mm: zswap: fix potential memory corruption on duplicate store

While stress-testing zswap a memory corruption was happening when writing
back pages.  __frontswap_store used to check for duplicate entries before
attempting to store a page in zswap, this was because if the store fails
the old entry isn't removed from the tree.  This change removes duplicate
entries in zswap_store before the actual attempt.

[cerasuolodomenico@gmail.com: add a warning and a comment, per Johannes]
  Link: https://lkml.kernel.org/r/20230925130002.1929369-1-cerasuolodomenico@gmail.com
Link: https://lkml.kernel.org/r/20230922172211.1704917-1-cerasuolodomenico@gmail.com
Fixes: 42c06a0e ("mm: kill frontswap")
Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zswap.c

@@ -1218,6 +1218,19 @@ bool zswap_store(struct folio *folio)
 	if (!zswap_enabled || !tree)
 		return false;
 
+	/*
+	 * If this is a duplicate, it must be removed before attempting to store
+	 * it, otherwise, if the store fails the old page won't be removed from
+	 * the tree, and it might be written back overriding the new data.
+	 */
+	spin_lock(&tree->lock);
+	dupentry = zswap_rb_search(&tree->rbroot, offset);
+	if (dupentry) {
+		zswap_duplicate_entry++;
+		zswap_invalidate_entry(tree, dupentry);
+	}
+	spin_unlock(&tree->lock);
+
 	/*
 	 * XXX: zswap reclaim does not work with cgroups yet. Without a
 	 * cgroup-aware entry LRU, we will push out entries system-wide based on
@@ -1333,7 +1346,14 @@ bool zswap_store(struct folio *folio)
 
 	/* map */
 	spin_lock(&tree->lock);
+	/*
+	 * A duplicate entry should have been removed at the beginning of this
+	 * function. Since the swap entry should be pinned, if a duplicate is
+	 * found again here it means that something went wrong in the swap
+	 * cache.
+	 */
 	while (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
+		WARN_ON(1);
 		zswap_duplicate_entry++;
 		zswap_invalidate_entry(tree, dupentry);
 	}