linux-kernel - Re: [PATCH v5 3/7] iommu: Validate that devices match domains

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Date:   Wed, 25 Oct 2023 13:15:05 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     joro@...tes.org, will@...nel.org, iommu@...ts.linux.dev,
        baolu.lu@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/7] iommu: Validate that devices match domains

On Wed, Oct 25, 2023 at 05:05:08PM +0100, Robin Murphy wrote:
> On 25/10/2023 1:55 pm, Jason Gunthorpe wrote:
> > On Wed, Oct 25, 2023 at 01:39:56PM +0100, Robin Murphy wrote:
> > > On 24/10/2023 7:52 pm, Jason Gunthorpe wrote:
> > > > On Wed, Oct 11, 2023 at 07:14:50PM +0100, Robin Murphy wrote:
> > > > 
> > > > > @@ -2279,10 +2280,16 @@ struct iommu_domain *iommu_get_dma_domain(struct device *dev)
> > > > >    static int __iommu_attach_group(struct iommu_domain *domain,
> > > > >    				struct iommu_group *group)
> > > > >    {
> > > > > +	struct device *dev;
> > > > > +
> > > > >    	if (group->domain && group->domain != group->default_domain &&
> > > > >    	    group->domain != group->blocking_domain)
> > > > >    		return -EBUSY;
> > > > > +	dev = iommu_group_first_dev(group);
> > > > > +	if (!dev_has_iommu(dev) || dev_iommu_ops(dev) != domain->owner)
> > > > > +		return -EINVAL;
> > > > 
> > > > I was thinking about this later, how does this work for the global
> > > > static domains? domain->owner will not be set?
> > > > 
> > > > 	if (alloc_type == IOMMU_DOMAIN_IDENTITY && ops->identity_domain)
> > > > 		return ops->identity_domain;
> > > > 	else if (alloc_type == IOMMU_DOMAIN_BLOCKED && ops->blocked_domain)
> > > > 		return ops->blocked_domain;
> > > > 
> > > > Seems like it will break everything?
> > > 
> > > I don't believe it makes any significant difference - as the commit message
> > > points out, this validation is only applied at the public interface
> > > boundaries of iommu_attach_group(), iommu_attach_device(),
> > 
> > Oh, making it only work for on domain type seems kind of hacky..
> > 
> > If that is the intention maybe the owner set should be moved into
> > iommu_domain_alloc() with a little comment noting that it is limited
> > to work in only a few cases?
> > 
> > I certainly didn't understand from the commit message to mean it was
> > only actually working for one domain type and this also blocks using
> > other types with the public interface.
> 
> It's not about one particular domain type, it's about the scope of what we
> consider valid usage. External API users should almost always be attaching
> to their own domain which they have allocated, however we also tolerate
> co-attaching additional groups to the same DMA domain in rare cases where
> it's reasonable. The fact is that those users cannot allocate blocking or
> identity domains, and I can't see that they would ever have any legitimate
> business trying to do anything with them anyway. So although yes, we
> technically lose some functionality once this intersects with the static
> domain optimisation, it's only questionable functionality which was never
> explicitly intended anyway.

I have no problem with that argument, I'm saying this is a subtle
emergent property. Lets document it, lets be more explicit. The owner
checks would do well to go along with specific domain type checks as
well to robustly enforce what you just explained.

Thanks,
Jason