
> 

How about dropping making a whole list of memory maps at the same
time, and how about rewriting the code so that it always has at most
one memory mapping by merging virtually consequtive chunks? If
possible, only 40 bytes is needed.

Thanks.
HATAYAMA, Daisuke