Re: [PATCH net] tipc: check minimum bearer MTU

From: Michal Kubecek
Date: Wed Nov 30 2016 - 05:24:40 EST

On Wed, Nov 30, 2016 at 10:57:02AM +0100, Michal Kubecek wrote:
> Qian Zhang (åè) 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.
> Fixes: b97bf3fd8f6a ("[TIPC] Initial merge")
> Signed-off-by: Michal Kubecek <mkubecek@xxxxxxx>
> Reported-by: Qian Zhang (åè) <zhangqian-c@xxxxxx>


Im sorry, while testing this, I overlooked that an attempt to change
MTU of an underlying device to low value issues a warning but it
succeeds anyway.

> @@ -624,6 +626,9 @@ static int tipc_l2_device_event(struct notifier_block *nb, unsigned long evt,
> tipc_reset_bearer(net, b);
> break;
> + if (tipc_check_mtu(dev, 0))
> + return -EINVAL;
> + b->mtu = dev->mtu;
> tipc_reset_bearer(net, b);
> break;

This is a notifier so that error value needs to be encoded into notifier
error. I'll send v2 after retesting

Michal Kubecek