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Eric Dumazet 008b807fe4 tcp: do not accept ACK of bytes we never sent 
[ Upstream commit 3d501dd326 ]

This patch is based on a detailed report and ideas from Yepeng Pan
and Christian Rossow.

ACK seq validation is currently following RFC 5961 5.2 guidelines:

   The ACK value is considered acceptable only if
   it is in the range of ((SND.UNA - MAX.SND.WND) <= SEG.ACK <=
   SND.NXT).  All incoming segments whose ACK value doesn't satisfy the
   above condition MUST be discarded and an ACK sent back.  It needs to
   be noted that RFC 793 on page 72 (fifth check) says: "If the ACK is a
   duplicate (SEG.ACK < SND.UNA), it can be ignored.  If the ACK
   acknowledges something not yet sent (SEG.ACK > SND.NXT) then send an
   ACK, drop the segment, and return".  The "ignored" above implies that
   the processing of the incoming data segment continues, which means
   the ACK value is treated as acceptable.  This mitigation makes the
   ACK check more stringent since any ACK < SND.UNA wouldn't be
   accepted, instead only ACKs that are in the range ((SND.UNA -
   MAX.SND.WND) <= SEG.ACK <= SND.NXT) get through.

This can be refined for new (and possibly spoofed) flows,
by not accepting ACK for bytes that were never sent.

This greatly improves TCP security at a little cost.

I added a Fixes: tag to make sure this patch will reach stable trees,
even if the 'blamed' patch was adhering to the RFC.

tp->bytes_acked was added in linux-4.2

Following packetdrill test (courtesy of Yepeng Pan) shows
the issue at hand:

0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0 bind(3, ..., ...) = 0
+0 listen(3, 1024) = 0

// ---------------- Handshake ------------------- //

// when window scale is set to 14 the window size can be extended to
// 65535 * (2^14) = 1073725440. Linux would accept an ACK packet
// with ack number in (Server_ISN+1-1073725440. Server_ISN+1)
// ,though this ack number acknowledges some data never
// sent by the server.

+0 < S 0:0(0) win 65535 <mss 1400,nop,wscale 14>
+0 > S. 0:0(0) ack 1 <...>
+0 < . 1:1(0) ack 1 win 65535
+0 accept(3, ..., ...) = 4

// For the established connection, we send an ACK packet,
// the ack packet uses ack number 1 - 1073725300 + 2^32,
// where 2^32 is used to wrap around.
// Note: we used 1073725300 instead of 1073725440 to avoid possible
// edge cases.
// 1 - 1073725300 + 2^32 = 3221241997

// Oops, old kernels happily accept this packet.
+0 < . 1:1001(1000) ack 3221241997 win 65535

// After the kernel fix the following will be replaced by a challenge ACK,
// and prior malicious frame would be dropped.
+0 > . 1:1(0) ack 1001

Fixes: 354e4aa391 ("tcp: RFC 5961 5.2 Blind Data Injection Attack Mitigation")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Yepeng Pan <yepeng.pan@cispa.de>
Reported-by: Christian Rossow <rossow@cispa.de>
Acked-by: Neal Cardwell <ncardwell@google.com>
Link: https://lore.kernel.org/r/20231205161841.2702925-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2023-12-13 18:39:11 +01:00
arch x86/tdx: Allow 32-bit emulation by default 2023-12-13 18:39:05 +01:00
block blk-core: use pr_warn_ratelimited() in bio_check_ro() 2023-11-20 11:52:17 +01:00
certs certs: Fix build error when PKCS#11 URI contains semicolon 2023-02-09 11:28:11 +01:00
crypto crypto: pcrypt - Fix hungtask for PADATA_RESET 2023-11-28 17:06:58 +00:00
Documentation dt-bindings: interrupt-controller: Allow #power-domain-cells 2023-12-13 18:39:10 +01:00
drivers octeontx2-af: Update Tx link register range 2023-12-13 18:39:10 +01:00
fs iomap: update ki_pos a little later in iomap_dio_complete 2023-12-08 08:51:20 +01:00
include netfilter: nft_exthdr: add boolean DCCP option matching 2023-12-13 18:39:10 +01:00
init proc: sysctl: prevent aliased sysctls from getting passed to init 2023-11-28 17:07:08 +00:00
io_uring io_uring: fix off-by one bvec index 2023-12-03 07:32:13 +01:00
ipc ipc: fix memory leak in init_mqueue_fs() 2022-12-31 13:32:01 +01:00
kernel hrtimers: Push pending hrtimers away from outgoing CPU earlier 2023-12-13 18:39:03 +01:00
lib zstd: Fix array-index-out-of-bounds UBSAN warning 2023-12-13 18:39:04 +01:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm,kfence: decouple kfence from page granularity mapping judgement 2023-12-03 07:32:08 +01:00
net tcp: do not accept ACK of bytes we never sent 2023-12-13 18:39:11 +01:00
rust rust: allocator: Prevent mis-aligned allocation 2023-08-11 12:08:18 +02:00
samples fprobe: Pass entry_data to handlers 2023-10-25 12:03:12 +02:00
scripts dt: dt-extract-compatibles: Don't follow symlinks when walking tree 2023-12-13 18:39:05 +01:00
security ima: detect changes to the backing overlay file 2023-11-28 17:07:12 +00:00
sound ALSA: hda/realtek: Add supported ALC257 for ChromeOS 2023-12-08 08:51:13 +01:00
tools selftests/net: mptcp: fix uninitialized variable warnings 2023-12-08 08:51:17 +01:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt kvm/vfio: ensure kvg instance stays around in kvm_vfio_group_add() 2023-09-13 09:42:46 +02:00
.clang-format inet: ping: use hlist_nulls rcu iterator during lookup 2022-12-01 12:42:46 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore Kbuild: add Rust support 2022-09-28 09:02:20 +02:00
.mailmap 9 hotfixes. 6 for MM, 3 for other areas. Four of these patches address 2022-12-10 17:10:52 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove Michal Marek from Kbuild maintainers 2022-11-16 14:53:00 +09:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS devlink: move code to a dedicated directory 2023-08-30 16:11:00 +02:00
Makefile Linux 6.1.67 2023-12-11 10:41:01 +01:00
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.