Re: [PATCH v3 4/9] mm: streamline COW logic in do_swap_page()

[Vlastimil Babka]

On 1/31/22 17:29, David Hildenbrand wrote:
> Currently we have a different COW logic when:
> * triggering a read-fault to swapin first and then trigger a write-fault
>   -> do_swap_page() + do_wp_page()
> * triggering a write-fault to swapin
>   -> do_swap_page() + do_wp_page() only if we fail reuse in do_swap_page()
> 
> The COW logic in do_swap_page() is different than our reuse logic in
> do_wp_page(). The COW logic in do_wp_page() -- page_count() == 1 --  makes
> currently sure that we certainly don't have a remaining reference, e.g.,
> via GUP, on the target page we want to reuse: if there is any unexpected
> reference, we have to copy to avoid information leaks.
> 
> As do_swap_page() behaves differently, in environments with swap enabled we
> can currently have an unintended information leak from the parent to the
> child, similar as known from CVE-2020-29374:
> 
> 	1. Parent writes to anonymous page
> 	-> Page is mapped writable and modified
> 	2. Page is swapped out
> 	-> Page is unmapped and replaced by swap entry
> 	3. fork()
> 	-> Swap entries are copied to child
> 	4. Child pins page R/O
> 	-> Page is mapped R/O into child
> 	5. Child unmaps page
> 	-> Child still holds GUP reference
> 	6. Parent writes to page
> 	-> Page is reused in do_swap_page()
> 	-> Child can observe changes
> 
> Exchanging 2. and 3. should have the same effect.
> 
> Let's apply the same COW logic as in do_wp_page(), conditionally trying to
> remove the page from the swapcache after freeing the swap entry, however,
> before actually mapping our page. We can change the order now that
> we use try_to_free_swap(), which doesn't care about the mapcount,
> instead of reuse_swap_page().
> 
> To handle references from the LRU pagevecs, conditionally drain the local
> LRU pagevecs when required, however, don't consider the page_count() when
> deciding whether to drain to keep it simple for now.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>