CVE-2026-68398 in Linux
Summary
by MITRE • 08/10/2026
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.
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Analysis
by VulDB Data Team • 08/10/2026
The vulnerability described represents a use-after-free condition in the Linux kernel's pppol2tp implementation that arises from improper synchronization during channel cleanup operations. This issue manifests specifically within the L2TP UDP-encapsulation receive path where pppol2tp_recv() processes incoming packets. The flaw occurs because the function executes under rcu_read_lock() while holding only an l2tp_session reference, but fails to maintain any reference to the internal PPP channel structure that ppp_input() subsequently dereferences. The channel structure in question is allocated separately from the socket and managed through a distinct memory allocation path that does not provide the necessary synchronization guarantees.
The technical implementation details reveal that while the pppox socket itself operates under RCU-safe conditions due to its SOCK_RCU_FREE flag, the internal channel structure remains vulnerable because ppp_release_channel() performs a direct kfree() operation without any grace period handling. This creates a race condition where an in-flight ppp_input() call executing on one CPU can attempt to access memory that has already been freed by a close() operation occurring on another CPU. The vulnerability specifically affects channels that are bound but not attached to a PPP unit and not bridged, as these paths skip the necessary synchronization mechanisms including synchronize_net() and synchronize_rcu() calls that would normally provide the required memory barriers.
The operational impact of this vulnerability is significant as it allows unprivileged users to trigger a use-after-free condition that could potentially lead to arbitrary code execution or system instability. The race condition occurs during normal channel teardown operations when the close() path does not wait for in-flight packet processing to complete before freeing memory. This scenario is particularly dangerous because it can be exploited through legitimate network operations without requiring elevated privileges, making it a serious security concern for systems running Linux kernels with pppol2tp support.
The fix implemented addresses this vulnerability by deferring the channel cleanup to an RCU callback mechanism using call_rcu() instead of direct kfree(). This approach ensures that any in-flight ppp_input() operations complete before memory is actually freed, providing the necessary grace period that rcu_read_lock() alone cannot guarantee. The solution maintains the performance characteristics of the close() path by avoiding stalling operations while still providing proper memory synchronization. This mitigation aligns with established best practices for RCU-based memory management and follows the pattern already implemented in other parts of the kernel's disconnect and unbridge teardown paths that properly utilize synchronize_net() and synchronize_rcu() calls. The fix essentially transforms the immediate free operation into a deferred cleanup that respects the RCU grace period requirements, preventing the race condition between packet processing and memory deallocation that led to the original vulnerability.
This vulnerability type falls under CWE-416, which specifically addresses Use After Free conditions, and represents a classic example of improper synchronization in concurrent systems. The ATT&CK framework would categorize this as a privilege escalation technique through kernel exploitation, potentially enabling an unprivileged user to gain elevated system privileges. The implementation follows the standard RCU memory management patterns that are commonly used throughout the Linux kernel for handling concurrent access to shared data structures while maintaining performance characteristics through asynchronous cleanup operations.