[PATCH] iommu/arm-smmu-v3: Set GBPA to abort all transactions

[Marc Zyngier]

On 12/04/18 11:17, Robin Murphy wrote:
> On 11/04/18 17:54, Marc Zyngier wrote:
>> Hi Sammer,
>>
>> On 11/04/18 16:58, Goel, Sameer wrote:
>>>
>>>
>>> On 3/28/2018 9:00 AM, Marc Zyngier wrote:
>>>> On 2018-03-28 15:39, Timur Tabi wrote:
>>>>> From: Sameer Goel <sgoel at codeaurora.org>
>>>>>
>>>>> Set SMMU_GBPA to abort all incoming translations during the SMMU reset
>>>>> when SMMUEN==0.
>>>>>
>>>>> This prevents a race condition where a stray DMA from the crashed primary
>>>>> kernel can try to access an IOVA address as an invalid PA when SMMU is
>>>>> disabled during reset in the crash kernel.
>>>>>
>>>>> Signed-off-by: Sameer Goel <sgoel at codeaurora.org>
>>>>> ---
>>>>>   drivers/iommu/arm-smmu-v3.c | 12 ++++++++++++
>>>>>   1 file changed, 12 insertions(+)
>>>>>
>>>>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
>>>>> index 3f2f1fc68b52..c04a89310c59 100644
>>>>> --- a/drivers/iommu/arm-smmu-v3.c
>>>>> +++ b/drivers/iommu/arm-smmu-v3.c
>>>>> @@ -2458,6 +2458,18 @@ static int arm_smmu_device_reset(struct
>>>>> arm_smmu_device *smmu, bool bypass)
>>>>>       if (reg & CR0_SMMUEN)
>>>>>           dev_warn(smmu->dev, "SMMU currently enabled! Resetting...\n");
>>>>>
>>>>> +    /*
>>>>> +     * Abort all incoming translations. This can happen in a kdump case
>>>>> +     * where SMMU is initialized when a prior DMA is pending. Just
>>>>> +     * disabling the SMMU in this case might result in writes to invalid
>>>>> +     * PAs.
>>>>> +     */
>>>>> +    ret = arm_smmu_update_gbpa(smmu, 1, GBPA_ABORT);
>>>>> +    if (ret) {
>>>>> +        dev_err(smmu->dev, "GBPA not responding to update\n");
>>>>> +        return ret;
>>>>> +    }
>>>>> +
>>>>>       ret = arm_smmu_device_disable(smmu);
>>>>>       if (ret)
>>>>>           return ret;
>>>>
>>>> A tangential question: can we reliably detect that the SMMU already
>>>> has valid mappings, which would indicate that we're in a pretty bad
>>>> shape already by the time we set that bit? For all we know, memory
>>>> could have been corrupted long before we hit this point, and this
>>>> patch barely narrows the window of opportunity.
>>>
>>> :) Yes that is correct. This only covers the kdump scenario. Trying
>>> to get some reliability when booting up the crash kernel. The system
>>> is already in a bad state. I don't think that this will happen in a
>>> normal scenario. But please point me to the GICv3 change and I'll
>>> have a look.
>>
>> See this:
>> https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/tree/drivers/irqchip/irq-gic-v3-its.c?h=irq/irqchip-4.17&id=6eb486b66a3094cdcd68dc39c9df3a29d6a51dd5#n3407
> 
> The nearest equivalent to that is probably the top-level SMMUEN check 
> that we already have (see the diff context above). To go beyond that 
> you'd have to chase the old stream table pointer and scan the whole 
> thing looking for valid contexts, then potentially walk page tables 
> within those contexts to check for live translations if you really 
> wanted to be sure. That would be a hell of a lot of work to do in the 
> boot path.
Yeah, feels a bit too involved for sanity. I'd simply suggest you taint
the kernel if you find the SMMU enabled, as you're already on shaky ground.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...