f2fs: avoid BUG_ON when mouting corrupted image having garbage blocks

If the disk has some garbage blocks, F2FS is able to face with BUG_ON when
recovering direct node blocks.
This patch detects the error case and avoids that prior to reaching BUG_ON.

Alexey Khoroshilov addressed the potential security issues as follows.
"An ability to trigger a BUG_ON assert by mounting a crafted image is
usually considered as a local denial of service [1-3]. As far as I
understand, the reason is that some kernel data may become inconsistent
that can lead to further problems.

[1] http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2011-3353
[2] http://www.openwall.com/lists/oss-security/2011/06/24/4
[3] http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2011-2928
etc."
Reported-by: Andrey Tsyvarev <tsyvarev@ispras.ru>
Cc: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -1609,6 +1609,11 @@ int recover_inode_page(struct f2fs_sb_info *sbi, struct page *page)
 	struct node_info old_ni, new_ni;
 	struct page *ipage;
 
+	get_node_info(sbi, ino, &old_ni);
+
+	if (unlikely(old_ni.blk_addr != NULL_ADDR))
+		return -EINVAL;
+
 	ipage = grab_cache_page(NODE_MAPPING(sbi), ino);
 	if (!ipage)
 		return -ENOMEM;
@@ -1616,7 +1621,6 @@ int recover_inode_page(struct f2fs_sb_info *sbi, struct page *page)
 	/* Should not use this inode  from free nid list */
 	remove_free_nid(NM_I(sbi), ino);
 
-	get_node_info(sbi, ino, &old_ni);
 	SetPageUptodate(ipage);
 	fill_node_footer(ipage, ino, ino, 0, true);