linux-kernel - Re: [RFT][PATCH v2 8/9] vfio/ccw: Add kmap_local

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Date:   Wed, 6 Jul 2022 15:17:04 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     kwankhede@...dia.com, corbet@....net, hca@...ux.ibm.com,
        gor@...ux.ibm.com, agordeev@...ux.ibm.com,
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Subject: Re: [RFT][PATCH v2 8/9] vfio/ccw: Add kmap_local_page() for memcpy

On Tue, Jul 05, 2022 at 11:27:58PM -0700, Nicolin Chen wrote:
> 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 a 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.
> 
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  drivers/s390/cio/vfio_ccw_cp.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason