Subject: CVE-2024-43887: net/tcp: Disable TCP-AO static key after RCU grace period

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

net/tcp: Disable TCP-AO static key after RCU grace period

The lifetime of TCP-AO static_key is the same as the last
tcp_ao_info. On the socket destruction tcp_ao_info ceases to be
with RCU grace period, while tcp-ao static branch is currently deferred
destructed. The static key definition is
: DEFINE_STATIC_KEY_DEFERRED_FALSE(tcp_ao_needed, HZ);

which means that if RCU grace period is delayed by more than a second
and tcp_ao_needed is in the process of disablement, other CPUs may
yet see tcp_ao_info which atent dead, but soon-to-be.
And that breaks the assumption of static_key_fast_inc_not_disabled().

See the comment near the definition:
> * The caller must make sure that the static key can't get disabled while
> * in this function. It doesn't patch jump labels, only adds a user to
> * an already enabled static key.

Originally it was introduced in commit eb8c507296f6 ("jump_label:
Prevent key->enabled int overflow"), which is needed for the atomic
contexts, one of which would be the creation of a full socket from a
request socket. In that atomic context, it's known by the presence
of the key (md5/ao) that the static branch is already enabled.
So, the ref counter for that static branch is just incremented
instead of holding the proper mutex.
static_key_fast_inc_not_disabled() is just a helper for such usage
case. But it must not be used if the static branch could get disabled
in parallel as it's not protected by jump_label_mutex and as a result,
races with jump_label_update() implementation details.

Happened on netdev test-bot[1], so not a theoretical issue:

[] jump_label: Fatal kernel bug, unexpected op at tcp_inbound_hash+0x1a7/0x870 [ffffffffa8c4e9b7] (eb 50 0f 1f 44 != 66 90 0f 1f 00)) size:2 type:1
[] ------------[ cut here ]------------
[] kernel BUG at arch/x86/kernel/jump_label.c:73!
[] Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN NOPTI
[] CPU: 3 PID: 243 Comm: kworker/3:3 Not tainted 6.10.0-virtme #1
[] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[] Workqueue: events jump_label_update_timeout
[] RIP: 0010:__jump_label_patch+0x2f6/0x350
...
[] Call Trace:
[]  <TASK>
[]  arch_jump_label_transform_queue+0x6c/0x110
[]  __jump_label_update+0xef/0x350
[]  __static_key_slow_dec_cpuslocked.part.0+0x3c/0x60
[]  jump_label_update_timeout+0x2c/0x40
[]  process_one_work+0xe3b/0x1670
[]  worker_thread+0x587/0xce0
[]  kthread+0x28a/0x350
[]  ret_from_fork+0x31/0x70
[]  ret_from_fork_asm+0x1a/0x30
[]  </TASK>
[] Modules linked in: veth
[] ---[ end trace 0000000000000000 ]---
[] RIP: 0010:__jump_label_patch+0x2f6/0x350

[1]: https://netdev-3.bots.linux.dev/vmksft-tcp-ao-dbg/results/696681/5-connect-deny-ipv6/stderr

The Linux kernel CVE team has assigned CVE-2024-43887 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 6.7 with commit 67fa83f7c86a and fixed in 6.10.5 with commit 954d55a59b25
	Issue introduced in 6.7 with commit 67fa83f7c86a and fixed in 6.11 with commit 14ab4792ee12

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-43887
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	net/ipv4/tcp_ao.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/954d55a59b2501f4a9bd693b40ce45a1c46cb2b3
	https://git.kernel.org/stable/c/14ab4792ee120c022f276a7e4768f4dcb08f0cdd