netfilter: x_tables: check for bogus target offset

netfilter: x_tables: check for bogus target offset

Orabug: 24690280
CVE: CVE-2016-3134

[ Upstream commit ce683e5f9d045e5d67d1312a42b359cb2ab2a13c ]

We're currently asserting that targetoff + targetsize <= nextoff.

Extend it to also check that targetoff is >= sizeof(xt_entry).
Since this is generic code, add an argument pointing to the start of the
match/target, we can then derive the base structure size from the delta.

We also need the e->elems pointer in a followup change to validate matches.

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 451e4403bc4abc51539376d4314baa739ab9e996)
Signed-off-by: Brian Maly <brian.maly@oracle.com>