Re: [PATCH v3 3/5] iommu: Add verisilicon IOMMU driver

Benjamin Gaignard
2025-06-20T10:15:51Z

Le 19/06/2025 à 18:59, Jason Gunthorpe a écrit :
> On Thu, Jun 19, 2025 at 06:27:52PM +0200, Benjamin Gaignard wrote:
>> Le 19/06/2025 à 15:47, Jason Gunthorpe a écrit :
>>> Ugh. This ignores the gfp flags that are passed into map because you
>>> have to force atomic due to the spinlock that shouldn't be there :(
>>> This means it does not set GFP_KERNEL_ACCOUNT when required. It would
>>> be better to continue to use the passed in GFP flags but override them
>>> to atomic mode.
>> I will add a gfp_t parameter and use it like that:
>> page_table = iommu_alloc_pages_sz(gfp | GFP_ATOMIC | GFP_DMA32, SPAGE_SIZE);
> This will or together GFP_ATOMIC and GFP_KERNEL, I don't think that
> works..

I have test it, I don't see conflicts or errors. What worries you ?

>
>>>> +static int vsi_iommu_attach_device(struct iommu_domain *domain,
>>>> + struct device *dev)
>>>> +{
>>>> + struct vsi_iommu *iommu = dev_iommu_priv_get(dev);
>>>> + struct vsi_iommu_domain *vsi_domain = to_vsi_domain(domain);
>>>> + unsigned long flags;
>>>> + int ret;
>>>> +
>>>> + if (WARN_ON(!iommu))
>>>> + return -ENODEV;
>>>> +
>>>> + /* iommu already attached */
>>>> + if (iommu->domain == domain)
>>>> + return 0;
>>>> +
>>>> + ret = vsi_iommu_identity_attach(&vsi_identity_domain, dev);
>>>> + if (ret)
>>>> + return ret;
>>> Hurm, this is actually quite bad, now that it is clear the HW is in an
>>> identity mode it is actually a security problem for VFIO to switch the
>>> translation to identity during attach_device. I'd really prefer new
>>> drivers don't make this mistake.
>>>
>>> It seems the main thing motivating this is the fact a linked list has
>>> only a single iommu->node so you can't attach the iommu to both the
>>> new/old domain and atomically update the page table base.
>>>
>>> Is it possible for the HW to do a blocking behavior? That would be an
>>> easy fix.. You should always be able to force this by allocating a
>>> shared top page table level during probe time and making it entirely
>>> empty while staying always in the paging mode. Maybe there is a less
>>> expensive way.
>>>
>>> Otherwise you probably have work more like the other drivers and
>>> allocate a struct for each attachment so you can have the iommu
>>> attached two domains during the switch over and never drop to an
>>> identity mode.
>> I will remove the switch to identity domain and it will works fine.
> You'll loose invalidations..
>
> Maybe the easiest thing is to squish vsi_iommu_enable() and reorganize
> it so that the spinlock is held across the register programming and
> then you can atomically under the lock change the registers and change
> the linked list. The register write cannot fail so this is fine.
>
> This should probably also flush the iotlb inside the lock.

I will try to summarize:
in vsi_iommu_attach_device() I should:
- take the lock
- do nothing if the domain is the same.
- if iommu is used (ie powered up):
- update the registers to enable the iommu
- flush
- update the link list
- update iommu->domain
- release the lock

in vsi_iommu_identity_attach() I should:
- take the lock
- do nothing if the domain is the same.
- if iommu is used (ie powered up):
- disable the iommu
- remove the node from the list
- update iommu->domain
- release the lock

vsi_iommu_suspend() and vsi_iommu_resume() will also have to take the lock
before calling vsi_iommu_disable() and vsi_iommu_enable().

sound good for you ?

Do I have to switch to identity domain in vsi_iommu_attach_device()
before applying the requested domain ?

Regards,
Benjamin

>
>>> Which will cause the core code to automatically run through to
>>> vsi_iommu_disable() prior to calling vsi_iommu_release_device(), which
>>> will avoid UAF problems.
>>>
>>> Also, should the probe functions be doing some kind of validation that
>>> there is only one struct device attached?
>> which kind of validation ?
> That only one device probed to the iommu?
>
> Jason