CVE-2026-64459: tcp: restore RCU grace period in tcp_ao_destroy_sock

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

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

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

tcp: restore RCU grace period in tcp_ao_destroy_sock

Commit 51e547e8c89c ("tcp: Free TCP-AO/TCP-MD5 info/keys without RCU")
removed the call_rcu() callback from tcp_ao_destroy_sock(), arguing that
"the destruction of info/keys is delayed until the socket destructor"
and therefore "no one can discover it anymore".

That argument does not hold for the call site in tcp_connect()
(net/ipv4/tcp_output.c:4327-4332). At that point the socket is in
TCP_SYN_SENT, has already been inserted into the inet ehash by
inet_hash_connect() in tcp_v4_connect(), and is therefore very much
discoverable: any softirq running tcp_v4_rcv() on another CPU can take
the socket out of the ehash, walk into tcp_inbound_hash(), and load
tp->ao_info via implicit RCU before bh_lock_sock_nested() is taken on
the destroying CPU.

The reader path then enters __tcp_ao_do_lookup() (net/ipv4/tcp_ao.c:208)
which re-loads tp->ao_info via rcu_dereference_check(); the re-load can
still observe the (about-to-be-freed) pointer because there is no
synchronize_rcu() between rcu_assign_pointer(tp->ao_info, NULL) and
tcp_ao_info_free() in tcp_ao_destroy_sock(). The captured pointer is
then walked at line 223:

	hlist_for_each_entry_rcu(key, &ao->head, node, ...)

The writer's synchronous kfree() is free to complete between the line
218 re-fetch and the line 223 hlist iteration. The slab is reused
(or simply LIST_POISON1-stamped if not yet reused) and the iteration
walks attacker-controlled or poison memory in softirq context.

Reproducer (no debug shim, stock x86_64 v7.1-rc2 SMP+KASAN, QEMU+KVM):
an unprivileged uid=1000 process inside CLONE_NEWUSER|CLONE_NEWNET
installs TCP_MD5SIG + TCP_AO_ADD_KEY on a TCP socket, sprays forged
TCP-AO segments toward its eventual 4-tuple via raw sockets, then
calls connect(). The md5-wins reconciliation in tcp_connect() fires
tcp_ao_destroy_sock(); the softirq backlog reader on the loopback
NAPI path crashes on the freed ao->head.first walk:

  Oops: general protection fault, probably for non-canonical
    address 0xfbd59c000000002f
  KASAN: maybe wild-memory-access in range
    [0xdead000000000178-0xdead00000000017f]
  CPU: 0 UID: 1000 PID: 100 Comm: repro_userns
  RIP: 0010:__tcp_ao_do_lookup+0x107/0x1c0
  Call Trace: <IRQ>
    __tcp_ao_do_lookup+0x107/0x1c0
    tcp_ao_inbound_lookup.constprop.0+0x12a/0x200
    tcp_inbound_ao_hash+0x5ea/0x1520
    tcp_inbound_hash+0x7ce/0x1240
    tcp_v4_rcv+0x1e7a/0x3e10
    ...

Restore the RCU grace period: re-add struct rcu_head to tcp_ao_info
and replace the synchronous tcp_ao_info_free() with a call_rcu()
callback. Readers that captured tp->ao_info before rcu_assign_pointer
NULLed it now see the object remain valid until rcu_read_unlock().
With the patch applied the reproducer runs cleanly for 2000 iterations
on the same kernel build.

The Linux kernel CVE team has assigned CVE-2026-64459 to this issue.


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

	Issue introduced in 6.18 with commit 51e547e8c89c661f6fbede4a28b1d33b13625683 and fixed in 6.18.39 with commit 657646c08c94ef7b9dbe468fe7828032216f9841
	Issue introduced in 6.18 with commit 51e547e8c89c661f6fbede4a28b1d33b13625683 and fixed in 7.1.4 with commit 4caf12c778fed3dc3824cf36263be5e2c491fbd0
	Issue introduced in 6.18 with commit 51e547e8c89c661f6fbede4a28b1d33b13625683 and fixed in 7.2-rc2 with commit 8bc4d43bccbd60efe85d0a44d5bf41762f2f0c30

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-2026-64459
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:
	include/net/tcp_ao.h
	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/657646c08c94ef7b9dbe468fe7828032216f9841
	https://git.kernel.org/stable/c/4caf12c778fed3dc3824cf36263be5e2c491fbd0
	https://git.kernel.org/stable/c/8bc4d43bccbd60efe85d0a44d5bf41762f2f0c30