Re: Hypercall fault injection (Was [PATCH 0/3] xen/domain: More structured teardown)

[Tamas K Lengyel <tamas@tklengyel.com>]

On Tue, Dec 22, 2020 at 12:18 PM Andrew Cooper
<andrew.cooper3@citrix.com> wrote:
>
> On 22/12/2020 15:47, Tamas K Lengyel wrote:
>
>
>
> On Tue, Dec 22, 2020 at 6:14 AM Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>>
>> On 22/12/2020 10:00, Jan Beulich wrote:
>> > On 21.12.2020 20:36, Andrew Cooper wrote:
>> >> Hello,
>> >>
>> >> We have some very complicated hypercalls, createdomain, and max_vcpus a
>> >> close second, with immense complexity, and very hard-to-test error handling.
>> >>
>> >> It is no surprise that the error handling is riddled with bugs.
>> >>
>> >> Random failures from core functions is one way, but I'm not sure that
>> >> will be especially helpful.  In particular, we'd need a way to exclude
>> >> "dom0 critical" operations so we've got a usable system to run testing on.
>> >>
>> >> As an alternative, how about adding a fault_ttl field into the hypercall?
>> >>
>> >> The exact paths taken in {domain,vcpu}_create() are sensitive to the
>> >> hardware, Xen Kconfig, and other parameters passed into the
>> >> hypercall(s).  The testing logic doesn't really want to care about what
>> >> failed; simply that the error was handled correctly.
>> >>
>> >> So a test for this might look like:
>> >>
>> >> cfg = { ... };
>> >> while ( xc_create_domain(xch, cfg) < 0 )
>> >>     cfg.fault_ttl++;
>> >>
>> >>
>> >> The pro's of this approach is that for a specific build of Xen on a
>> >> piece of hardware, it ought to check every failure path in
>> >> domain_create(), until the ttl finally gets higher than the number of
>> >> fail-able actions required to construct a domain.  Also, the test
>> >> doesn't need changing as the complexity of domain_create() changes.
>> >>
>> >> The main con will mostly likely be the invasiveness of code in Xen, but
>> >> I suppose any fault injection is going to be invasive to a certain extent.
>> > While I like the idea in principle, the innocent looking
>> >
>> > cfg = { ... };
>> >
>> > is quite a bit of a concern here as well: Depending on the precise
>> > settings, paths taken in the hypervisor may heavily vary, and hence
>> > such a test will only end up being useful if it covers a wide
>> > variety of settings. Even if the number of tests to execute turned
>> > out to still be manageable today, it may quickly turn out not
>> > sufficiently scalable as we add new settings controllable right at
>> > domain creation (which I understand is the plan).
>>
>> Well - there are two aspects here.
>>
>> First, 99% of all VMs in practice are one of 3 or 4 configurations.  An
>> individual configuration is O(n) time complexity to test with fault_ttl,
>> depending on the size of Xen's logic, and we absolutely want to be able
>> to test these deterministically and to completion.
>>
>> For the plethora of other configurations, I agree that it is infeasible
>> to test them all.  However, a hypercall like this is easy to wire up
>> into a fuzzing harness.
>>
>> TBH, I was thinking of something like
>> https://github.com/intel/kernel-fuzzer-for-xen-project with a PVH Xen
>> and XTF "dom0" poking just this hypercall.  All the other complicated
>> bits of wiring AFL up appear to have been done.
>>
>> Perhaps when we exhaust that as a source of bugs, we move onto fuzzing
>> the L0 Xen, because running on native will give it more paths to
>> explore.  We'd need some way of reporting path/trace data back to AFL in
>> dom0 which might require a bit plumbing.
>
>
> This is a pretty cool idea, I would be very interested in trying this out. If running Xen nested in a HVM domain is possible (my experiments with nested setups using Xen have only worked on ancient hw last time I tried) then running the fuzzer would be entirely possible using VM forks. You don't even need a special "dom0", you could just add the fuzzer's CPUID harness to Xen's hypercall handler and the only thing needed from the nested dom0 would be to trigger the hypercall with a normal config. The fuzzer would take it from there and replace the config with the fuzzed version directly in VM forks. Defining what to report as a "crash" to AFL would still need to be defined manually for Xen as the current sink points are Linux specific (https://github.com/intel/kernel-fuzzer-for-xen-project/blob/master/src/sink.h), but that should be straight forward.
>
> Also, running the fuzzer with PVH guests hasn't been tested but since all VM forking needs is EPT it should work.
>
>
> Xen running inside Xen definitely works, and is even supported as far as PV-Shim goes (i.e. no nested virt).  That would limit testing to just creation of PV guests at L1, which is plenty to get started with.
>
> Xen nested under Xen does work to a first approximation, and for the purposes of fuzzing areas other than the nested-virt logic, might even be ok.  (I use this configuration for a fair chunk of hvm development).
>

Sounds like there is no blocker on fuzzing any hypercall handler then.
Just have to add the harness, define what code-path needs to be
defined as a sink, and off you go. Should work with PT-based coverage
no problem. If you need to fuzz multiple hypercalls that may require
some tweaking as the PT based coverage doesn't support a change in CR3
right now (it's just a limitation of libxdc that does the PT
decoding). You could always fall-back to the
disassemble/breakpoint/singlestep coverage option but would need to
add vmcall/vmenter instruction to the control-flow instruction list
here https://github.com/intel/kernel-fuzzer-for-xen-project/blob/master/src/tracer.c#L46.

Tamas