Re: [PATCH v2 5/6] qmp: Added new command to retrieve eBPF blob.

[Markus Armbruster <armbru@redhat.com>]

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

> On Tue, May 16, 2023 at 12:23:28PM +0200, Markus Armbruster wrote:
>> Daniel P. Berrangé <berrange@redhat.com> writes:
>> 
>> > On Tue, May 16, 2023 at 10:47:52AM +0200, Markus Armbruster wrote:
>> 
>> [...]
>> 
>> >> So, this is basically a way to retrieve an eBPF program by some
>> >> well-known name.
>> >> 
>> >> Ignorant question: how are these programs desposited?
>> >
>> > The eBPF code blob is linked into QEMU at build time. THis API lets
>> > libvirt fetch it from QEMU, in base64 format. When libvirt later
>> > creates NICs, it can attach the eBPF code blob to the TAP device (which
>> > requires elevated privilleges that QEMU lacks). NB, libvirt would fetch
>> > the eBPF code from QEMU when probing capabilities, as once a VM is
>> > running it is untrusted.
>> 
>> Okay, I can see how that helps.  I trust the blob is in a read-only
>> segment.  Ideally, libvirt fetches it before the guest runs.
>
> Whether the blob is in a read-only segment or not isn't important,
> because it transits writable memory in the QMP command marshalling.

True.  We could bypass marshalling.  Unclean hack.  Or we could sign the
bits cryptograhically.  Key management headaches.  Not worth it, because
fetching it before QEMU becomes untrusted is easier.

However, I now wonder why we fetch it from QEMU.  Why not ship it with
QEMU?

> IOW, if we're trying to mitigate against compromised QEMU, we
> *must* fetch it before vCPUs are started. If we're super paranoid,
> we would want to fetch it before even opening untrusted disk images
> too.  It might push towards fetching it while probing capabilities
> from a throw-away QEMU with "-m none"

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