linux-kernel - Re: [PATCH v2 13/22] iommufd: Add mmap interface

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Message-ID: <aBvMC7dnYghoX5Aq@nvidia.com>
Date: Wed, 7 May 2025 14:09:31 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Pranjal Shrivastava <praan@...gle.com>, <kevin.tian@...el.com>,
	<corbet@....net>, <will@...nel.org>, <bagasdotme@...il.com>,
	<robin.murphy@....com>, <joro@...tes.org>, <thierry.reding@...il.com>,
	<vdumpa@...dia.com>, <jonathanh@...dia.com>, <shuah@...nel.org>,
	<jsnitsel@...hat.com>, <nathan@...nel.org>, <peterz@...radead.org>,
	<yi.l.liu@...el.com>, <mshavit@...gle.com>, <zhangzekun11@...wei.com>,
	<iommu@...ts.linux.dev>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-tegra@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
	<patches@...ts.linux.dev>, <mochs@...dia.com>, <alok.a.tiwari@...cle.com>,
	<vasant.hegde@....com>
Subject: Re: [PATCH v2 13/22] iommufd: Add mmap interface

On Wed, May 07, 2025 at 09:39:01AM -0300, Jason Gunthorpe wrote:
> On Tue, May 06, 2025 at 12:30:54PM -0700, Nicolin Chen wrote:
> 
> > So, if I understand it correctly, what we want to achieve is to
> > have maple tree to manage all PFN ranges. And each range holds
> > the same entry, a structure that we can use to verify the sanity
> > of an mmap? Let's say for PFNs A->B, the tree should store the
> > structure between index A and index B (inclusive)?
> 
> And tell you what has been mmap'd.
> 
> > If this is correct, mtree_alloc_range() that is given a range of
> > [0, ULONG_MAX] would allocate the PFN range from the lowest index
> > (i.e. 0) instead of PFN A?
> 
> mtree_alloc_range() returns a new range of PFNs that does not overlap
> with any existing range. It should always be called on O->U32_MAX (for
> 32bit uapi compat) and it should always pick the range to use.

Ah, so it's like an address translation table. mtree_alloc_range()
just gives us a virtual address range (i.e the cookie) for mmap()
to translate back to the real PFN range.

I have another question: while I don't think my code is handling
this well either, how should we validate the input address is an
allowed one?

Because mmap() is per ictx, i.e. it's a global translation table.
So, the following might happen: let's say we have two vIOMMUs in
the same ictx. Either vIOMMU has reported a cookie to index the
mtree for the real PFN range: call them PFN_RANGE0 (for vIOMMU0)
and PFN_RANGE1 (for vIOMMU1). What if a buggy VMM inverted these
cookies between the two vIOMMUs, so it would end up with vIOMMU0
accessing PFN_RANGE1?

Thanks
Nicolin