iommu/arm-smmu-v3: Prevent any devices access to memory without registration

Stream bypass is a potential security hole since a malicious device can be
hotplugged in without matching any drivers, yet be granted the ability to
access all of physical memory.

Now that we attach devices to domains by default, we can toggle the
disable_bypass default to "on", preventing DMA from unknown devices.
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

drivers/iommu/arm-smmu-v3.c

@@ -366,7 +366,7 @@
 #define MSI_IOVA_BASE			0x8000000
 #define MSI_IOVA_LENGTH			0x100000
 
-static bool disable_bypass;
+static bool disable_bypass = 1;
 module_param_named(disable_bypass, 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.");