[PATCH net 2/2] ipv4: avoid using shared IP generator for connected sockets

[Eric Dumazet <eric.dumazet@gmail.com>]

From: Eric Dumazet <edumazet@google.com>

ip_select_ident_segs() has been very conservative about using
the connected socket private generator only for packets with IP_DF
set, claiming it was needed for some VJ compression implementations.

As mentioned in this referenced document, this can be abused.
(Ref: Off-Path TCP Exploits of the Mixed IPID Assignment)

Before switching to pure random IPID generation and possibly hurt
some workloads, lets use the private inet socket generator.

Not only this will remove one vulnerability, this will also
improve performance of TCP flows using pmtudisc==IP_PMTUDISC_DONT

Fixes: 73f156a6e8c1 ("inetpeer: get rid of ip_id_count")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Ray Che <xijiache@gmail.com>
Cc: Willy Tarreau <w@1wt.eu>
---
 include/net/ip.h | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/include/net/ip.h b/include/net/ip.h
index 81e23a102a0d5edec859b78239b81e6dcd82c54d..b51bae43b0ddb00735a09718530aa3fff4a04872 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -525,19 +525,18 @@ static inline void ip_select_ident_segs(struct net *net, struct sk_buff *skb,
 {
 	struct iphdr *iph = ip_hdr(skb);
 
+	/* We had many attacks based on IPID, use the private
+	 * generator as much as we can.
+	 */
+	if (sk && inet_sk(sk)->inet_daddr) {
+		iph->id = htons(inet_sk(sk)->inet_id);
+		inet_sk(sk)->inet_id += segs;
+		return;
+	}
 	if ((iph->frag_off & htons(IP_DF)) && !skb->ignore_df) {
-		/* This is only to work around buggy Windows95/2000
-		 * VJ compression implementations.  If the ID field
-		 * does not change, they drop every other packet in
-		 * a TCP stream using header compression.
-		 */
-		if (sk && inet_sk(sk)->inet_daddr) {
-			iph->id = htons(inet_sk(sk)->inet_id);
-			inet_sk(sk)->inet_id += segs;
-		} else {
-			iph->id = 0;
-		}
+		iph->id = 0;
 	} else {
+		/* Unfortunately we need the big hammer to get a suitable IPID */
 		__ip_select_ident(net, iph, segs);
 	}
 }
-- 
2.35.0.rc0.227.g00780c9af4-goog