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
