linux-kernel - Re: [RFT][PATCH v1 5/6] vfio/ccw: Add kmap_local

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Message-ID: <20220624224245.GC4147@nvidia.com>
Date:   Fri, 24 Jun 2022 19:42:45 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     Christoph Hellwig <hch@...radead.org>, kwankhede@...dia.com,
        corbet@....net, hca@...ux.ibm.com, gor@...ux.ibm.com,
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Subject: Re: [RFT][PATCH v1 5/6] vfio/ccw: Add kmap_local_page() for memcpy

On Fri, Jun 24, 2022 at 01:12:56PM -0700, Nicolin Chen wrote:

> > The kmap_local_page() arose because the code doing memcpy had to be
> > updated to go from a struct page to a void * for use with memcpy and
> > the kmap_local_page() is the correct API to use for that.
> > 
> > The existing code which casts a pfn to a void * is improper.
> 
> Yes.
> 
> If I understand everything correctly:
> 
> A PFN is not secure enough to promise that the memory is not IO. And
> direct access via memcpy() that only handles CPU memory will crash on
> S390 if the PFN is an IO PFN, as we have to use the memcpy_to/fromio()
> that uses the special S390 IO access instructions. On the other hand,
> a "struct page *" is always a CPU coherent thing that fits memcpy().
> 
> Also, casting a PFN to "void *" for memcpy() is not an proper practice,
> kmap_local_page() is the correct API to call here, though S390 doesn't
> use highmem, which means kmap_local_page() is a NOP.
> 
> There's a following patch changing the vfio_pin_pages() API to return
> a list of "struct page *" instead of PFNs. It will block any IO memory
> from ever getting into this call path, for such a security purpose. In
> this patch, add kmap_local_page() to prepare for that.

Yes, basically

Jason