Re: [PATCH RFC v2 5/9] mm/huge_memory: streamline COW logic in do_huge_pmd_wp_page()

[Yang Shi]

On Wed, Jan 26, 2022 at 2:00 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> We currently have a different COW logic for anon THP than we have for
> ordinary anon pages in do_wp_page(): the effect is that the issue reported
> in CVE-2020-29374 is currently still possible for anon THP: an unintended
> information leak from the parent to the child.
>
> Let's apply the same logic (page_count() == 1), with similar
> optimizations to remove additional references first as we really want to
> avoid PTE-mapping the THP and copying individual pages best we can.
>
> If we end up with a page that has page_count() != 1, we'll have to PTE-map
> the THP and fallback to do_wp_page(), which will always copy the page.
>
> Note that KSM does not apply to THP.
>
> I. Interaction with the swapcache and writeback
>
> While a THP is in the swapcache, the swapcache holds one reference on each
> subpage of the THP. So with PageSwapCache() set, we expect as many
> additional references as we have subpages. If we manage to remove the
> THP from the swapcache, all these references will be gone.
>
> Usually, a THP is not split when entered into the swapcache and stays a
> compound page. However, try_to_unmap() will PTE-map the THP and use PTE
> swap entries. There are no PMD swap entries for that purpose, consequently,
> we always only swapin subpages into PTEs.
>
> Removing a page from the swapcache can fail either when there are remaining
> swap entries (in which case COW is the right thing to do) or if the page is
> currently under writeback.
>
> Having a locked, R/O PMD-mapped THP that is in the swapcache seems to be
> possible only in corner cases, for example, if try_to_unmap() failed
> after adding the page to the swapcache. However, it's comparatively easy to
> handle.
>
> As we have to fully unmap a THP before starting writeback, and swapin is
> always done on the PTE level, we shouldn't find a R/O PMD-mapped THP in the
> swapcache that is under writeback. This should at least leave writeback
> out of the picture.
>
> II. Interaction with GUP references
>
> Having a R/O PMD-mapped THP with GUP references (i.e., R/O references)
> will result in PTE-mapping the THP on a write fault. Similar to ordinary
> anon pages, do_wp_page() will have to copy sub-pages and result in a
> disconnect between the GUP references and the pages actually mapped into
> the page tables. To improve the situation in the future, we'll need
> additional handling to mark anonymous pages as definitely exclusive to a
> single process, only allow GUP pins on exclusive anon pages, and
> disallow sharing of exclusive anon pages with GUP pins e.g., during
> fork().
>
> III. Interaction with references from LRU pagevecs
>
> Similar to ordinary anon pages, we can have LRU pagevecs referencing our
> THP. Reliably removing such references requires draining LRU pagevecs on
> all CPUs -- lru_add_drain_all() -- a possibly expensive operation that can
> sleep. For now, similar do do_wp_page(), let's conditionally drain the
> local LRU pagevecs only if we detect !PageLRU().
>
> IV. Interaction with speculative/temporary references
>
> Similar to ordinary anon pages, other speculative/temporary references on
> the THP, for example, from the pagecache or page migration code, will
> disallow exclusive reuse of the page. We'll have to PTE-map the THP.
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 406a3c28c026..b6ba88a98266 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1286,6 +1286,7 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>         struct page *page;
>         unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
>         pmd_t orig_pmd = vmf->orig_pmd;
> +       int swapcache_refs = 0;
>
>         vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
>         VM_BUG_ON_VMA(!vma->anon_vma, vma);
> @@ -1303,7 +1304,6 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>         page = pmd_page(orig_pmd);
>         VM_BUG_ON_PAGE(!PageHead(page), page);
>
> -       /* Lock page for reuse_swap_page() */
>         if (!trylock_page(page)) {
>                 get_page(page);
>                 spin_unlock(vmf->ptl);
> @@ -1319,10 +1319,20 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>         }
>
>         /*
> -        * We can only reuse the page if nobody else maps the huge page or it's
> -        * part.
> +        * See do_wp_page(): we can only map the page writable if there are
> +        * no additional references.
>          */
> -       if (reuse_swap_page(page)) {
> +       if (PageSwapCache(page))
> +               swapcache_refs = thp_nr_pages(page);
> +       if (page_count(page) > 1 + swapcache_refs + !PageLRU(page))
> +               goto unlock_fallback;
> +       if (!PageLRU(page))
> +               lru_add_drain();

IMHO, draining lru doesn't help out too much for THP since THP will be
drained to LRU immediately once it is added into pagevec.

> +       if (page_count(page) > 1 + swapcache_refs)
> +               goto unlock_fallback;
> +       if (swapcache_refs)
> +               try_to_free_swap(page);
> +       if (page_count(page) == 1) {
>                 pmd_t entry;
>                 entry = pmd_mkyoung(orig_pmd);
>                 entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> @@ -1333,6 +1343,7 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
>                 return VM_FAULT_WRITE;
>         }
>
> +unlock_fallback:
>         unlock_page(page);
>         spin_unlock(vmf->ptl);
>  fallback:
> --
> 2.34.1
>