Re: [PATCH 3/5] qmp: Added the helper stamp check.

[Toke Høiland-Jørgensen]

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

> On Wed, Mar 01, 2023 at 03:53:47PM +0100, Toke Høiland-Jørgensen wrote:
>> Daniel P. Berrangé <berrange@redhat.com> writes:
>> 
>> > On Tue, Feb 28, 2023 at 11:21:56PM +0100, Toke Høiland-Jørgensen wrote:
>> >> Daniel P. Berrangé <berrange@redhat.com> writes:
>> >> 
>> >> > On Tue, Feb 28, 2023 at 08:01:51PM +0100, Toke Høiland-Jørgensen wrote:
>> >> >> Daniel P. Berrangé <berrange@redhat.com> writes:
>> >> >> 
>> >> >> Just to interject a note on this here: the skeleton code is mostly a
>> >> >> convenience feature used to embed BPF programs into the calling binary.
>> >> >> It is perfectly possible to just have the BPF object file itself reside
>> >> >> directly in the file system and just use the regular libbpf APIs to 
>> >> >> load
>> >> >> it. Some things get a bit more cumbersome (mostly setting values of
>> >> >> global variables, if the BPF program uses those).
>> >> >> 
>> >> >> So the JSON example above could just be a regular compiled-from-clang
>> >> >> BPF object file, and the management program can load that, inspect its
>> >> >> contents using the libbpf APIs and pass the file descriptors on to 
>> >> >> Qemu.
>> >> >> It's even possible to embed version information into this so that Qemu
>> >> >> can check if it understands the format and bail out if it doesn't - 
>> >> >> just
>> >> >> stick a version field in the configuration map as the first entry :)
>> >> >
>> >> > If all you have is the BPF object file is it possible to interrogate
>> >> > it to get a list of all the maps, and get FDs associated for them ?
>> >> > I had a look at the libbpf API and wasn't sure about that, it seemed
>> >> > like you had to know the required maps upfront ?  If it is possible
>> >> > to auto-discover everything you need, soley from the BPF object file
>> >> > as input, then just dealing with that in isolation would feel simpler.
>> >> 
>> >> It is. You load the object file, and bpf_object__for_each_map() lets you
>> >> discover which maps it contains, with the different bpf_map__*() APIs
>> >> telling you the properties of that map (and you can modify them too
>> >> before loading the object if needed).
>> >> 
>> >> The only thing that's not in the object file is any initial data you
>> >> want to put into the map(s). But except for read-only maps that can be
>> >> added by userspace after loading the maps, so you could just let Qemu do
>> >> that...
>> >> 
>> >> > It occurrs to me that exposing the BPF program as data rather than
>> >> > via binary will make more practical to integrate this into KubeVirt's
>> >> > architecture. In their deployment setup both QEMU and libvirt are
>> >> > running unprivileged inside a container. For any advanced nmetworking
>> >> > a completely separate component creates the TAP device and passes it
>> >> > into the container running QEMU. I don't think that the separate
>> >> > precisely matched helper binary would be something they can use, but
>> >> > it might be possible to expose a data file providing the BPF program
>> >> > blob and describing its maps.
>> >> 
>> >> Well, "a data file providing the BPF program blob and describing its
>> >> maps" is basically what a BPF .o file is. It just happens to be encoded
>> >> in ELF format :)
>> >> 
>> >> You can embed it into some other data structure and have libbpf load it
>> >> from a blob in memory as well as from the filesystem, though; that is
>> >> basically what the skeleton file does (notice the big character string
>> >> at the end, that's just the original .o file contents).
>> >
>> > Ok, in that case I'm really wondering why any of this helper program
>> > stuff was proposed. I recall the rationale was that it was impossible
>> > for an external program to load the BPF object on behalf of QEMU,
>> > because it would not know how todo that without QEMU specific
>> > knowledge.
>> 
>> I'm not sure either. Was there some bits that initially needed to be set
>> before the program was loaded (read-only maps or something)? Also,
>> upstream does encourage the use of skeletons for embedding into
>> applications, so it's not an unreasonable thing to start with if you
>> don't have the kind of deployment constraints that Qemu does in this
>> case.
>> 
>> > It looks like we can simply expose the BPF object blob to mgmt apps
>> > directly and get rid of this helper program entirely.
>> 
>> I believe so, yes. You'd still need to be sure that the BPF object file
>> itself comes from a trusted place, but hopefully it should be enough to
>> load it from a known filesystem path? (Sorry if this is a stupid
>> question, I only have a fuzzy idea of how all the pieces fit together
>> here).
>
> It could be from a well known location on the filesystem, but might
> be better to make it possible to query it from QMP, which is mostly
> safe *provided* you've not yet started guest CPUs running. It could
> be queried at startup and then cached for future use.

Right, I don't have a strong opinion about the exact mechanism, just
wanted to convey a general "loading an untrusted BPF program is bad"
kind of vibe ;)

-Toke