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

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Message-ID: <YrI2Ul/u6pRvt0rT@Asurada-Nvidia>
Date:   Tue, 21 Jun 2022 14:21:22 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     Jason Gunthorpe <jgg@...dia.com>, <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 Sun, Jun 19, 2022 at 11:32:07PM -0700, Christoph Hellwig wrote:
> On Sun, Jun 19, 2022 at 11:57:26PM -0300, Jason Gunthorpe wrote:
> > 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.
> 
> Yes.  The same is true for various other architectures, inluding arm64
> under the right circumstances.
> 
> > 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..
> 
> Prevent as in crash when trying to convert it to a page?
> 
> > 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?
> 
> Ira has been lookin into the protection keys, although they don't
> apply to s390.  Either way I don't object to using kmap, but the
> commit log doesn't make much sense to me.

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.