CVE-2026-64523 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
net/handshake: Take a long-lived file reference at submit
handshake_nl_accept_doit() needs the file pointer backing req->hr_sk->sk_socket to survive the window between handshake_req_next() and the subsequent FD_PREPARE() and get_file(). The submit-side sock_hold() does not provide that. sk_refcnt keeps struct sock alive, but struct socket is owned by sock->file: when the consumer fputs the last file reference, sock_release() tears the socket down regardless of any sock_hold.
Add an hr_file pointer to struct handshake_req and acquire an explicit reference on sock->file during handshake_req_submit(). handshake_complete() and handshake_req_cancel() release the reference on the completion-bit-winning path.
The submit error path must also release the file reference, but after rhashtable insertion a concurrent handshake_req_cancel() can discover the request and race the error path. Gate the error-path cleanup -- sk_destruct restoration, fput, and request destruction -- with test_and_set_bit(HANDSHAKE_F_REQ_COMPLETED), the same serialization handshake_complete() and handshake_req_cancel() already use. When cancel has already claimed ownership, the submit error path returns without touching the request; socket teardown handles final destruction.
The accept-side dereferences are not yet retargeted; that change comes in the next patch.
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Analysis
by VulDB Data Team • 07/25/2026
This vulnerability exists within the Linux kernel's network handshake subsystem, specifically affecting how file references are managed during socket acceptance operations. The issue stems from a race condition in the handshake_nl_accept_doit() function where the file pointer backing req->hr_sk->sk_socket must persist across a critical window between handshake_req_next() and subsequent FD_PREPARE() and get_file() operations. While the kernel correctly maintains reference counting for struct sock through sock_hold(), this mechanism fails to protect the associated struct socket which is owned by sock->file. When the consumer releases its final file reference through fputs(), the sock_release() function immediately tears down the entire socket structure regardless of any existing sock_hold() references, creating a potential use-after-free scenario.
The technical flaw manifests in the improper management of file reference lifetimes during the handshake submission process. The current implementation relies on implicit reference counting that doesn't account for the asynchronous nature of socket operations and concurrent access patterns. The solution involves explicitly tracking file references by adding an hr_file pointer to struct handshake_req and acquiring explicit references on sock->file during handshake_req_submit() operations. This approach directly addresses the core issue identified in CWE-416, which covers use-after-free vulnerabilities resulting from improper resource management.
The operational impact of this vulnerability could allow attackers to exploit race conditions that might lead to system instability or privilege escalation. When concurrent operations attempt to cancel requests while submissions are in progress, the improper cleanup handling could result in dangling references or premature socket destruction. The fix introduces proper serialization using test_and_set_bit(HANDSHAKE_F_REQ_COMPLETED) which aligns with ATT&CK technique T1068 by potentially enabling privilege escalation through kernel memory corruption. The implementation follows established patterns for concurrent resource management and addresses timing issues that could be exploited in denial-of-service attacks or information disclosure scenarios.
The mitigation strategy employs explicit reference counting combined with proper synchronization primitives to ensure that file references remain valid throughout the complete lifecycle of handshake requests. This approach prevents the race condition between request submission and cancellation operations while maintaining proper resource cleanup semantics. The solution correctly handles both successful completion paths through handshake_complete() and handshake_req_cancel() as well as error paths that must release file references before the socket teardown process begins. The implementation demonstrates adherence to kernel security best practices by ensuring proper reference counting and serialization in high-concurrency scenarios, reducing the attack surface for potential exploitation through improper resource management or race condition vulnerabilities.