CVE-2026-64045 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
ovpn: tcp - use cached peer pointer in ovpn_tcp_close()
ovpn_tcp_close() loads the ovpn_socket via rcu_dereference_sk_user_data() under rcu_read_lock(), takes a reference on sock->peer, caches the peer pointer in a local, and drops the read lock. It then passes sock->peer (rather than the cached local) to ovpn_peer_del(), re-dereferencing the ovpn_socket after the RCU read section has ended.
Unlike ovpn_tcp_sendmsg(), which uses the same "load under RCU, use after unlock" pattern but is protected by lock_sock() held across the function, ovpn_tcp_close() runs without the socket lock: inet_release() invokes sk_prot->close() without taking lock_sock first.
ovpn_socket_release() can therefore complete its kref_put -> detach -> synchronize_rcu -> kfree(sock) sequence concurrently, in the window after ovpn_tcp_close() drops rcu_read_lock() but before it dereferences sock->peer. The synchronize_rcu() in ovpn_socket_release() protects readers that use the dereferenced pointer inside the RCU read section, not those that escape the pointer to a local and use it afterwards.
A reproducer follows the pattern of commit 94560267d6c4 ("ovpn: tcp - don't deref NULL sk_socket member after tcp_close()"): trigger a peer removal (keepalive expiration or netlink OVPN_CMD_DEL_PEER) at the same moment userspace closes the TCP fd. That commit fixed the detach-side of the same race window; this one fixes the close-side at a different victim.
Tighten the entry block to read sock->peer exactly once into the cached peer local, and route all subsequent uses (the hold check, the ovpn_peer_del() call, and the prot->close() invocation) through that local. sock->peer is only ever written once in ovpn_socket_new() under lock_sock(), before rcu_assign_sk_user_data() publishes the ovpn_socket, and is never reassigned afterwards - but the previous multi-read pattern made that invariant implicit rather than explicit. The same multi-read shape exists in ovpn_tcp_recvmsg(), ovpn_tcp_sendmsg(), ovpn_tcp_data_ready() and ovpn_tcp_write_space(); those will be cleaned up via a dedicated helper in a follow-up net-next series.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability described involves a race condition in the Linux kernel's OpenVPN TCP implementation within the ovpn_tcp_close() function, which can lead to use-after-free scenarios and potential system instability. This issue specifically affects the OpenVPN over TCP transport mechanism where socket operations are handled through the kernel's networking subsystem. The flaw occurs during the cleanup phase of TCP socket closure when the kernel attempts to manage peer references and perform necessary cleanup operations.
The technical root cause stems from improper handling of RCU (Read-Copy-Update) protected data structures within the ovpn_tcp_close() function. During execution, the function loads the ovpn_socket using rcu_dereference_sk_user_data() while holding an RCU read lock, then caches a reference to sock->peer in a local variable before releasing the read lock. However, subsequent operations continue to reference the original sock->peer pointer rather than the cached local copy, creating a window where concurrent operations can invalidate the pointer between the release of the RCU lock and the final dereference. This pattern differs from other similar functions like ovpn_tcp_sendmsg() which properly hold socket locks across the entire operation sequence.
The operational impact of this vulnerability is significant as it creates a potential exploitation vector for privilege escalation or denial-of-service attacks. The race condition can be triggered when userspace closes a TCP file descriptor simultaneously with peer removal operations such as keepalive expiration or explicit netlink commands to delete peers. This timing window allows the ovpn_socket_release() function to complete its synchronization and memory deallocation sequence while ovpn_tcp_close() is still attempting to use invalidated peer references. The vulnerability directly relates to CWE-362, which describes race conditions in concurrent programming where operations can be interrupted between check and use phases.
Security implications extend beyond simple instability as the flawed pointer handling could potentially allow attackers to manipulate kernel memory structures through carefully crafted timing attacks. The kernel's synchronize_rcu() mechanism provides protection for readers operating within RCU read sections but does not protect against subsequent uses of cached pointers that escape these protected contexts, creating a fundamental mismatch in memory safety guarantees. This vulnerability pattern is consistent with ATT&CK technique T1068, which involves exploiting local privilege escalation through kernel vulnerabilities.
The fix implemented addresses the core issue by enforcing single-read semantics for sock->peer within the function entry block and routing all subsequent operations through the cached local variable. This change ensures that the pointer remains stable throughout the function execution regardless of concurrent modifications or deallocations. The solution explicitly enforces the invariant that sock->peer is only written once during socket creation under lock_sock() before publication via rcu_assign_sk_user_data(), making this behavior explicit rather than implicit. While the patch resolves the immediate issue in ovpn_tcp_close(), similar multi-read patterns exist in other functions like ovpn_tcp_recvmsg(), ovpn_tcp_sendmsg(), ovpn_tcp_data_ready(), and ovpn_tcp_write_space(), which will be addressed separately through a dedicated helper function in future kernel updates. This approach maintains consistency with established kernel coding practices for handling RCU-protected data structures while ensuring proper memory lifecycle management during socket operations.
The vulnerability demonstrates the critical importance of careful synchronization when working with RCU-protected data in kernel space, where even seemingly benign pointer caching can create security-relevant race conditions. The fix reinforces proper kernel development practices by ensuring that all references to potentially volatile data are properly anchored and managed throughout function execution, preventing scenarios where cached pointers become invalid due to concurrent modification or deallocation. This aligns with security best practices for kernel module development and highlights the need for thorough code reviews of concurrent access patterns in low-level system components.