Re: Maximum QMP reply size

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Fri, Sep 16, 2022 at 04:12:00PM +0100, Peter Maydell wrote:
>> On Thu, 15 Sept 2022 at 16:21, Dr. David Alan Gilbert
>> <dgilbert@redhat.com> wrote:
>> >
>> > * Peter Maydell (peter.maydell@linaro.org) wrote:
>> > > On Tue, 6 Sept 2022 at 20:41, John Snow <jsnow@redhat.com> wrote:
>> > > > Hi, I suspect I have asked this before, but I didn't write it down in
>> > > > a comment, so I forget my justification...
>> > > >
>> > > > In the QMP lib, we need to set a buffering limit for how big a QMP
>> > > > message can be -- In practice, I found that the largest possible
>> > > > response was the QAPI schema reply, and I set the code to this:
>> > > >
>> > > >     # Maximum allowable size of read buffer
>> > > >     _limit = (64 * 1024)
>> > > >
>> > > > However, I didn't document if this was a reasonable limit or just a
>> > > > "worksforme" one. I assume that there's no hard limit for the protocol
>> > > > or the implementation thereof in QEMU. Is there any kind of value here
>> > > > that would be more sensible than another?
>> > > >
>> > > > I'm worried that if replies get bigger in the future (possibly in some
>> > > > degenerate case I am presently unaware of) that the library default
>> > > > will become nonsensical.
>> > >
>> > > There are some QMP commands which return lists of things
>> > > where we put no inherent limit on how many things there
>> > > are in the list, like qom-list-types. We'd have to be getting
>> > > a bit enthusiastic about defining types for that to get
>> > > up towards 64K's worth of response, but it's not inherently
>> > > impossible. I think using human-monitor-command to send
>> > > an 'xp' HMP command is also a way to get back an arbitrarily
>> > > large string (just ask for a lot of memory to be dumped).
>> >
>> > We could put size limits on xp; most Humans will only dump a few kB
>> > maximum like that, any larger and you can dump to file.
>> 
>> Sure, we could, but why? It's not clear to me why a consumer
>> of QMP needs to impose a maximum message size limit on it:
>> I thought it was just JSON. Fixed buffer sizes are very 1980s :-)
>
> Well even if they parse the JSON as it streams in, rather than
> reading the whole doc and then parsing it in one go, you still
> need to have limits on any sane QMP client.
>
> The QEMU process is an untrusted component in the stack, talking
> to a trusted mgmt layer. If the QEMU process sends a 1 TB JSON
> message as a QMP reply, the mgmt layer must not try to parse
> that as they'll let loose the kraken^H^H^H^H^H OOM killer.
>
> To be robust against either a malicious or mis-behaving QEMU
> they need to impose a limit on the size of QMP response they'll
> be willing to process. The challenge is figuring out what limit
> is big enough to handle any conceivable valid message, while
> being small enough to minimize denial of service risks.

Yes.

QEMU does this for QMP input.

Trying to defend against malicious QMP input would of course be
pointless; if you can send QMP input, you "own" QEMU anyway.  It's
defense against *accidents*.

The limits are extremely (overly?) generous: each command is limited to
1024 levels of nesting to protect the stack, 64MiB of total token size
and 2Mi[*] tokens to protect the heap.

> NB, that's not the only thing clients need todo to protect from
> a bad QEMU. Rate limiting consumption is potentially important too
> lest a bad QEMU inflict a DoS on the CPU by sending such frequent
> messages that the QMP client is burning 100% CPU just parsing
> them.  I've not seen any QMP client do this in practice though,
> not even libvirt has attempted it.

What could a management application do when it detects it can't /
doesn't want to keep up with QMP output?

>> If this is a common requirement then should we define something
>> in the protocol where the client says "I can support messages
>> up to this big" and then the server has to split things up?
>
> Splitting doesn't help protect against the DoS, because the QMP
> client would have to reassemble the pieces afterwards to process
> the reply / async event.

Yes.

Can we estimate limits that should suffice?  Documenting them could help
management applications.


[*] If you're curious about this value, see commit df649835fe "qjson:
Limit number of tokens in addition to total size".