netfilter: x_tables: validate targets of jumps

netfilter: x_tables: validate targets of jumps

Orabug: 24690280
CVE: CVE-2016-3134

[ Upstream commit 36472341017529e2b12573093cc0f68719300997 ]

When we see a jump also check that the offset gets us to beginning of
a rule (an ipt_entry).

The extra overhead is negible, even with absurd cases.

300k custom rules, 300k jumps to 'next' user chain:
[ plus one jump from INPUT to first userchain ]:

Before:
real    0m24.874s
user    0m7.532s
sys     0m16.076s

After:
real    0m27.464s
user    0m7.436s
sys     0m18.840s

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
(cherry picked from commit 8163327a3a927c36f7c032bb67957e6c0f4ec27d)
Signed-off-by: Brian Maly <brian.maly@oracle.com>