linux-kernel - Re: [PATCH v1 15/16] iommu/tegra241-cmdqv: Add user-space use support

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Message-ID: <aApcwKmb2PPPR8hf@Asurada-Nvidia>
Date: Thu, 24 Apr 2025 08:46:08 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: "Tian, Kevin" <kevin.tian@...el.com>, "corbet@....net" <corbet@....net>,
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Subject: Re: [PATCH v1 15/16] iommu/tegra241-cmdqv: Add user-space use support

On Thu, Apr 24, 2025 at 10:40:49AM -0300, Jason Gunthorpe wrote:
> On Wed, Apr 23, 2025 at 11:51:53PM -0700, Nicolin Chen wrote:
> > On Wed, Apr 23, 2025 at 08:13:33PM -0300, Jason Gunthorpe wrote:
> > > On Wed, Apr 23, 2025 at 11:31:29AM -0700, Nicolin Chen wrote:
> > > 
> > > > > It also needs to act like a mdev and lock down the part of the IOAS
> > > > > that provides that memory so the pin can't be released and UAF things.
> > > > 
> > > > If I capture this correctly, the GPA->PA mapping is already done
> > > > at the IOAS level for the S2 HWPT/domain, i.e. pages are already
> > > > pinned. So we just need to a pair of for-driver APIs to validate
> > > > the contiguity and refcount pages calling iopt_area_add_access().
> > > 
> > > Yes, adding an access is the key thing, the access will give you a
> > > page list which you can validate, but it also provides a way to
> > > synchronize if a hostile userspace does an unmap.
> > 
> > The new APIs are very like iommufd_access_pin/unpin_pages(). But
> > to reduce the amount of code that we have to share with driver.o,
> > I added a smaller iopt_area_get/put_access() that gets an access
> > and increases/decreases the refcounts only.
> 
> Maybe the access should be obtained by the core code to avoid the
> driver.o bloating? All the cmdq types need a memory buffer, right?
> Perhaps that should just be generalized

Yes. AMD just confirmed that they needed a similar setup. I think
we can do that!

> > Meanwhile, I am thinking if we could use the known S2 domain to
> > translate the GPAs to PAs for the contiguity test, which feels a
> > little cleaner to do in an IOMMU driver v.s. with a page list?
> 
> You still need the access, and the access already generates a page
> list..

Right. I was thinking it could save a few lines that fetches the
page list, but then that would need another around of translation,
which is unnecessary.

Thanks
Nicolin