Re: [PATCH] iommu/vt-d: Add a fix for devices need extra dtlb flush

[Baolu Lu <baolu.lu@linux.intel.com>]

On 11/23/22 7:32 PM, Robin Murphy wrote:
> On 2022-11-23 05:18, Tian, Kevin wrote:
>>> From: Baolu Lu <baolu.lu@linux.intel.com>
>>> Sent: Wednesday, November 23, 2022 1:04 PM
>>>
>>> On 2022/11/23 9:02, Tian, Kevin wrote:
>>>>> From: Robin Murphy <robin.murphy@arm.com>
>>>>> Sent: Wednesday, November 23, 2022 1:49 AM
>>>>>
>>>>>> +
>>>>>> +/* Impacted QAT device IDs ranging from 0x4940 to 0x4943 */
>>>>>> +#define BUGGY_QAT_DEVID_MASK 0x494c
>>>>>> +static bool dev_needs_extra_dtlb_flush(struct pci_dev *pdev)
>>>>>> +{
>>>>>> +    if (pdev->vendor != PCI_VENDOR_ID_INTEL)
>>>>>> +        return false;
>>>>>> +
>>>>>> +    if ((pdev->device & 0xfffc) != BUGGY_QAT_DEVID_MASK)
>>>>>> +        return false;
>>>>>> +
>>>>>> +    if (risky_device(pdev))
>>>>>> +        return false;
>>>>>
>>>>> Hmm, I'm not sure that that makes much sense to me - what privilege 
>>>>> can
>>>>> the device gain from being told to invalidate things twice? Why 
>>>>> would we
>>>>> want to implicitly *allow* a device to potentially keep using a stale
>>>>> translation if for some bizarre reason firmware has marked it as
>>>>> external, surely that's worse?
>>>
>>>   From the perspective of IOMMU, any quirk is only applicable to trusted
>>> devices. If the IOMMU driver detects that a quirk is being applied to an
>>> untrusted device, it is already buggy or malicious. The IOMMU driver
>>> should let the users know by:
>>>
>>>     pci_info(pdev,
>>>          "Skipping IOMMU quirk for dev [%04X:%04X] on untrusted
>>> PCI link\n",
>>>          pdev->vendor, pdev->device);
>>>     pci_info(pdev, "Please check with your BIOS/Platform vendor about
>>> this\n");
>>>
>>> and stop applying any quirk.
>>>
>>
>> A quirk usually relaxes something then you want it only on trusted 
>> devices.
>>
>> but the quirk in this patch is trying to fix a vulnerability. In 
>> concept it's
>> weird to skip it on untrusted devices. This iiuc was the part causing 
>> confusion
>> to Robin.
> 
> Right, it's that reasoning in general that seems bogus to me. Clearly 
> any quirk that effectively grants additional privileges, like an 
> identity mapping quirk, should not be applied to untrusted external 
> devices which may be spoofing an affected VID/DID to gain that 
> privilege, but not all quirks imply privilege. If, say, a WiFI 
> controller needs something innocuous like a DMA alias or address width 
> quirk to function correctly, it will still need that regardless of 
> whether it's soldered to a motherboard or to a removable expansion card, 
> and it would do nobody any good to deny correct functionality based on 
> that unnecessary distinction. Yes, I appreciate that in practice many of 
> those kind of quirks will be applied in other layers anyway, but I still 
> think it's wrong to make a sweeping assumption that all IOMMU-level 
> quirks are precious treasure not to be shared with outsiders, rather 
> than assess their impact individually. The detriment in this case is 
> small (just needless code churn), but even that's still not nothing.

Fair enough. I agreed here.

Can we put some comments here so that people can still easily read the
discussion here after a long time?

Best regards,
baolu