netdev - Re: [PATCH net 1/4] tcp: limit payload size of sacked skbs

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Date:   Mon, 17 Jun 2019 10:14:12 -0700
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Eric Dumazet" <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        "Eric Dumazet" <eric.dumazet@...il.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Jonathan Looney" <jtl@...flix.com>,
        "Neal Cardwell" <ncardwell@...gle.com>,
        "Tyler Hicks" <tyhicks@...onical.com>,
        "Yuchung Cheng" <ycheng@...gle.com>,
        "Bruce Curtis" <brucec@...flix.com>
Subject: Re: [PATCH net 1/4] tcp: limit payload size of sacked skbs



On 17 Jun 2019, at 10:03, Eric Dumazet wrote:

> Jonathan Looney reported that TCP can trigger the following crash
> in tcp_shifted_skb() :
>
> 	BUG_ON(tcp_skb_pcount(skb) < pcount);
>
> This can happen if the remote peer has advertized the smallest
> MSS that linux TCP accepts : 48
>
> An skb can hold 17 fragments, and each fragment can hold 32KB
> on x86, or 64KB on PowerPC.
>
> This means that the 16bit witdh of TCP_SKB_CB(skb)->tcp_gso_segs
> can overflow.
>
> Note that tcp_sendmsg() builds skbs with less than 64KB
> of payload, so this problem needs SACK to be enabled.
> SACK blocks allow TCP to coalesce multiple skbs in the retransmit
> queue, thus filling the 17 fragments to maximal capacity.
>
> CVE-2019-11477 -- u16 overflow of TCP_SKB_CB(skb)->tcp_gso_segs
>
> Fixes: 832d11c5cd07 ("tcp: Try to restore large SKBs while SACK 
> processing")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Jonathan Looney <jtl@...flix.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> Reviewed-by: Tyler Hicks <tyhicks@...onical.com>
Acked-by: Jonathan Lemon <jonathan.lemon@...il.com>