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

[Nate Watterson]

Hi Mark,

On 4/12/2018 7:56 AM, Marc Zyngier wrote:
> 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.

Finding the SMMU already enabled does not necessarily indicate that
anything catastrophic has occurred.

For instance, to support OSes without an SMMUv3 driver, boot FW may have
enabled the SMMU and installed 1-to-1 mappings for DDR and MSI target
addr(s) to compensate for a MSI-capable master whose default DMA attrs
needed tweaking (ex: non-coherent -> coherent).

If such a configuration warrants tainting the kernel, then we should
similarly check GBPA for attr overrides and taint the kernel if any are
found there.

> 
> Thanks,
> 
> 	M.
> 

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