Re: [RFT][PATCH v1 5/6] vfio/ccw: Add kmap_local_page() for memcpy

[Jason Gunthorpe]

On Fri, Jun 24, 2022 at 12:22:36PM -0700, Nicolin Chen wrote:
> On Fri, Jun 24, 2022 at 10:56:15AM -0300, Jason Gunthorpe wrote:
> 
> > > 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.
> 
> I thought the whole purpose of switching to "struct page *" was to use
> kmap_local_page() for the memcpy call, and the combination of these two
> does the protection. Do you mind explaining how the switching part does
> the protection?

A 'struct page' (ignoring ZONE_DEVICE) cannot represent IO memory
inherently because a 'struct page' is always a CPU coherent thing.

So, when VFIO returns only struct pages it also is promising that the
memory is not IO.

The kmap_local_page() arose because the code doing memcpy had to be
updated to go from a struct page to a void * for use with memcpy and
the kmap_local_page() is the correct API to use for that.

The existing code which casts a pfn to a void * is improper.

Jason