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

Jacob Pan
2022-04-18T18:08:06Z

Hi Kevin,

On Mon, 18 Apr 2022 06:34:19 +0000, "Tian, Kevin" <kevin.tian@intel.com>
wrote:

> > From: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > Sent: Saturday, April 16, 2022 5:00 AM
> >
> > Hi zhangfei.gao@foxmail.com,
> >
> > On Fri, 15 Apr 2022 19:52:03 +0800, "zhangfei.gao@foxmail.com"
> > <zhangfei.gao@foxmail.com> wrote:
> >
> > > >>> A PASID might be still used even though it is freed on mm exit.
> > > >>>
> > > >>> process A:
> > > >>> sva_bind();
> > > >>> ioasid_alloc() = N; // Get PASID N for the mm
> > > >>> fork(): // spawn process B
> > > >>> exit();
> > > >>> ioasid_free(N);
> > > >>>
> > > >>> process B:
> > > >>> device uses PASID N -> failure
> > > >>> sva_unbind();
> > > >>>
> > > >>> Dave Hansen suggests to take a refcount on the mm whenever
> > > >>> binding
> > the
> > > >>> PASID to a device and drop the refcount on unbinding. The mm
> > > >>> won't
> > be
> > > >>> dropped if the PASID is still bound to it.
> > > >>>
> > > >>> Fixes: 701fac40384f ("iommu/sva: Assign a PASID to mm on PASID
> > > >>> allocation and free it on mm exit")
> > > >>>
> > Is process A's mm intended to be used by process B? Or you really should
> > use PASID N on process B's mm? If the latter, it may work for a while
> > until B changes mapping.
> >
> > It seems you are just extending the life of a defunct mm?
> >
>
> IMHO the intention is not to allow B to access A's mm.
>
> The problem is that PASID N is released on exit() of A and then
> reallocated to B before iommu driver gets the chance to quiesce
> the device and clear the PASID entry. According to the discussion
> the quiesce operation must be done when driver calls unbind()
> instead of in mm exit. In this case a failure is reported when
> B tries to call bind() on PASID N due to an already-present entry.
>
> Dave's patch extending the life of A's mm until unbind() is called.
> With it B either gets a different PASID before A's unbind() is
> completed or same PASID N pointing to B's mm after A's unbind().
>
As long as B gets a different PASID, that is fine. It seems PASID N has no
use then.

> Thanks
> Kevin

Thanks,

Jacob