exfat: fix the new buffer was not zeroed before writing

Before writing, if a buffer_head marked as new, its data must
be zeroed, otherwise uninitialized data in the page cache will
be written.

So this commit uses folio_zero_new_buffers() to zero the new
buffers before ->write_end().

Fixes: 6630ea4 ("exfat: move extend valid_size into ->page_mkwrite()")
Reported-by: syzbot+91ae49e1c1a2634d20c0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=91ae49e1c1a2634d20c0
Tested-by: syzbot+91ae49e1c1a2634d20c0@syzkaller.appspotmail.com
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

Author: YuezhangMo

fs/exfat/file.c

@@ -545,6 +545,7 @@ static int exfat_extend_valid_size(struct file *file, loff_t new_valid_size)
 	while (pos < new_valid_size) {
 		u32 len;
 		struct folio *folio;
+		unsigned long off;
 
 		len = PAGE_SIZE - (pos & (PAGE_SIZE - 1));
 		if (pos + len > new_valid_size)
@@ -554,6 +555,9 @@ static int exfat_extend_valid_size(struct file *file, loff_t new_valid_size)
 		if (err)
 			goto out;
 
+		off = offset_in_folio(folio, pos);
+		folio_zero_new_buffers(folio, off, off + len);
+
 		err = ops->write_end(file, mapping, pos, len, len, folio, NULL);
 		if (err < 0)
 			goto out;
@@ -563,6 +567,8 @@ static int exfat_extend_valid_size(struct file *file, loff_t new_valid_size)
 		cond_resched();
 	}
 
+	return 0;
+
 out:
 	return err;
 }