netdev - [PATCH net v3 2/3] tcp: tighten acceptance of ACKs not matching a child socket

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Message-ID: <05f412281ffe11a603260c849851df39c0e8c952.1575595670.git.gnault@redhat.com>
Date:   Fri, 6 Dec 2019 02:49:55 +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 v3 2/3] 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.

In practice, using last_overflow as lower bound is problematic.
If the synflood timestamp is concurrently updated between the time
we read jiffies and the moment we store the timestamp in
'last_overflow', then 'now' becomes smaller than 'last_overflow' and
tcp_synq_no_recent_overflow() returns true, potentially dropping a
valid syncookie.

Reading jiffies after loading the timestamp could fix the problem,
but that'd require a memory barrier. Let's just accommodate for
potential timestamp growth instead and extend the interval using
'last_overflow - HZ' as lower bound.

Suggested-by: Eric Dumazet <eric.dumazet@...il.com>
Signed-off-by: Guillaume Nault <gnault@...hat.com>
---
 include/net/tcp.h | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 7d734ba391fc..43e04e14c41e 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -518,13 +518,23 @@ static inline bool tcp_synq_no_recent_overflow(const struct sock *sk)
 		reuse = rcu_dereference(sk->sk_reuseport_cb);
 		if (likely(reuse)) {
 			last_overflow = READ_ONCE(reuse->synq_overflow_ts);
-			return time_after32(now, last_overflow +
-					    TCP_SYNCOOKIE_VALID);
+			return !time_between32(now, last_overflow - HZ,
+					       last_overflow +
+					       TCP_SYNCOOKIE_VALID);
 		}
 	}

 	last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
-	return time_after32(now, last_overflow + TCP_SYNCOOKIE_VALID);
+
+	/* If last_overflow <= jiffies <= last_overflow + TCP_SYNCOOKIE_VALID,
+	 * then we're under synflood. However, we have to use
+	 * 'last_overflow - HZ' as lower bound. That's because a concurrent
+	 * tcp_synq_overflow() could update .ts_recent_stamp after we read
+	 * jiffies but before we store .ts_recent_stamp into last_overflow,
+	 * which could lead to rejecting a valid syncookie.
+	 */
+	return !time_between32(now, last_overflow - HZ,
+			       last_overflow + TCP_SYNCOOKIE_VALID);
 }

 static inline u32 tcp_cookie_time(void)
-- 
2.21.0