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iommu/arm-smmu: Break insecure users by disabling bypass by default

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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>
Reviewed-by: Marc Gonzalez <marc.w.gonzalez@free.fr>
Tested-by: Marc Gonzalez <marc.w.gonzalez@free.fr>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
This commit is contained in:
Douglas Anderson 2019-03-01 11:20:17 -08:00 committed by Will Deacon
parent 79a3aaa7b8
commit 954a03be03
2 changed files with 27 additions and 1 deletions
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25
drivers/iommu/Kconfig
View File

@ -359,6 +359,31 @@ config ARM_SMMU
Say Y here if your SoC includes an IOMMU device implementing
the ARM SMMU architecture.
config ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT
bool "Default to disabling bypass on ARM SMMU v1 and v2"
depends on ARM_SMMU
default y
help
Say Y here to (by default) disable bypass streams such that
incoming transactions from devices that are not attached to
an iommu domain will report an abort back to the device and
will not be allowed to pass through the SMMU.
Any old kernels that existed before this KConfig was
introduced would default to _allowing_ bypass (AKA the
equivalent of NO for this config). However the default for
this option is YES because the old behavior is insecure.
There are few reasons to allow unmatched stream bypass, and
even fewer good ones. If saying YES here breaks your board
you should work on fixing your board. This KConfig option
is expected to be removed in the future and we'll simply
hardcode the bypass disable in the code.
NOTE: the kernel command line parameter
'arm-smmu.disable_bypass' will continue to override this
config.
config ARM_SMMU_V3
bool "ARM Ltd. System MMU Version 3 (SMMUv3) Support"
depends on ARM64

3
drivers/iommu/arm-smmu.c
View File

@ -110,7 +110,8 @@ static int force_stage;
module_param(force_stage, int, S_IRUGO);
MODULE_PARM_DESC(force_stage,
"Force SMMU mappings to be installed at a particular stage of translation. A value of '1' or '2' forces the corresponding stage. All other values are ignored (i.e. no stage is forced). Note that selecting a specific stage will disable support for nested translation.");
static bool disable_bypass;
static bool disable_bypass =
IS_ENABLED(CONFIG_ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT);
module_param(disable_bypass, bool, S_IRUGO);
MODULE_PARM_DESC(disable_bypass,
"Disable bypass streams such that incoming transactions from devices that are not attached to an iommu domain will report an abort back to the device and will not be allowed to pass through the SMMU.");