f2fs: fix NULL pointer dereference in f2fs_verity_work()

If both compression and fsverity feature is on, generic/572 will
report below NULL pointer dereference bug.

 BUG: kernel NULL pointer dereference, address: 0000000000000018
 RIP: 0010:f2fs_verity_work+0x60/0x90 [f2fs]
 #PF: supervisor read access in kernel mode
 Workqueue: fsverity_read_queue f2fs_verity_work [f2fs]
 RIP: 0010:f2fs_verity_work+0x60/0x90 [f2fs]
 Call Trace:
  process_one_work+0x16c/0x3f0
  worker_thread+0x4c/0x440
  ? rescuer_thread+0x350/0x350
  kthread+0xf8/0x130
  ? kthread_unpark+0x70/0x70
  ret_from_fork+0x35/0x40

There are two issue in f2fs_verity_work():
- it needs to traverse and verify all pages in bio.
- if pages in bio belong to non-compressed cluster, accessing
decompress IO context stored in page private will cause NULL
pointer dereference.

Fix them.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/compress.c

@@ -477,6 +477,8 @@ void f2fs_decompress_pages(struct bio *bio, struct page *page, bool verity)
 out_vunmap_rbuf:
 	vunmap(dic->rbuf);
 out_free_dic:
+	if (verity)
+		refcount_add(dic->nr_cpages - 1, &dic->ref);
 	if (!verity)
 		f2fs_decompress_end_io(dic->rpages, dic->cluster_size,
 								ret, false);

fs/f2fs/data.c

@@ -187,12 +187,37 @@ static void f2fs_verify_pages(struct page **rpages, unsigned int cluster_size)
 
 static void f2fs_verify_bio(struct bio *bio)
 {
-	struct page *page = bio_first_page_all(bio);
-	struct decompress_io_ctx *dic =
-			(struct decompress_io_ctx *)page_private(page);
+	struct bio_vec *bv;
+	struct bvec_iter_all iter_all;
+
+	bio_for_each_segment_all(bv, bio, iter_all) {
+		struct page *page = bv->bv_page;
+		struct decompress_io_ctx *dic;
+
+		dic = (struct decompress_io_ctx *)page_private(page);
+
+		if (dic) {
+			if (refcount_dec_not_one(&dic->ref))
+				continue;
+			f2fs_verify_pages(dic->rpages,
+						dic->cluster_size);
+			f2fs_free_dic(dic);
+			continue;
+		}
+
+		if (bio->bi_status || PageError(page))
+			goto clear_uptodate;
 
-	f2fs_verify_pages(dic->rpages, dic->cluster_size);
-	f2fs_free_dic(dic);
+		if (fsverity_verify_page(page)) {
+			SetPageUptodate(page);
+			goto unlock;
+		}
+clear_uptodate:
+		ClearPageUptodate(page);
+		ClearPageError(page);
+unlock:
+		unlock_page(page);
+	}
 }
 #endif