Re: [PATCH net] dccp: fix undefined behavior with 'cwnd' shift in ccid2_cwnd_restart()

[David Miller]

From: Alexey Kodanev <alexey.koda...@oracle.com>
Date: Thu,  2 Aug 2018 19:22:05 +0300

> Make sure that the value of "(now - hc->tx_lsndtime) / hc->tx_rto" is
> properly limited when shifting 'u32 cwnd' with it, otherwise we can get:
 ...
> Fixes: 113ced1f52e5 ("dccp ccid-2: Perform congestion-window validation")
> Signed-off-by: Alexey Kodanev <alexey.koda...@oracle.com>
 ...
> @@ -234,7 +234,7 @@ static void ccid2_cwnd_restart(struct sock *sk, const u32 
> now)
>  
>       /* don't reduce cwnd below the initial window (IW) */
>       restart_cwnd = min(cwnd, iwnd);
> -     cwnd >>= (now - hc->tx_lsndtime) / hc->tx_rto;
> +     cwnd >>= min((now - hc->tx_lsndtime) / hc->tx_rto, 31U);
>       hc->tx_cwnd = max(cwnd, restart_cwnd);
>  
>       hc->tx_cwnd_stamp = now;

Better to mimick the TCP cwnd validation code, something like:

        s32 delta = now - hc->tx_lsndtime;
        while ((delta -= hc->tx_rto) > 0 && cwnd > restart_cwnd)
                cwnd >>= 1;

Thanks.