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Eric Dumazet 970a5a3ea8 ipv4: tcp: send zero IPID in SYNACK messages 
In commit 431280eebe ("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: 1da177e4c3 ("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>
2022-01-27 08:37:02 -08:00
arch Networking fixes for 5.17-rc1, including fixes from netfilter, bpf. 2022-01-20 10:57:05 +02:00
block SCSI misc on 20220113 2022-01-14 14:37:34 +01:00
certs certs: move scripts/extract-cert to certs/ 2022-01-08 18:28:21 +09:00
crypto lib/crypto: add prompts back to crypto libraries 2022-01-18 13:03:55 +01:00
Documentation dt-bindings: can: tcan4x5x: fix mram-cfg RX FIFO config 2022-01-24 18:27:43 +01:00
drivers net: stmmac: dwmac-sun8i: use return val of readl_poll_timeout() 2022-01-27 13:51:10 +00:00
fs Merge branch 'akpm' (patches from Andrew) 2022-01-20 10:41:01 +02:00
include Revert "ipv6: Honor all IPv6 PIO Valid Lifetime values" 2022-01-27 13:35:14 +00:00
init Kbuild updates for v5.17 2022-01-19 11:15:19 +02:00
ipc shm: extend forced shm destroy to support objects from several IPC nses 2021-11-20 10:35:54 -08:00
kernel Networking fixes for 5.17-rc1, including fixes from netfilter, bpf. 2022-01-20 10:57:05 +02:00
lib Networking fixes for 5.17-rc1, including fixes from netfilter, bpf. 2022-01-20 10:57:05 +02:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm Merge branch 'akpm' (patches from Andrew) 2022-01-20 10:41:01 +02:00
net ipv4: tcp: send zero IPID in SYNACK messages 2022-01-27 08:37:02 -08:00
samples Merge branch 'akpm' (patches from Andrew) 2022-01-20 10:41:01 +02:00
scripts Merge branch 'akpm' (patches from Andrew) 2022-01-20 10:41:01 +02:00
security fs.idmapped.v5.17 2022-01-11 14:26:55 -08:00
sound virtio,vdpa,qemu_fw_cfg: features, cleanups, fixes 2022-01-18 10:05:48 +02:00
tools selftests: net: ioam: expect support for Queue depth data 2022-01-21 19:26:39 -08:00
usr kbuild: rename cmd_{bzip2,lzma,lzo,lz4,xzkern,zstd22} 2022-01-14 02:54:05 +09:00
virt RISCV: 2022-01-16 16:15:14 +02:00
.clang-format genirq/msi: Make interrupt allocation less convoluted 2021-12-16 22:22:20 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore only top-level modules.builtin 2021-05-02 00:43:35 +09:00
.mailmap mailmap: update email address of Brian Silverman 2022-01-24 18:27:23 +01:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Removing Ohad from remoteproc/rpmsg maintenance 2021-12-08 10:09:40 -07:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: add missing IPv4/IPv6 header paths 2022-01-27 13:58:28 +00:00
Makefile Kbuild updates for v5.17 2022-01-19 11:15:19 +02:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.