Re: [PATCH net 1/2] ipv4: tcp: send zero IPID in SYNACK messages

[David Ahern <dsahern@gmail.com>]

On 1/26/22 1:05 PM, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> 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>
> Cc: Geoff Alexander <alexandg@cs.unm.edu>
> Cc: Willy Tarreau <w@1wt.eu>
> ---
>  net/ipv4/ip_output.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 


Reviewed-by: David Ahern <dsahern@kernel.org>