Subject: Re: [PATCH net v2 2/2] tcp: tighten acceptance of ACKs not matching a child socket

From: Eric Dumazet <[email-masked]>
Date: Wed, 4 Dec 2019 19:08:49 -0800

On 12/4/19 4:59 PM, Guillaume Nault wrote:
> When no synflood occurs, the synflood timestamp isn't updated.
> Therefore it can be so old that time_after32() can consider it to be
> in the future.
> 
> That's a problem for tcp_synq_no_recent_overflow() as it may report
> that a recent overflow occurred while, in fact, it's just that jiffies
> has grown past 'last_overflow' + TCP_SYNCOOKIE_VALID + 2^31.
> 
> Spurious detection of recent overflows lead to extra syncookie
> verification in cookie_v[46]_check(). At that point, the verification
> should fail and the packet dropped. But we should have dropped the
> packet earlier as we didn't even send a syncookie.
> 
> Let's refine tcp_synq_no_recent_overflow() to report a recent overflow
> only if jiffies is within the
> [last_overflow, last_overflow + TCP_SYNCOOKIE_VALID] interval. This
> way, no spurious recent overflow is reported when jiffies wraps and
> 'last_overflow' becomes in the future from the point of view of
> time_after32().
> 
> However, if jiffies wraps and enters the
> [last_overflow, last_overflow + TCP_SYNCOOKIE_VALID] interval (with
> 'last_overflow' being a stale synflood timestamp), then
> tcp_synq_no_recent_overflow() still erroneously reports an
> overflow. In such cases, we have to rely on syncookie verification
> to drop the packet. We unfortunately have no way to differentiate
> between a fresh and a stale syncookie timestamp.
> 
> Signed-off-by: Guillaume Nault <[email-masked]>
> ---
>  include/net/tcp.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index f0eae83ee555..005d4c691543 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -520,12 +520,14 @@ static inline bool tcp_synq_no_recent_overflow(const struct sock *sk)
>  		if (likely(reuse)) {
>  			last_overflow = READ_ONCE(reuse->synq_overflow_ts);
>  			return time_after32(now, last_overflow +
> -					    TCP_SYNCOOKIE_VALID);
> +					    TCP_SYNCOOKIE_VALID) ||
> +				time_before32(now, last_overflow);
>  		}
>  	}
>  
>  	last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
> -	return time_after32(now, last_overflow + TCP_SYNCOOKIE_VALID);
> +	return time_after32(now, last_overflow + TCP_SYNCOOKIE_VALID) ||
> +		time_before32(now, last_overflow);
>  }

There is a race I believe here.

CPU1                                 CPU2
 
now = jiffies.
    ...
                                     jiffies++
                                     ...
                                     SYN received, last_overflow is updated to the new jiffies.

CPU1 
 timer_before32(now, last_overflow) is true, because last_overflow was set to now+1

I suggest some cushion here.

Also we TCP uses between() macro, we might add a time_between32(a, b, c) macro
to ease code review.

->
  return !time_between32(last_overflow - HZ, now, last_overflow + TCP_SYNCOOKIE_VALID);