[kernel-hardening] Re: [PATCH v5 03/10] swiotlb: Map the buffer if it was unmapped by XPFO

[Dave Hansen <dave.hansen@intel.com>]

On 08/09/2017 01:07 PM, Tycho Andersen wrote:
> --- a/lib/swiotlb.c
> +++ b/lib/swiotlb.c
> @@ -420,8 +420,9 @@ static void swiotlb_bounce(phys_addr_t orig_addr, phys_addr_t tlb_addr,
>  {
>  	unsigned long pfn = PFN_DOWN(orig_addr);
>  	unsigned char *vaddr = phys_to_virt(tlb_addr);
> +	struct page *page = pfn_to_page(pfn);
>  
> -	if (PageHighMem(pfn_to_page(pfn))) {
> +	if (PageHighMem(page) || xpfo_page_is_unmapped(page)) {
>  		/* The buffer does not have a mapping.  Map it in and copy */
>  		unsigned int offset = orig_addr & ~PAGE_MASK;
>  		char *buffer;

This is a little scary.  I wonder how many more of these are in the
kernel, like:

> static inline void *skcipher_map(struct scatter_walk *walk)
> {
>         struct page *page = scatterwalk_page(walk);
> 
>         return (PageHighMem(page) ? kmap_atomic(page) : page_address(page)) +
>                offset_in_page(walk->offset);
> }

Is there any better way to catch these?  Like, can we add some debugging
to check for XPFO pages in __va()?