ipv4: tcp: send zero IPID in SYNACK messages
In commit 431280ee ("ipv4: tcp: send zero IPID for RST and ACK sent in SYN-RECV and TIME-WAIT state") we took care of some ctl packets sent by TCP. It turns out we need to use a similar strategy for SYNACK packets. By default, they carry IP_DF and IPID==0, but there are ways to ask them to use the hashed IP ident generator and thus be used to build off-path attacks. (Ref: Off-Path TCP Exploits of the Mixed IPID Assignment) One of this way is to force (before listener is started) echo 1 >/proc/sys/net/ipv4/ip_no_pmtu_disc Another way is using forged ICMP ICMP_FRAG_NEEDED with a very small MTU (like 68) to force a false return from ip_dont_fragment() In this patch, ip_build_and_send_pkt() uses the following heuristics. 1) Most SYNACK packets are smaller than IPV4_MIN_MTU and therefore can use IP_DF regardless of the listener or route pmtu setting. 2) In case the SYNACK packet is bigger than IPV4_MIN_MTU, we use prandom_u32() generator instead of the IPv4 hashed ident one. Fixes: 1da177e4 ("Linux-2.6.12-rc2") Signed-off-by:Eric Dumazet <edumazet@google.com> Reported-by:
Ray Che <xijiache@gmail.com> Reviewed-by:
David Ahern <dsahern@kernel.org> Cc: Geoff Alexander <alexandg@cs.unm.edu> Cc: Willy Tarreau <w@1wt.eu> Signed-off-by:
Jakub Kicinski <kuba@kernel.org>
parent
153a0d18
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