From: Florian Westphal <fw@strlen.de>
Subject: netfilter: bridge: ebt_among: add missing match size
 checks
Patch-mainline: v4.16-rc5
Git-commit: c4585a2823edf4d1326da44d1524ecbfda26bb37
References: git-fixes

ebt_among is special, it has a dynamic match size and is exempt
from the central size checks.

Therefore it must check that the size of the match structure
provided from userspace is sane by making sure em->match_size
is at least the minimum size of the expected structure.

The module has such a check, but its only done after accessing
a structure that might be out of bounds.

tested with: ebtables -A INPUT ... \
--among-dst fe:fe:fe:fe:fe:fe
--among-dst fe:fe:fe:fe:fe:fe --among-src fe:fe:fe:fe:ff:f,fe:fe:fe:fe:fe:fb,fe:fe:fe:fe:fc:fd,fe:fe:fe:fe:fe:fd,fe:fe:fe:fe:fe:fe
--among-src fe:fe:fe:fe:ff:f,fe:fe:fe:fe:fe:fa,fe:fe:fe:fe:fe:fd,fe:fe:fe:fe:fe:fe,fe:fe:fe:fe:fe:fe

Reported-by: <syzbot+fe0b19af568972814355@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Denis Kirjanov <dkirjanov@suse.com>
---
 net/bridge/netfilter/ebt_among.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/net/bridge/netfilter/ebt_among.c b/net/bridge/netfilter/ebt_among.c
index 279527f8b1fe..59baaecd3e54 100644
--- a/net/bridge/netfilter/ebt_among.c
+++ b/net/bridge/netfilter/ebt_among.c
@@ -172,18 +172,35 @@ ebt_among_mt(const struct sk_buff *skb, struct xt_action_param *par)
 	return true;
 }
 
+static bool poolsize_invalid(const struct ebt_mac_wormhash *w)
+{
+	return w && w->poolsize >= (INT_MAX / sizeof(struct ebt_mac_wormhash_tuple));
+}
+
 static int ebt_among_mt_check(const struct xt_mtchk_param *par)
 {
 	const struct ebt_among_info *info = par->matchinfo;
 	const struct ebt_entry_match *em =
 		container_of(par->matchinfo, const struct ebt_entry_match, data);
-	int expected_length = sizeof(struct ebt_among_info);
+	unsigned int expected_length = sizeof(struct ebt_among_info);
 	const struct ebt_mac_wormhash *wh_dst, *wh_src;
 	int err;
 
+	if (expected_length > em->match_size)
+		return -EINVAL;
+
 	wh_dst = ebt_among_wh_dst(info);
-	wh_src = ebt_among_wh_src(info);
+	if (poolsize_invalid(wh_dst))
+		return -EINVAL;
+
 	expected_length += ebt_mac_wormhash_size(wh_dst);
+	if (expected_length > em->match_size)
+		return -EINVAL;
+
+	wh_src = ebt_among_wh_src(info);
+	if (poolsize_invalid(wh_src))
+		return -EINVAL;
+
 	expected_length += ebt_mac_wormhash_size(wh_src);
 
 	if (em->match_size != EBT_ALIGN(expected_length)) {
-- 
2.12.3