[PATCH v2] jfs: fix divide error in dbNextAG

[Pavel Skripkin <paskripkin@gmail.com>]

Syzbot reported divide error in dbNextAG(). The problem was in missing
validation check for malicious image.

Syzbot crafted an image with bmp->db_numag equal to 0. There wasn't any
validation checks, but dbNextAG() blindly use bmp->db_numag in divide
expression

Fix it by validating bmp->db_numag in dbMount() and return an error if
image is malicious

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-and-tested-by: syzbot+46f5c25af73eb8330eb6@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
---

Changes since v1:
	- Add missing clean up in case of error

---
 fs/jfs/jfs_dmap.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index 91f4ec93dab1..d8502f4989d9 100644
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -148,6 +148,7 @@ static const s8 budtab[256] = {
  *	0	- success
  *	-ENOMEM	- insufficient memory
  *	-EIO	- i/o error
+ *	-EINVAL - wrong bmap data
  */
 int dbMount(struct inode *ipbmap)
 {
@@ -179,6 +180,12 @@ int dbMount(struct inode *ipbmap)
 	bmp->db_nfree = le64_to_cpu(dbmp_le->dn_nfree);
 	bmp->db_l2nbperpage = le32_to_cpu(dbmp_le->dn_l2nbperpage);
 	bmp->db_numag = le32_to_cpu(dbmp_le->dn_numag);
+	if (!bmp->db_numag) {
+		release_metapage(mp);
+		kfree(bmp);
+		return -EINVAL;
+	}
+
 	bmp->db_maxlevel = le32_to_cpu(dbmp_le->dn_maxlevel);
 	bmp->db_maxag = le32_to_cpu(dbmp_le->dn_maxag);
 	bmp->db_agpref = le32_to_cpu(dbmp_le->dn_agpref);
-- 
2.35.1