[PATCH RFC v2 4/9] mm: streamline COW logic in do_swap_page()

[David Hildenbrand]

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>
---
 mm/memory.c | 55 +++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 43 insertions(+), 12 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index ab3153252cfe..ba23d13b8410 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3499,6 +3499,25 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
 	return 0;
 }
 
+static inline bool should_try_to_free_swap(struct page *page,
+					   struct vm_area_struct *vma,
+					   unsigned int fault_flags)
+{
+	if (!PageSwapCache(page))
+		return false;
+	if (mem_cgroup_swap_full(page) || (vma->vm_flags & VM_LOCKED) ||
+	    PageMlocked(page))
+		return true;
+	/*
+	 * If we want to map a page that's in the swapcache writable, we
+	 * have to detect via the refcount if we're really the exclusive
+	 * owner. Try freeing the swapcache to get rid of the swapcache
+	 * reference in case it's likely that we will succeed.
+	 */
+	return (fault_flags & FAULT_FLAG_WRITE) && !PageKsm(page) &&
+		page_count(page) == 2;
+}
+
 /*
  * We enter with non-exclusive mmap_lock (to exclude vma changes,
  * but allow concurrent faults), and pte mapped but not yet locked.
@@ -3640,6 +3659,16 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 			page = swapcache;
 			goto out_page;
 		}
+
+		/*
+		 * If we want to map a page that's in the swapcache writable, we
+		 * have to detect via the refcount if we're really the exclusive
+		 * owner. Try removing the extra reference from the local LRU
+		 * pagevecs if required.
+		 */
+		if ((vmf->flags & FAULT_FLAG_WRITE) && page == swapcache &&
+		    !PageKsm(page) && !PageLRU(page))
+			lru_add_drain();
 	}
 
 	cgroup_throttle_swaprate(page, GFP_KERNEL);
@@ -3658,19 +3687,25 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	}
 
 	/*
-	 * The page isn't present yet, go ahead with the fault.
-	 *
-	 * Be careful about the sequence of operations here.
-	 * To get its accounting right, reuse_swap_page() must be called
-	 * while the page is counted on swap but not yet in mapcount i.e.
-	 * before page_add_anon_rmap() and swap_free(); try_to_free_swap()
-	 * must be called after the swap_free(), or it will never succeed.
+	 * Remove the swap entry and conditionally try to free up the swapcache.
+	 * We're already holding a reference on the page but haven't mapped it
+	 * yet.
 	 */
+	swap_free(entry);
+	if (should_try_to_free_swap(page, vma, vmf->flags))
+		try_to_free_swap(page);
 
 	inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
 	dec_mm_counter_fast(vma->vm_mm, MM_SWAPENTS);
 	pte = mk_pte(page, vma->vm_page_prot);
-	if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
+
+	/*
+	 * Same logic as in do_wp_page(); however, optimize for fresh pages
+	 * that are certainly not shared because we just allocated them without
+	 * exposing them to the swapcache.
+	 */
+	if ((vmf->flags & FAULT_FLAG_WRITE) && !PageKsm(page) &&
+	    (page != swapcache || page_count(page) == 1)) {
 		pte = maybe_mkwrite(pte_mkdirty(pte), vma);
 		vmf->flags &= ~FAULT_FLAG_WRITE;
 		ret |= VM_FAULT_WRITE;
@@ -3696,10 +3731,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
 	arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
 
-	swap_free(entry);
-	if (mem_cgroup_swap_full(page) ||
-	    (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
-		try_to_free_swap(page);
 	unlock_page(page);
 	if (page != swapcache && swapcache) {
 		/*
-- 
2.34.1