[libvirt] [PATCH v2] lib: Add note that bulk stats API queries may overrun RPC buffers

Michal Privoznik mprivozn at redhat.com
Wed May 24 15:12:06 UTC 2017

On 05/24/2017 04:58 PM, Richard W.M. Jones wrote:
> On Wed, May 24, 2017 at 04:16:45PM +0200, Michal Privoznik wrote:
>> On 05/24/2017 02:47 PM, Richard W.M. Jones wrote:
>>> On Wed, May 24, 2017 at 12:49:58PM +0200, Michal Privoznik wrote:
>>>> That's quite exact. I mean the word 'guessing'. We can't really provide
>>>> reliable way of dealing with what you're suggesting (unless we cut the
>>>> limit really small) nor we can guarantee atomicity. Therefore I think it
>>>> would be a waste of time to work on this. Yes, it can be done, but the
>>>> benefits are pretty small IMO.
>>> Why is atomicity a problem?
>> The atomicity steps in depending on what level we are talking about
>> serialization. If we do it the way Martin suggested (= on public API
>> level) then the list of domains may change between two iterations of the
>> suggested loop. Thus the result obtained would not reflect the reality.
>> However, if we are talking on RPC level, then there's no atomicity
>> problem as RPC is 'blind' to the data (it doesn't care what data is sent).
>>> Just structure the libvirtd
>>> messages so that you have:
>>>     - saves the stats into an internal buffer in libvirtd
>>>       and returns a handle and a number of stat items
>>>     - returns partial subset of previously collected stats,
>>>       called multiple times to transfer the data back to libvirt
>>>     - free the internal buffer
>> This is exactly what I was proposing in my e-mail that I linked in the
>> other thread. You just wrote it more cute. Yet again, what's the
>> gain/advantage of this over one big message?
> In the libguestfs case there is an actual security concern.  The
> daemon runs in the untrusted context of the guest, where a malicious
> guest filesystem or program could (in some cases quite easily) send
> back arbitrarily large messages to the library, tieing up infinite
> resources on the host.
> In libvirt the danger is possibly more on the other side (modified
> library sends infinitely large message to libvirtd).  There's also an
> issue of libvirt connecting to a compromised remote host.
> Whether having a maximum message size prevents all such attacks I'm
> less clear about, but it probably helps against simple ones.

I'm not a security expert but I view both approach the same from
security POV. I mean, I'm all up for limits. Don't get me wrong there.
It's just that we have two options:

a) one API call = one RPC call
b) one API call split across multiple RPC messages

So, it's obvious that in case a) we need bigger limit for the RPC
message to fit all the data. In case b) the limit for the RPC message
can be smaller, but in turn we need limit for the maximum messages per
one API call. Both of these approaches consume about the same memory on
src & dst (I think option b) does consume slightly more because of all
the extra headers sent with each message, but that's not the point).
Now, if I were an attacker I can very well send one big message as well
as N small messages to fill up the buffers. Therefore I think the attack
surface is the same for both of these approaches.

a) is easier to implement IMO.


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