mm/z3fold: fix z3fold_page_migrate races with z3fold_map

Think about the below scenario:

CPU1				CPU2
 z3fold_page_migrate		z3fold_map
  z3fold_page_trylock
  ...
  z3fold_page_unlock
  /* slots still points to old zhdr*/
				 get_z3fold_header
				  get slots from handle
				  get old zhdr from slots
				  z3fold_page_trylock
				  return *old* zhdr
  encode_handle(new_zhdr, FIRST|LAST|MIDDLE)
  put_page(page) /* zhdr is freed! */
				 but zhdr is still used by caller!

z3fold_map can map freed z3fold page and lead to use-after-free bug.  To
fix it, we add PAGE_MIGRATED to indicate z3fold page is migrated and soon
to be released.  So get_z3fold_header won't return such page.

Link: https://lkml.kernel.org/r/20220429064051.61552-10-linmiaohe@huawei.com
Fixes: 1f862989b04a ("mm/z3fold.c: support page migration")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/z3fold.c b/mm/z3fold.c
index a7769be..f41f8b0 100644 (file)
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -181,6 +181,7 @@ enum z3fold_page_flags {
        NEEDS_COMPACTING,
        PAGE_STALE,
        PAGE_CLAIMED, /* by either reclaim or free */
+       PAGE_MIGRATED, /* page is migrated and soon to be released */
 };
 
 /*
@@ -270,8 +271,13 @@ static inline struct z3fold_header *get_z3fold_header(unsigned long handle)
                        zhdr = (struct z3fold_header *)(addr & PAGE_MASK);
                        locked = z3fold_page_trylock(zhdr);
                        read_unlock(&slots->lock);
-                       if (locked)
-                               break;
+                       if (locked) {
+                               struct page *page = virt_to_page(zhdr);
+
+                               if (!test_bit(PAGE_MIGRATED, &page->private))
+                                       break;
+                               z3fold_page_unlock(zhdr);
+                       }
                        cpu_relax();
                } while (true);
        } else {
@@ -389,6 +395,7 @@ static struct z3fold_header *init_z3fold_page(struct page *page, bool headless,
        clear_bit(NEEDS_COMPACTING, &page->private);
        clear_bit(PAGE_STALE, &page->private);
        clear_bit(PAGE_CLAIMED, &page->private);
+       clear_bit(PAGE_MIGRATED, &page->private);
        if (headless)
                return zhdr;
 
@@ -1576,7 +1583,7 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
        new_zhdr = page_address(newpage);
        memcpy(new_zhdr, zhdr, PAGE_SIZE);
        newpage->private = page->private;
-       page->private = 0;
+       set_bit(PAGE_MIGRATED, &page->private);
        z3fold_page_unlock(zhdr);
        spin_lock_init(&new_zhdr->page_lock);
        INIT_WORK(&new_zhdr->work, compact_page_work);
@@ -1606,7 +1613,8 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
 
        queue_work_on(new_zhdr->cpu, pool->compact_wq, &new_zhdr->work);
 
-       clear_bit(PAGE_CLAIMED, &page->private);
+       /* PAGE_CLAIMED and PAGE_MIGRATED are cleared now. */
+       page->private = 0;
        put_page(page);
        return 0;
 }