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Message-Id: <20220201180822.835171440@linuxfoundation.org>
Date: Tue, 1 Feb 2022 19:16:45 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
stable@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
Ray Che <xijiache@...il.com>, David Ahern <dsahern@...nel.org>,
Geoff Alexander <alexandg@...unm.edu>,
Willy Tarreau <w@....eu>, Jakub Kicinski <kuba@...nel.org>,
Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.4 21/25] ipv4: tcp: send zero IPID in SYNACK messages
From: Eric Dumazet <edumazet@...gle.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@...gle.com>
Reported-by: Ray Che <xijiache@...il.com>
Reviewed-by: David Ahern <dsahern@...nel.org>
Cc: Geoff Alexander <alexandg@...unm.edu>
Cc: Willy Tarreau <w@....eu>
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.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
@@ -155,12 +155,19 @@ 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) {
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