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

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Message-ID: <20220620025726.GA5219@nvidia.com>
Date:   Sun, 19 Jun 2022 23:57:26 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Nicolin Chen <nicolinc@...dia.com>, kwankhede@...dia.com,
        corbet@....net, hca@...ux.ibm.com, gor@...ux.ibm.com,
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        svens@...ux.ibm.com, zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
        jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
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Subject: Re: [RFT][PATCH v1 5/6] vfio/ccw: Add kmap_local_page() for memcpy

On Fri, Jun 17, 2022 at 01:44:30AM -0700, Christoph Hellwig wrote:
> On Thu, Jun 16, 2022 at 04:52:11PM -0700, Nicolin Chen wrote:
> > The pinned PFN list returned from vfio_pin_pages() is simply converted
> > using page_to_pfn() without protection, so direct access via memcpy()
> > will crash on S390 if the PFN is an IO PFN. Instead, the pages should
> > be touched using kmap_local_page().
> 
> I don't see how this helps.  kmap_local_page only works for either
> pages in the kernel direct map or highmem, but not for memory that needs
> to be ioremapped.  And there is no highmem on s390.

The remark about io memory is because on s390 memcpy() will crash even
on ioremapped memory, you have to use the memcpy_to/fromio() which
uses the special s390 io access instructions.

This helps because we now block io memory from ever getting into these
call paths. I'm pretty sure this is a serious security bug, but would
let the IBM folks remark as I don't know it all that well..

As for the kmap, I thought it was standard practice even if it is a
non-highmem? Aren't people trying to use this for other security
stuff these days?

Jason