RE: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment

[Tian, Kevin]

> From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Wednesday, February 15, 2023 1:51 PM
> 
> On 2/13/23 10:19 PM, Jason Gunthorpe wrote:
> > On Mon, Feb 13, 2023 at 03:49:39PM +0800, Lu Baolu wrote:
> >> @@ -2992,6 +2987,14 @@ static ssize_t iommu_group_store_type(struct
> iommu_group *group,
> >>   	else
> >>   		return -EINVAL;
> >>
> >> +	if (req_type != IOMMU_DOMAIN_DMA_FQ ||
> >> +	    group->default_domain->type != IOMMU_DOMAIN_DMA) {
> >> +		ret = iommu_group_claim_dma_owner(group, (void *)buf);
> >> +		if (ret)
> >> +			return ret;
> >> +		group_owner_claimed = true;
> >> +	}
> >
> > I don't get it, this should be done unconditionally. If we couldn't
> > take ownership then we simply can't progress.
> 
> The existing code allows the user to switch the default domain from
> strict to lazy invalidation mode. The default domain is not changed,
> hence it should be seamless and transparent to the device driver.

Is there real usage relying on this transition for a bound device?

In concept strict->lazy transition implies relaxed DMA security. It's hard
to think of a motivation of doing so while the device might be doing
in-fly DMAs.

Presumably such perf/security tradeoff should be planned way before
binding device/driver together.

btw if strict->lazy is allowed why lazy->strict is prohibited?

> 
> > which also means this needs to be
> > an externally version of iommu_group_claim_dma_owner()
> 
> Sorry! What does "an externally version of
> iommu_group_claim_dma_owner()" mean?
> 
> My understanding is that we should limit iommu_group_claim_dma_owner()
> use in the driver context. For this non-driver context, we should not
> use iommu_group_claim_dma_owner() directly, but hold the group->mutex
> and check the group->owner_cnt directly:
> 
>          mutex_lock(&group->mutex);
>          if (group->owner_cnt) {
>                  ret = -EPERM;
>                  goto unlock_out;
>          }
> 
> the group->mutex should be held until everything is done.
> 

I guess you two meant the same thing.

	mutex_lock(&group->mutex);
	iommu_group_claim_dma_owner_unlocked();
	//blah blah
	mutex_unlock(&group->mutex);