fs/jfs: Add check for negative db_l2nbperpage

l2nbperpage is log2(number of blks per page), and the minimum legal
value should be 0, not negative.

In the case of l2nbperpage being negative, an error will occur
when subsequently used as shift exponent.

Syzbot reported this bug:

UBSAN: shift-out-of-bounds in fs/jfs/jfs_dmap.c:799:12
shift exponent -16777216 is negative

Reported-by: syzbot+debee9ab7ae2b34b0307@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=debee9ab7ae2b34b0307
Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>

diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index 88afd108c2dd2e04c63ee2bb7348c8173bc6d26b..3a1842348112dedc4a87b2ba11646f8d6b83abb6 100644 (file)
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -180,7 +180,8 @@ 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);
-       if (bmp->db_l2nbperpage > L2PSIZE - L2MINBLOCKSIZE) {
+       if (bmp->db_l2nbperpage > L2PSIZE - L2MINBLOCKSIZE ||
+               bmp->db_l2nbperpage < 0) {
                err = -EINVAL;
                goto err_release_metapage;
        }