netdev - Re: [PATCH net-next 03/13] sctp: remove an if() that is always true

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Message-ID: <20180427105050.GA22078@hmswarspite.think-freely.org>
Date:   Fri, 27 Apr 2018 06:50:50 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
        Vlad Yasevich <vyasevich@...il.com>,
        Xin Long <lucien.xin@...il.com>
Subject: Re: [PATCH net-next 03/13] sctp: remove an if() that is always true

On Thu, Apr 26, 2018 at 04:58:52PM -0300, Marcelo Ricardo Leitner wrote:
> As noticed by Xin Long, the if() here is always true as PMTU can never
> be 0.
> 
> Reported-by: Xin Long <lucien.xin@...il.com>
> Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> ---
>  net/sctp/associola.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/net/sctp/associola.c b/net/sctp/associola.c
> index b3aa95222bd52113295cb246c503c903bdd5c353..c5ed09cfa8423b17546e3d45f6d06db03af66384 100644
> --- a/net/sctp/associola.c
> +++ b/net/sctp/associola.c
> @@ -1397,10 +1397,8 @@ void sctp_assoc_sync_pmtu(struct sctp_association *asoc)
>  			pmtu = t->pathmtu;
>  	}
>  
> -	if (pmtu) {
> -		asoc->pathmtu = pmtu;
> -		asoc->frag_point = sctp_frag_point(asoc, pmtu);
> -	}
> +	asoc->pathmtu = pmtu;
> +	asoc->frag_point = sctp_frag_point(asoc, pmtu);
>  
Can you double check this?  Looking at it, it seems far fetched, but if someone
sends a crafted icmp dest unreach message to the host, pmtu_sending might be
able to get set for an association (which may have no transports established
yet), and if so, on the first packet send sctp_assoc_sync_pmtu can be called,
leading to a fall through in the loop over all transports, and pmtu being zero.
It seems like a far fetched set of circumstances, I know, but if it can happen,
I think you might see a crash in sctp_frag_point due to an underflow of the frag
value

Neil

>  	pr_debug("%s: asoc:%p, pmtu:%d, frag_point:%d\n", __func__, asoc,
>  		 asoc->pathmtu, asoc->frag_point);
> -- 
> 2.14.3
> 
>