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

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Date:   Thu, 3 Oct 2019 11:27:37 -0700
From:   Tim Harvey <tharvey@...eworks.com>
To:     Douglas Anderson <dianders@...omium.org>,
        Robin Murphy <robin.murphy@....com>,
        Tirumalesh Chalamarla <tchalamarla@...iumnetworks.com>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will.deacon@....com>,
        linux-arm-msm@...r.kernel.org, evgreen@...omium.org,
        tfiga@...omium.org, Rob Clark <robdclark@...il.com>,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] iommu/arm-smmu: Break insecure users by disabling
 bypass by default

On Fri, Mar 1, 2019 at 11:21 AM Douglas Anderson <dianders@...omium.org> 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@....com>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---

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?

Best Regards,

Tim