High [CVE-2026-68398] Linux kernel: PPP over L2TP Use-After-Free vulnerability
This high-severity Red Hat Linux advisory covers CVE-2026-68398 affecting Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9.
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Summary
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.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · high-confidence parse· fetched 4 hours ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 4 hours ago·verify at source
Mitigation checklist
- If PPP over L2TP functionality is not required, prevent the 'pppol2tp' kernel module from loading to mitigate this vulnerability. Create a file named `/etc/modprobe.d/disable-pppol2tp.conf` with the following content: ``` install pppol2tp /bin/true ``` After creating the file, regenerate the initramfs and reboot the system for the changes to take effect. This action may impact services relying on PPP over L2TP.
Official advisory · high-confidence parse· fetched 4 hours ago·verify at source
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