CVE-2026-72139 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
tcp: defer md5sig_info kfree past RCU grace period in tcp_connect
The md5+ao reconciliation in tcp_connect() (net/ipv4/tcp_output.c) has two symmetric branches:
if (needs_md5) {
tcp_ao_destroy_sock(sk, false); } else if (needs_ao) {
tcp_clear_md5_list(sk); kfree(rcu_replace_pointer(tp->md5sig_info, NULL, ...)); }
Both branches free a per-socket auth-info object while the socket is in TCP_SYN_SENT and is already on the inet ehash (inserted by inet_hash_connect() in tcp_v4_connect()). Both branches are reachable by softirq RX-path readers that load the corresponding info pointer via implicit RCU before bh_lock_sock_nested() is taken.
The needs_md5 branch is fixed in the prior patch by re-introducing the call_rcu() free in tcp_ao_destroy_sock(): the equivalent per-key loop runs inside tcp_ao_info_free_rcu(), the RCU callback, so by the time it frees each tcp_ao_key all softirq readers that captured the container have already completed rcu_read_unlock().
The needs_ao branch is not symmetric in the same way. The container free can be deferred via kfree_rcu(md5sig, rcu) -- struct tcp_md5sig_info already has the required rcu member (include/net/tcp.h:1999-2002), and the rest of the tree already does this in the tcp_md5sig_info_add() rollback paths (net/ipv4/tcp_ipv4.c:1410, 1436). But the per-key teardown is done by tcp_clear_md5_list() in process context BEFORE the container's RCU grace period: it walks &md5sig->head and frees each tcp_md5sig_key with bare hlist_del + kfree. A concurrent softirq reader in __tcp_md5_do_lookup() / __tcp_md5_do_lookup_exact() (tcp_ipv4.c:1253, 1298) walks the same list via hlist_for_each_entry_rcu() and races with that bare kfree on the keys themselves -- a per-key slab use-after-free of the same class as the TCP-AO bug, on the same race window.
Fix this in two halves:
1. Convert the bare kfree() in tcp_connect() to kfree_rcu() so the md5sig_info container joins the rest of the md5sig lifecycle. The local-variable lift is mechanical and required because kfree_rcu() is a macro that expects an lvalue.
2. Make tcp_clear_md5_list() RCU-safe by replacing hlist_del + kfree(key) with hlist_del_rcu + kfree_rcu(key, rcu). struct tcp_md5sig_key already carries the rcu member (include/net/tcp.h:1995) and tcp_md5_do_del() (net/ipv4/tcp_ipv4.c:1456) already uses kfree_rcu, so this restores the lifecycle invariant the rest of the file follows rather than introducing a one-off.
The other caller of tcp_clear_md5_list() is tcp_md5_destruct_sock() (net/ipv4/tcp.c:412), which runs from the sock destructor when the socket is already unhashed and unreachable; the extra grace period there is unnecessary but harmless. Making the helper unconditionally RCU-safe is the cleaner contract.
The needs_ao branch is not reachable by the userns reproducer used to demonstrate the AO-side splat (the repro installs both keys but ends up in the needs_md5 branch because the connect peer matches the MD5 key, not the AO key); however the symmetric race exists and a maintainer touching this code should not have to think about which branch escapes RCU and which one does not.
[also credits to Qihang, who found that this races with tcp-diag]
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability in question involves a race condition within the Linux kernel's TCP implementation, specifically in how authentication information is managed during socket connection establishment. This flaw manifests in the tcp_connect function where two distinct code paths handle MD5 and AO (Authentication Option) authentication mechanisms. The issue arises when both branches attempt to free per-socket authentication information while the socket is in the TCP_SYN_SENT state and already present in the inet ehash table, a condition that occurs during connection establishment.
The technical core of the vulnerability stems from improper handling of RCU (Read-Copy-Update) grace periods when freeing memory structures. In the needs_md5 branch, the fix has already been implemented by reintroducing call_rcu() free mechanisms within tcp_ao_destroy_sock(), ensuring that the RCU callback tcp_ao_info_free_rcu() properly handles per-key cleanup before readers complete their operations. However, the needs_ao branch presents a symmetric but unresolved race condition where the container structure md5sig_info is freed using kfree_rcu() as required, but the individual keys within this structure are cleaned up by tcp_clear_md5_list() in process context before the RCU grace period completes.
This creates a use-after-free scenario that follows the same pattern as previously identified TCP-AO bugs, where concurrent softirq readers accessing the tcp_md5sig_key structures via hlist_for_each_entry_rcu() race with the direct kfree operations performed by tcp_clear_md5_list(). The race occurs during __tcp_md5_do_lookup() and __tcp_md5_do_lookup_exact() functions, which traverse the same hash list structure that is being modified by the cleanup process. This represents a classic RCU memory safety issue where readers may access freed memory after the cleanup phase has begun but before the grace period completes.
The proposed fix addresses this through two distinct mechanisms that align with established kernel patterns and standards. First, the bare kfree() operation in tcp_connect() is converted to kfree_rcu() to ensure consistent lifecycle management of the md5sig_info container structure. This change requires a mechanical local variable lift since kfree_rcu() operates on lvalues rather than expressions. Second, tcp_clear_md5_list() is made RCU-safe by replacing hlist_del + kfree(key) with the proper hlist_del_rcu + kfree_rcu(key, rcu) sequence, which follows existing patterns within the kernel codebase where struct tcp_md5sig_key already contains the required rcu member and tcp_md5_do_del() already implements this safe cleanup pattern.
The fix maintains consistency with industry standards such as CWE-415 (Double Free) and CWE-416 (Use After Free) while addressing ATT&CK techniques related to kernel memory corruption and privilege escalation. The solution ensures that both the container and individual key structures follow the same RCU lifecycle, making tcp_clear_md5_list() unconditionally safe for use in RCU contexts regardless of which code path is taken during connection establishment. This approach eliminates the asymmetric race condition that existed between the two branches and ensures that all cleanup operations respect proper RCU ordering semantics, preventing potential kernel crashes or memory corruption that could be exploited by malicious actors.
The fix also addresses a broader class of issues identified in tcp-diag related to concurrent access patterns, as noted by the original researcher Qihang. The solution makes the helper function universally safe rather than maintaining special cases for different execution contexts, which improves code maintainability and reduces the likelihood of similar issues being introduced during future modifications to this critical kernel subsystem.