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

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Date:   Sun, 19 Jun 2022 23:32:07 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Nicolin Chen <nicolinc@...dia.com>, kwankhede@...dia.com,
        corbet@....net, hca@...ux.ibm.com, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com,
        svens@...ux.ibm.com, zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
        jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        rodrigo.vivi@...el.com, tvrtko.ursulin@...ux.intel.com,
        airlied@...ux.ie, daniel@...ll.ch, farman@...ux.ibm.com,
        mjrosato@...ux.ibm.com, pasic@...ux.ibm.com, vneethv@...ux.ibm.com,
        oberpar@...ux.ibm.com, freude@...ux.ibm.com,
        akrowiak@...ux.ibm.com, jjherne@...ux.ibm.com,
        alex.williamson@...hat.com, cohuck@...hat.com,
        kevin.tian@...el.com, jchrist@...ux.ibm.com, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [RFT][PATCH v1 5/6] vfio/ccw: Add kmap_local_page() for memcpy

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.