tipc: check minimum bearer MTU

[ Upstream commit 3de81b75 ]

Qian Zhang (张谦) reported a potential socket buffer overflow in
tipc_msg_build() which is also known as CVE-2016-8632: due to
insufficient checks, a buffer overflow can occur if MTU is too short for
even tipc headers. As anyone can set device MTU in a user/net namespace,
this issue can be abused by a regular user.

As agreed in the discussion on Ben Hutchings' original patch, we should
check the MTU at the moment a bearer is attached rather than for each
processed packet. We also need to repeat the check when bearer MTU is
adjusted to new device MTU. UDP case also needs a check to avoid
overflow when calculating bearer MTU.

References: CVE-2016-8632
Fixes: b97bf3fd ("[TIPC] Initial merge")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reported-by: Qian Zhang (张谦) <zhangqian-c@360.cn>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Conflicts:
	net/tipc/bearer.c
	net/tipc/bearer.h
due to 1a90632d: tipc: eliminate remnants of hungarian notation
and b1c29f6b tipc: simplify resetting and disabling of bearers

Signed-off-by: Philipp Hahn <hahn@univention.de>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>