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

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Date:   Fri, 24 Jun 2022 12:22:36 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Christoph Hellwig <hch@...radead.org>, <kwankhede@...dia.com>,
        <corbet@....net>, <hca@...ux.ibm.com>, <gor@...ux.ibm.com>,
        <agordeev@...ux.ibm.com>, <borntraeger@...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 10:56:15AM -0300, Jason Gunthorpe wrote:

> > How about the updated commit log below? Thanks.
> > 
> > The pinned PFN list returned from vfio_pin_pages() is converted using
> > page_to_pfn(), so direct access via memcpy() will crash on S390 if the
> > PFN is an IO PFN, as we have to use the memcpy_to/fromio(), which uses
> > the special s390 IO access instructions.
> > 
> > As a standard practice for security purpose, add kmap_local_page() to
> > block any IO memory from ever getting into this call path.
> 
> The kmap_local_page is not about the IO memory, the switch to struct
> page is what is protecting against IO memory.
> 
> Use kmap_local_page() is just the correct way to convert a struct page
> into a CPU address to use with memcpy and it is a NOP on S390 because
> it doesn't use highmem/etc.

I thought the whole purpose of switching to "struct page *" was to use
kmap_local_page() for the memcpy call, and the combination of these two
does the protection. Do you mind explaining how the switching part does
the protection?

Thanks!