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

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Date:   Thu, 5 Dec 2019 01:59:13 +0100
From:   Guillaume Nault <gnault@...hat.com>
To:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net v2 2/2] tcp: tighten acceptance of ACKs not matching a
 child socket

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 <gnault@...hat.com>
---
 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);
 }

 static inline u32 tcp_cookie_time(void)
-- 
2.21.0