CVE-2026-68398: ppp: defer channel free to an RCU grace period to fix pppol2tp RX UAF

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

ppp: defer channel free to an RCU grace period to fix pppol2tp RX UAF

pppol2tp_recv() runs in the L2TP UDP-encap softirq RX path:

 l2tp_udp_encap_recv() -> l2tp_recv_common() -> pppol2tp_recv()
   -> ppp_input(&po->chan)

It runs under rcu_read_lock() holding only an l2tp_session reference and
takes NO reference on the internal PPP channel (struct channel,
chan->ppp) that ppp_input() dereferences.

The pppox socket is SOCK_RCU_FREE, so 'po' and the embedded ppp_channel
are RCU-safe.  But the internal struct channel is a separate allocation
that ppp_release_channel() frees with a plain kfree():

 close(data socket) -> pppol2tp_release() -> pppox_unbind_sock()
   -> ppp_unregister_channel() -> ppp_release_channel() -> kfree(pch)

For a channel that is bound (PPPIOCGCHAN) but not attached to a ppp unit
(no PPPIOCCONNECT, pch->ppp == NULL) and not bridged, teardown skips
both ppp_disconnect_channel()'s synchronize_net() and
ppp_unbridge_channels()'s synchronize_rcu(), so the kfree() has no grace
period.  rcu_read_lock() in pppol2tp_recv() does not protect against a
plain kfree(), so an in-flight ppp_input() on one CPU can dereference
the channel just freed by close() on another CPU.

The bug is reachable by an unprivileged user.

Defer the channel free to an RCU callback via call_rcu() so the grace
period fences any in-flight ppp_input(). The disconnect and unbridge
teardown paths already fence with synchronize_net()/synchronize_rcu();
call_rcu() does the same here without stalling the close() path.

The Linux kernel CVE team has assigned CVE-2026-68398 to this issue.


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

	Issue introduced in 4.15 with commit ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 and fixed in 6.6.148 with commit 4bb84e964ff0fe0a171c965362de72f9820dbce9
	Issue introduced in 4.15 with commit ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 and fixed in 6.12.101 with commit 3ab32218d7182705dae5c86f13925f458072da2c
	Issue introduced in 4.15 with commit ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 and fixed in 6.18.42 with commit c9574b8a8edeb4edd3ac6472c27ef7184bdb2baa
	Issue introduced in 4.15 with commit ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 and fixed in 7.1.6 with commit 06213c85d8c0994f786c093b8b2a517987943ca6
	Issue introduced in 4.15 with commit ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 and fixed in 7.2-rc4 with commit ec4215683e47424c9c4762fd3c60f552a3119142
	Issue introduced in 3.2.99 with commit d36e5ba7bbed5d7bd26e8609ffed503c2def401b
	Issue introduced in 3.16.54 with commit 9bcc0508576b2d50efd958f2ea1c5906749c2c89
	Issue introduced in 4.4.225 with commit c2984681fe15cfb803a9132aaaf1140ab20a72c1
	Issue introduced in 4.9.225 with commit 5803ecd7f6ac6f747582e775caa62ac9d0489261
	Issue introduced in 4.14.182 with commit 26f8819ddd10141ebe7bbce700fbab36bfa5f478

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-2026-68398
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:
	drivers/net/ppp/ppp_generic.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/4bb84e964ff0fe0a171c965362de72f9820dbce9
	https://git.kernel.org/stable/c/3ab32218d7182705dae5c86f13925f458072da2c
	https://git.kernel.org/stable/c/c9574b8a8edeb4edd3ac6472c27ef7184bdb2baa
	https://git.kernel.org/stable/c/06213c85d8c0994f786c093b8b2a517987943ca6
	https://git.kernel.org/stable/c/ec4215683e47424c9c4762fd3c60f552a3119142