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

From: Will Deacon <[email-masked]>
Date: Wed, 24 Apr 2019 12:52:31 +0100

On Wed, Apr 24, 2019 at 01:36:58PM +0200, Marc Gonzalez wrote:
> On 04/04/2019 17:00, Will Deacon wrote:
> 
> > On Fri, Mar 01, 2019 at 11:20:17AM -0800, Douglas Anderson 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 <[email-masked]>
> >> Signed-off-by: Douglas Anderson <[email-masked]>
> >> ---
> >>
> >> Changes in v2:
> >> - Flipped default to 'yes' and changed comments a lot.
> >>
> >>  drivers/iommu/Kconfig    | 25 +++++++++++++++++++++++++
> >>  drivers/iommu/arm-smmu.c |  3 ++-
> >>  2 files changed, 27 insertions(+), 1 deletion(-)
> > 
> > Cheers, I'll pick this one up for 5.2.
> 
> Hello Will,
> 
> You haven't pushed this patch out to linux-next AFAICT.
> 
> Is that expected?

It's on my branch for Joerg:

https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/log/?h=for-joerg/arm-smmu/updates

which I'll send to him today. My SMMU stuff doesn't go directly into -next.

Will

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