linux-kernel - Re: [PATCH net] tipc: check minimum bearer MTU

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Message-ID: <1480550709.16599.81.camel@decadent.org.uk>
Date:   Thu, 01 Dec 2016 00:05:09 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Michal Kubecek <mkubecek@...e.cz>,
        Jon Maloy <jon.maloy@...csson.com>,
        Ying Xue <ying.xue@...driver.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        tipc-discussion@...ts.sourceforge.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Qian Zhang <zhangqian-c@....cn>
Subject: Re: [PATCH net] tipc: check minimum bearer MTU

On Wed, 2016-11-30 at 11:24 +0100, Michal Kubecek wrote:
> On Wed, Nov 30, 2016 at 10:57:02AM +0100, Michal Kubecek wrote:
> > 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.
> > 
> > Fixes: b97bf3fd8f6a ("[TIPC] Initial merge")
> > Signed-off-by: Michal Kubecek <mkubecek@...e.cz>
> > Reported-by: Qian Zhang (张谦) <zhangqian-c@....cn>
> 
> Self-NACK.
> 
> 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.
[...]

I'm not sure that TIPC should block the MTU change, anyway.  For IPv4
and IPv6 we disable the protocol on a device if its MTU is reduced
below the minimum.  I think TIPC should behave the same way.

Ben.

-- 
Ben Hutchings
Never attribute to conspiracy what can adequately be explained by
stupidity.


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