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

[Will Deacon <will.deacon@arm.com>]

On Thu, May 02, 2019 at 12:59:12PM +0200, Thierry Reding wrote:
> On Wed, Apr 24, 2019 at 12:52:31PM +0100, Will Deacon wrote:
> > 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 <robin.murphy@arm.com>
> > > >> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> > > >> ---
> > > >>
> > > >> 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.
> 
> This made it to linux-next yesterday (less than a week before the merge
> window opens) and deliberately breaks existing configurations. That's a
> little rude.
> 
> At least give people a fair heads-up and a chance to fix things before
> you start break things.

Sorry about the inconvenience here.

This patch has been floating around for a while (albeit not in -next, since
I send my stuff via Joerg) and is heading for 5.3, so you have ages to fix
up your config! It would, of course, be better to configure the IOMMU to
provide mappings for your DMA peripherals, but the trivial config change
will be enough to keep things working. We won't remove that as long as
people are relying on it.

I don't expect most people to run into problems with this change (the new
behaviour matches what SMMUv3 does already).

Will

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