From: Eric Dumazet edumazet@google.com
[ Upstream commit 970a5a3ea86da637471d3cd04d513a0755aba4bf ]
In commit 431280eebed9 ("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: 1da177e4c3f4 ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/ip_output.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
--- a/net/ipv4/ip_output.c +++ b/net/ipv4/ip_output.c @@ -159,12 +159,19 @@ int ip_build_and_send_pkt(struct sk_buff iph->daddr = (opt && opt->opt.srr ? opt->opt.faddr : daddr); iph->saddr = saddr; iph->protocol = sk->sk_protocol; - if (ip_dont_fragment(sk, &rt->dst)) { + /* Do not bother generating IPID for small packets (eg SYNACK) */ + if (skb->len <= IPV4_MIN_MTU || ip_dont_fragment(sk, &rt->dst)) { iph->frag_off = htons(IP_DF); iph->id = 0; } else { iph->frag_off = 0; - __ip_select_ident(net, iph, 1); + /* TCP packets here are SYNACK with fat IPv4/TCP options. + * Avoid using the hashed IP ident generator. + */ + if (sk->sk_protocol == IPPROTO_TCP) + iph->id = (__force __be16)prandom_u32(); + else + __ip_select_ident(net, iph, 1); }
if (opt && opt->opt.optlen) {