[Codel] [RFC PATCH] Don't give malicious CE senders a free ride

[Dave Taht]

After doing myself in with BBR not respecting CE marks, I'm now thinking
that perhaps the right thing for red/fq_codel/cake/pie/etc to do is actually
*drop* stuff that it sees that is already CE marked, when it too wants
to mark, instead of giving it a free ride.

It's an unfriendly world out there.

Not happy with the name of the function... and to me - it looks like
the existing  call sites for IP_ECN_set_ce would actually be saner if
they always returned 0 instead of !ecn.

diff --git a/include/net/inet_ecn.h b/include/net/inet_ecn.h
index dce2d58..3742d84 100644
--- a/include/net/inet_ecn.h
+++ b/include/net/inet_ecn.h
@@ -71,6 +71,36 @@ static inline void INET_ECN_dontxmit(struct sock *sk)
                (label) |= htonl(INET_ECN_ECT_0 << 20);                 \
     } while (0)


+/* When CE is already asserted, sometimes it is saner to drop the packet
+   in order to not give malicious CE senders as much of a free ride. */
+
+static inline int IP_ECN_safe_set_ce(struct iphdr *iph)
+{
+       u32 check = (__force u32)iph->check;
+       u32 ecn = (iph->tos + 1) & INET_ECN_MASK;
+
+       /*
+        * After the last operation we have (in binary):
+        * INET_ECN_NOT_ECT => 01
+        * INET_ECN_ECT_1   => 10
+        * INET_ECN_ECT_0   => 11
+        * INET_ECN_CE      => 00
+        */
+       if (!(ecn & 2))
+               return 0;
+
+       /*
+        * The following gives us:
+        * INET_ECN_ECT_1 => check += htons(0xFFFD)
+        * INET_ECN_ECT_0 => check += htons(0xFFFE)
+        */
+       check += (__force u16)htons(0xFFFB) + (__force u16)htons(ecn);
+
+       iph->check = (__force __sum16)(check + (check>=0xFFFF));
+       iph->tos |= INET_ECN_CE;
+       return 1;
+}
+

-- 
Dave Täht
Let's go make home routers and wifi faster! With better software!
http://blog.cerowrt.org