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

Overview

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.

Affected

• Linux >= ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 < 4e47f1ac188ece11d6fdabe44166a2776cc5bd4e
• Linux >= ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 < 110b765744b147c63882f5e9cb12931c5dc8d85f
• Linux >= ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 < 4bb84e964ff0fe0a171c965362de72f9820dbce9
• Linux >= ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 < 3ab32218d7182705dae5c86f13925f458072da2c
• Linux >= ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 < c9574b8a8edeb4edd3ac6472c27ef7184bdb2baa
• Linux >= ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 < 06213c85d8c0994f786c093b8b2a517987943ca6
• Linux >= ee40fb2e1eb5bc0ddd3f2f83c6e39a454ef5a741 < ec4215683e47424c9c4762fd3c60f552a3119142
• Linux d36e5ba7bbed5d7bd26e8609ffed503c2def401b
• Linux 9bcc0508576b2d50efd958f2ea1c5906749c2c89
• Linux c2984681fe15cfb803a9132aaaf1140ab20a72c1
• Linux 5803ecd7f6ac6f747582e775caa62ac9d0489261
• Linux 26f8819ddd10141ebe7bbce700fbab36bfa5f478
• Linux >= 3.2.99 < 3.3
• Linux >= 3.16.54 < 3.17
• Linux >= 4.4.225 < 4.5
• Linux >= 4.9.225 < 4.10
• Linux >= 4.14.182 < 4.15
• Linux 4.15

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.