Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

Hi,

On Tue, Apr 12, 2022 at 07:36:21AM -0700, Dave Hansen wrote:
> On 4/12/22 00:04, zhangfei.gao@foxmail.com wrote:
> > master process quit, mmput ->  mm_pasid_drop->ioasid_free
> > But this ignore driver's iommu_sva_unbind_device function,
> > iommu_sva_bind_device and iommu_sva_unbind_device are not pair,  So
> > driver does not know ioasid is freed.
> > 
> > Any suggestion?
> 
> It sounds like you're saying that the device is still abound to the
> PASID even though the mm has exited and freed the PASID.  This is
> essentially a use-after-free for the PASID.  Right?
> 
> The right thing to do here is to have the PASID code hold a reference on
> the mm.  The mm "owns" the PASID for its entire lifetime and if anything
> needs the PASID to live longer, its only recourse for doing that is via
> an mmget().  I _think_ mmget() is the right thing as opposed to mmgrab()
> because the PASID users actually need the page tables to be around.
> 
> This would still be nice to confirm with some traces of fork()/exit()
> and the iommu_sva_{bind,unbind} and ioasid_{alloc,free} functions.
> 
> I wonder if the Intel and ARM IOMMU code differ in the way they keep
> references to the mm, or if this affects Intel as well, but we just
> haven't tested the code enough.

The Arm code was written expecting the PASID to be freed on unbind(), not
mm exit. I missed the change of behavior, sorry (I thought your plan was
to extend PASID lifetime, not shorten it?) but as is it seems very broken.
For example in the iommu_sva_unbind_device(), we have
arm_smmu_mmu_notifier_put() clearing the PASID table entry for
"mm->pasid", which is going to end badly if the PASID has been cleared or
reallocated. We can't clear the PASID entry in mm exit because at that
point the device may still be issuing DMA for that PASID and we need to
quiesce the entry rather than deactivate it. We can only deactivate it
once the device driver has properly stopped the device, at which point it
can call unbind(). There may be other issues but I can't check it
thoroughly until next week.

Thanks,
Jean