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

> + 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?

> + /* 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.

> + 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?

> + lookup_page_ext(page)->keyid = 0;
> + page++;
> + }
> +}
> --
> 2.20.1
>

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