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

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Date:   Fri, 24 Jun 2022 10:56:15 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     Christoph Hellwig <hch@...radead.org>, kwankhede@...dia.com,
        corbet@....net, hca@...ux.ibm.com, gor@...ux.ibm.com,
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        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 Tue, Jun 21, 2022 at 02:21:22PM -0700, Nicolin Chen wrote:
> 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.

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.

Jason