Re: [PATCH v2] iommu/arm-smmu: Break insecure users by disabling bypass by default

[Tim Harvey]

On Thu, Oct 3, 2019 at 1:42 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> Hi Tim,
>
> On 2019-10-03 7:27 pm, Tim Harvey wrote:
> > On Fri, Mar 1, 2019 at 11:21 AM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
> >>
> >> If you're bisecting why your peripherals stopped working, it's
> >> probably this CL.  Specifically if you see this in your dmesg:
> >>    Unexpected global fault, this could be serious
> >> ...then it's almost certainly this CL.
> >>
> >> Running your IOMMU-enabled peripherals with the IOMMU in bypass mode
> >> is insecure and effectively disables the protection they provide.
> >> There are few reasons to allow unmatched stream bypass, and even fewer
> >> good ones.
> >>
> >> This patch starts the transition over to make it much harder to run
> >> your system insecurely.  Expected steps:
> >>
> >> 1. By default disable bypass (so anyone insecure will notice) but make
> >>     it easy for someone to re-enable bypass with just a KConfig change.
> >>     That's this patch.
> >>
> >> 2. After people have had a little time to come to grips with the fact
> >>     that they need to set their IOMMUs properly and have had time to
> >>     dig into how to do this, the KConfig will be eliminated and bypass
> >>     will simply be disabled.  Folks who are truly upset and still
> >>     haven't fixed their system can either figure out how to add
> >>     'arm-smmu.disable_bypass=n' to their command line or revert the
> >>     patch in their own private kernel.  Of course these folks will be
> >>     less secure.
> >>
> >> Suggested-by: Robin Murphy <robin.murphy@xxxxxxx>
> >> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> >> ---
> >
> > Hi Doug / Robin,
> >
> > I ran into this breaking things on OcteonTx boards based on CN80XX
> > CPU. The IOMMU configuration is a bit beyond me and I'm hoping you can
> > offer some advice. The IOMMU here is cavium,smmu-v2 as defined in
> > https://github.com/Gateworks/dts-newport/blob/master/cn81xx-linux.dtsi
> >
> > Booting with 'arm-smmu.disable_bypass=n' does indeed work around the
> > breakage as the commit suggests.
> >
> > Any suggestions for a proper fix?
>
> Ah, you're using the old "mmu-masters" binding (and in a way which isn't
> well-defined - it's never been specified what the stream ID argument(s)
> would mean for a PCI host bridge, and Linux just ignores them). The
> ideal thing would be to update the DT to generic "iommu-map" properties
> - it's been a long time since I last played with a ThunderX, but I
> believe the SMMU stream IDs should just be the same as the ITS device
> IDs (which is how the "mmu-masters" mapping would have played out anyway).
>
> The arm-smmu driver support for the old binding has always relied on
> implicit bypass - there are technical reasons why we can't realistically
> support the full functionality offered to the generic bindings, but it
> would be possible to add some degree of workaround to prevent it
> interacting quite so poorly with disable_bypass, if necessary. Do you
> have deployed systems with DTs that can't be updated, but still might
> need to run new kernels?
>

Robin,

Thanks for the response. I don't care too much about supporting new
kernels with the current DT - I'm good with fixing this with a DT
change. Would you be able to give me an example? I would love to see
Cavium mainline an cn81xx dts/dtsi in arch/arm64/boot/dts to be used
as a base as the only thing we have to go off of currently is the
Cavium SDK which has fairly old kernel support.

Thanks,

Tim