Re: [PATCH v1 0/7] Implement support for external IPT monitoring

["Michał Leszczyński" <michal.leszczynski@cert.pl>]

----- 17 cze 2020 o 18:27, Tamas K Lengyel tamas.k.lengyel@gmail.com napisał(a):

> On Wed, Jun 17, 2020 at 10:19 AM Andrew Cooper
> <andrew.cooper3@citrix.com> wrote:
>>
>> On 17/06/2020 04:02, Tamas K Lengyel wrote:
>> > On Tue, Jun 16, 2020 at 2:17 PM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> >> On 16/06/2020 19:47, Michał Leszczyński wrote:
>> >>> ----- 16 cze 2020 o 20:17, Andrew Cooper andrew.cooper3@citrix.com napisał(a):
>> >>>
>> >>>> Are there any restrictions on EPT being enabled in the first place?  I'm
>> >>>> not aware of any, and in principle we could use this functionality for
>> >>>> PV guests as well (using the CPL filter).  Therefore, I think it would
>> >>>> be helpful to not tie the functionality to HVM guests, even if that is
>> >>>> the only option enabled to start with.
>> >>> I think at the moment it's not required to have EPT. This patch series doesn't
>> >>> use any translation feature flags, so the output address is always a machine
>> >>> physical address, regardless of context. I will check if it could be easily
>> >>> used with PV.
>> >> If its trivial to add PV support then please do.  If its not, then don't
>> >> feel obliged, but please do at least consider how PV support might look
>> >> in the eventual feature.
>> >>
>> >> (Generally speaking, considering "how would I make this work in other
>> >> modes where it is possible" leads to a better design.)
>> >>
>> >>>> The buffer mapping and creation logic is fairly problematic.  Instead of
>> >>>> fighting with another opencoded example, take a look at the IOREQ
>> >>>> server's use of "acquire resource" which is a mapping interface which
>> >>>> supports allocating memory on behalf of the guest, outside of the guest
>> >>>> memory, for use by control tools.
>> >>>>
>> >>>> I think what this wants is a bit somewhere in domain_create to indicate
>> >>>> that external tracing is used for this domain (and allocate whatever
>> >>>> structures/buffers are necessary), acquire resource to map the buffers
>> >>>> themselves, and a domctl for any necessary runtime controls.
>> >>>>
>> >>> I will check this out, this sounds like a good option as it would remove lots of
>> >>> complexity from the existing ipt_enable domctl.
>> >> Xen has traditionally opted for a "and turn this extra thing on
>> >> dynamically" model, but this has caused no end of security issues and
>> >> broken corner cases.
>> >>
>> >> You can see this still existing in the difference between
>> >> XEN_DOMCTL_createdomain and XEN_DOMCTL_max_vcpus, (the latter being
>> >> required to chose the number of vcpus for the domain) and we're making
>> >> good progress undoing this particular wart (before 4.13, it was
>> >> concerning easy to get Xen to fall over a NULL d->vcpu[] pointer by
>> >> issuing other hypercalls between these two).
>> >>
>> >> There is a lot of settings which should be immutable for the lifetime of
>> >> the domain, and external monitoring looks like another one of these.
>> >> Specifying it at createdomain time allows for far better runtime
>> >> behaviour (you are no longer in a situation where the first time you try
>> >> to turn tracing on, you end up with -ENOMEM because another VM booted in
>> >> the meantime and used the remaining memory), and it makes for rather
>> >> more simple code in Xen itself (at runtime, you can rely on it having
>> >> been set up properly, because a failure setting up will have killed the
>> >> domain already).
>> > I'm not in favor of this being a flag that gets set during domain
>> > creation time. It could certainly be the case that some users would
>> > want this being on from the start till the end but in other cases you
>> > may want to enable it intermittently only for some time in-between
>> > particular events. If it's an on/off flag during domain creation you
>> > pretty much force that choice on the users and while the overhead of
>> > PT is better than say MTF it's certainly not nothing. In case there is
>> > an OOM situation enabling IPT dynamically the user can always just
>> > pause the VM and wait till memory becomes available.
>>
>> There is nothing wrong with having "turn tracing on/off at runtime"
>> hypercalls.  It is specifically what I suggested two posts up in this
>> thread, but it should be limited to the TraceEn bit in RTIT_CTL.
>>
>> What isn't ok is trying to allocate the buffers, write the TOPA, etc on
>> first-enable or first-map, because the runtime complexity of logic like
>> this large, and far too easy to get wrong in security relevant ways.
>>
>> The domain create flag would mean "I wish to use tracing with this
>> domain", and not "I want tracing enabled from the getgo".
> 
> Gotcha, that's reasonable.
> 


I think I also agree with this, i.e. to alloc buffers on domain creation and just enable/disable the feature in runtime. This would remove some complexity from runtime. I think it's usually (always?) known in advance whether we would like to use external monitoring on a domain or not.

I will try to adapt this approach in patch v2.


>>
>> >>>> What semantics do you want for the buffer becoming full?  Given that
>> >>>> debugging/tracing is the goal, I presume "pause vcpu on full" is the
>> >>>> preferred behaviour, rather than drop packets on full?
>> >>>>
>> >>> Right now this is a ring-style buffer and when it would become full it would
>> >>> simply wrap and override the old data.
>> >> How does the consumer spot that the data has wrapped?  What happens if
>> >> data starts getting logged, but noone is listening?  What happens if the
>> >> consumer exits/crashes/etc and stops listening as a consequence?
>> >>
>> >> It's fine to simply state what will happen, and possibly even "don't do
>> >> that then", but the corner cases do at least need thinking about.
>> > AFAIU the current use-case is predominantly to be used in conjunction
>> > with VMI events where you want to be able to see the trace leading up
>> > to a particular vmexit. So in the case when the buffer is wrapped
>> > in-between events and data is lost that's not really of concern.
>>
>> That's all fine.  I imagine the output here is voluminous, and needs
>> help being cut down as much as possible.
>>
>> On a tangent, I presume you'd like to include VM-fork eventually, which
>> ought to include copying the trace buffer on fork?
> 
> I would eventually like to use it to reconstruct the branch history so
> we can update AFL's coverage map with that instead of having to do the
> current breakpoint-singlestep dance. But for that I would only care
> about the trace starting after the fork, so copying the parent's PT
> buffer is not needed. We'll also probably only use PT if the branch
> history is larger than what LBR can hold. I asked Michal to name the
> hypercall interface "vmtrace" for this reason so we can add other
> stuff like LBR later using the same interface (which I already
> implemented in https://github.com/tklengyel/xen/commits/lbr).
> 
> Tamas