Re: [PATCH v5 12/16] PCI: Add pci_iomap_host_shared(), pci_iomap_host_shared_range()

Michael S. Tsirkin
2021-10-11T12:00:01Z

On Sun, Oct 10, 2021 at 03:22:39PM -0700, Andi Kleen wrote:
>
> > To which Andi replied
> > One problem with removing the ioremap opt-in is that
> > it's still possible for drivers to get at devices without going through probe.
> >
> > To which Greg replied:
> > https://lore.kernel.org/all/YVXBNJ431YIWwZdQ@kroah.com/
> > If there are in-kernel PCI drivers that do not do this, they need to be
> > fixed today.
> >
> > Can you guys resolve the differences here?
>
>
> I addressed this in my other mail, but we may need more discussion.

Hopefully Greg will reply to that one.

>
> >
> > And once they are resolved, mention this in the commit log so
> > I don't get to re-read the series just to find out nothing
> > changed in this respect?
> >
> > I frankly do not believe we are anywhere near being able to harden
> > an arbitrary kernel config against attack.
>
> Why not? Device filter and the opt-ins together are a fairly strong
> mechanism.

Because it does not end with I/O operations, that's a trivial example.
module unloading is famous for being racy: I just re-read that part of
virtio drivers and sure enough we have bugs there, this is after
they have presumably been audited, so a TDX guest is better off
just disabling hot-unplug completely, and hotplug isn't far behind.
Malicious filesystems can exploit many linux systems unless
you take pains to limit what is mounted and how.
Networking devices tend to get into the default namespaces and can
do more or less whatever CAP_NET_ADMIN can.
Etc.
I am not saying this makes the effort worthless, I am saying userspace
better know very well what it's doing, and kernel better be
configured in a very specific way.

> And it's not that they're a lot of code or super complicated either.
>
> You're essentially objecting to a single line change in your subsystem here.

Well I commented on the API patch, not the virtio patch.
If it's a way for a driver to say "I am hardened
and audited" then I guess it should at least say so. It has nothing
to do with host or sharing, that's an implementation detail,
and it obscures the actual limitations of the approach,
in that eventually in an ideal world all drivers would be secure
and use this API.

Yes, if that's the API that PCI gains then virtio will use it.

> > How about creating a defconfig that makes sense for TDX then?
>
> TDX can be used in many different ways, I don't think a defconfig is
> practical.
>
> In theory you could do some Kconfig dependency (at the pain point of having
> separate kernel binariees), but why not just do it at run time then if you
> maintain the list anyways. That's much easier and saner for everyone. In the
> past we usually always ended up with runtime mechanism for similar things
> anyways.
>
> Also it turns out that the filter mechanisms are needed for some arch
> drivers which are not even configurable, so alone it's probably not enough,

I guess they aren't really needed though right, or you won't try to
filter them? So make them configurable?

>
> > Anyone deviating from that better know what they are doing,
> > this API tweaking is just putting policy into the kernel ...
>
> Hardening drivers is kernel policy. It cannot be done anywhere else.
>
>
> -Andi

To clarify, the policy is which drivers to load into the kernel.

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MST