Re: [PATCH, RFC 13/62] x86/mm: Add hooks to allocate and free encrypted pages

[Kirill A. Shutemov]

On Fri, Jun 14, 2019 at 11:34:09AM +0200, Peter Zijlstra wrote:
> On Wed, May 08, 2019 at 05:43:33PM +0300, Kirill A. Shutemov wrote:
> 
> > +/* Prepare page to be used for encryption. Called from page allocator. */
> > +void __prep_encrypted_page(struct page *page, int order, int keyid, bool zero)
> > +{
> > +	int i;
> > +
> > +	/*
> > +	 * The hardware/CPU does not enforce coherency between mappings
> > +	 * of the same physical page with different KeyIDs or
> > +	 * encryption keys. We are responsible for cache management.
> > +	 */
> 
> On alloc we should flush the unencrypted (key=0) range, while on free
> (below) we should flush the encrypted (key!=0) range.
> 
> But I seem to have missed where page_address() does the right thing
> here.

As you've seen by now, it will be addressed later in the patchset. I'll
update the changelog to indicate that page_address() handles KeyIDs
correctly.

> > +	clflush_cache_range(page_address(page), PAGE_SIZE * (1UL << order));
> > +
> > +	for (i = 0; i < (1 << order); i++) {
> > +		/* All pages coming out of the allocator should have KeyID 0 */
> > +		WARN_ON_ONCE(lookup_page_ext(page)->keyid);
> > +		lookup_page_ext(page)->keyid = keyid;
> > +
> 
> So presumably page_address() is affected by this keyid, and the below
> clear_highpage() then accesses the 'right' location?

Yes. clear_highpage() -> kmap_atomic() -> page_address().

> > +		/* Clear the page after the KeyID is set. */
> > +		if (zero)
> > +			clear_highpage(page);
> > +
> > +		page++;
> > +	}
> > +}
> > +
> > +/*
> > + * Handles freeing of encrypted page.
> > + * Called from page allocator on freeing encrypted page.
> > + */
> > +void free_encrypted_page(struct page *page, int order)
> > +{
> > +	int i;
> > +
> > +	/*
> > +	 * The hardware/CPU does not enforce coherency between mappings
> > +	 * of the same physical page with different KeyIDs or
> > +	 * encryption keys. We are responsible for cache management.
> > +	 */
> 
> I still don't like that comment much; yes the hardware doesn't do it,
> and yes we have to do it, but it doesn't explain the actual scheme
> employed to do so.

Fair enough. I'll do better.

> > +	clflush_cache_range(page_address(page), PAGE_SIZE * (1UL << order));
> > +
> > +	for (i = 0; i < (1 << order); i++) {
> > +		/* Check if the page has reasonable KeyID */
> > +		WARN_ON_ONCE(lookup_page_ext(page)->keyid > mktme_nr_keyids);
> 
> It should also check keyid > 0, so maybe:
> 
> 	(unsigned)(keyid - 1) > keyids-1
> 
> instead?

Makes sense.

-- 
 Kirill A. Shutemov