From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-72123][IMPORTANT] can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF Date: Sun, 16 Aug 2026 01:58:48 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-72123 X-AL-KERNEL-Priority: IMPORTANT X-AL-KERNEL-Severity: IMPORTANT X-AL-KERNEL-Base-Severity: IMPORTANT X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 6 X-AL-KERNEL-ActionableScore-Lower: 5 X-AL-KERNEL-Commit: 3cf4fd5316f449811d8baf1bc6978ef5a7b743a9 List-Id: CVE: CVE-2026-72123 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF Commit: 3cf4fd5316f449811d8baf1bc6978ef5a7b743a9 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3cf4fd5316f449811d8baf1bc6978ef5a7b743a9 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72123 Analysis date: Sun, 16 Aug 2026 01:58:48 -0400 ActionableScore: 6 ActionableScore lower bound: 5 Actionable bucket: Actionable Moderate at minimum / Strong Important candidate for manual review Manual review required: YES Summary: A race in CAN_BCM RX operation teardown can leave thrtimer armed after bcm_op is freed through RCU, causing a timer callback use-after-free that is locally triggerable through SocketCAN and may lead to kernel crash or possible privilege escalation. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-72123 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72123 The original announcement does not normally provide a security severity estimate, CVSS assessment, or enough information to determine whether the reported kernel bug represents a practically relevant security issue. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. It combines an autonomous classifier with LLM-assisted technical analysis and a separate ActionableScore mechanism. The purpose of this report is to prioritize Linux kernel CVEs before manual review, identify cases that require prompt investigation, and support automatic closure of issues that are unlikely to have meaningful security impact. Published priority for this report: IMPORTANT Manual review required: YES A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-72123 IMPORTANT CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in the CAN BCM RX teardown path can leave thrtimer armed after bcm_op has been scheduled for RCU freeing. When the RCU grace period completes, the object can be freed while a later hrtimer callback still dereferences the old operation and its socket pointer, resulting in a use-after-free. For the CVSS the PR:L is used for the paranoid score because a local user or local process with access to CAN_BCM sockets can create and tear down BCM operations without full administrator privileges in some deployments, although creating CAN interfaces usually requires CAP_NET_ADMIN. The issue is not directly reachable over IP networks and is primarily triggered through the local SocketCAN control path, with incoming CAN traffic only helping the race timing. Impact is at least local denial of service via kernel crash and in the worst case may allow privilege escalation because this is a real kernel use-after-free in a timer callback.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum / Strong Important candidate for manual review (with actual score 7) YES DANGER INIT NOMEMCHK RACE UAF IMPROVEONLY TIMER LINUS KPANIC PACKET YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=6 ActionableScoreLower=5 ## 1. ActionableScore * Conservative score: 5 * Paranoid score: 6 * Final recommended bucket: **Actionable Moderate at minimum / Strong Important candidate for manual review** ## 2. Signal breakdown Triggered signals: * Local unprivileged trigger: +1 CAN_BCM sockets can be used by a local process when SocketCAN and a CAN or vcan interface are available. Creating the interface may require CAP_NET_ADMIN, but use of an existing interface may be available to less privileged local users depending on deployment. * Memory corruption, strong corruption primitive: +2 The commit explicitly describes a real use-after-free where thrtimer later dereferences a freed bcm_op. * Real lifetime corruption: +1 This is an RCU and timer lifetime bug. The fix defers freeing to a workqueue and adds sock_hold and sock_put to protect op->sk lifetime. * Privilege escalation plausible: +1 conservative, +2 paranoid Conservative scoring uses weak to moderate LPE concern because no attacker-controlled reclaim or overwrite primitive is demonstrated. Paranoid scoring uses strong LPE concern because the stale timer callback dereferences a freed kernel object and socket pointer after an attacker-influenced teardown and rearm race. * Reliable kernel crash / strong DoS: +1 A later hrtimer callback dereferencing freed memory is a credible kernel crash path. Subtracted signals: * Hard or unreliable race / special timing required: -1 The bug requires a teardown vs packet RX fast-path race and timer rearm timing. * Configuration dependent SocketCAN exposure: -1 This is not ordinary IP remote exposure. It requires CAN_BCM support and a usable CAN or vcan setup. Call-site confidence: high. The patch and commit show the relevant lifetime path: bcm_rx_handler, bcm_rx_update_and_send, call_rcu, hrtimer callback, bcm_free_op_rcu, deferred workqueue free, and socket reference handling. ## 3. Reachability analysis The bug is locally reachable through the SocketCAN CAN_BCM control path. A local process can create BCM RX operations, tear them down, and rely on concurrent CAN RX activity to rearm thrtimer during the RCU grace-period window. It is not directly reachable over normal IP networking. Namespaces and containers matter. If a container or reduced-privilege environment is given access to a CAN or vcan interface, the practical privilege requirement may be closer to PR:L rather than PR:H. If no CAN interface exists and the attacker must create vcan or configure CAN devices, CAP_NET_ADMIN is typically required. The affected path is not a broad default remote attack surface, but it is a real kernel lifetime bug in a networking protocol family. Realistic exploitation is timing dependent, but the UAF is explicit and the timer callback makes the issue more actionable than a simple NULL dereference or resource leak. ## 4. Severity interpretation This should not be auto-closed as an ordinary Moderate. The realistic impact is at least local denial of service through kernel crash. The theoretical impact includes local privilege escalation because the stale hrtimer callback can access a freed bcm_op and op->sk after RCU teardown, although the patch does not demonstrate controlled reclaim, arbitrary write, or a reliable exploitation primitive. Overall this is best treated as Actionable Moderate at minimum, with Strong Important candidate handling for manual review. ## 5. One-sentence report phrase A race in CAN_BCM RX operation teardown can leave thrtimer armed after bcm_op is freed through RCU, causing a timer callback use-after-free that is locally triggerable through SocketCAN and may lead to kernel crash or possible privilege escalation. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: this is an explicit kernel UAF involving RCU lifetime, hrtimer callbacks, and socket lifetime. Even though exploitation is race dependent and not remotely reachable over IP, the stale callback into freed kernel state makes it important enough for manual review. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF Commit: 3cf4fd5316f449811d8baf1bc6978ef5a7b743a9 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3cf4fd5316f449811d8baf1bc6978ef5a7b743a9 Commit description: Commit f1b4e32 ("can: bcm: use call_rcu() instead of costly synchronize_rcu()") replaced synchronize_rcu() in bcm_delete_rx_op() with call_rcu() and introduced the RX_NO_AUTOTIMER flag. However, this flag check was omitted for thrtimer in the packet rx fast-path. During BCM RX operation teardown, a concurrent RCU reader (bcm_rx_handler) can race and re-arm thrtimer via bcm_rx_update_and_send() after call_rcu() has been scheduled. Once the RCU grace period elapses, bcm_op is freed. The subsequently firing thrtimer then dereferences the deallocated op, causing a UAF. Adding flag checks to the rx fast-path (bcm_rx_update_and_send) does not fully close the TOCTOU race and introduces latency for every CAN frame. Conversely, calling hrtimer_cancel() directly inside the RCU callback (softirq context) is fatal as hrtimer_cancel() can sleep, triggering a "scheduling while atomic" panic. Resolve this by deferring the timer cancellation and memory free to a dedicated unbound workqueue (bcm_wq). The RCU callback now queues a work item to bcm_wq, which safely cancels both timers and deallocates memory in sleepable process context. A dedicated workqueue is used to prevent system-wide WQ saturation and is cleanly flushed/destroyed on module unload to avoid rmmod page faults. Since the deferred work can now outlive the calling context by an unbounded amount, also take a reference on op->sk when it is assigned and drop it only once the deferred work has cancelled both timers, so a socket can no longer be freed out from under a still-armed timer whose callback (bcm_send_to_user()) dereferences op->sk. Fixes: f1b4e32 ("can: bcm: use call_rcu() instead of costly synchronize_rcu()") Tested-by: Feng Xue Tested-by: Oliver Hartkopp Signed-off-by: Lee Jones Signed-off-by: Oliver Hartkopp Link: https://patch.msgid.link/20260714-bcm_fixes-v15-1-562f7e3e42da@hartkopp.net Cc: stable@kernel.org Signed-off-by: Marc Kleine-Budde Signed-off-by: Greg Kroah-Hartman Changed files: net/can/bcm.c net/sock.h net/net_namespace.h Diff excerpt: Not included in this email. See the upstream URL for the full patch. Full patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3cf4fd5316f449811d8baf1bc6978ef5a7b743a9 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-72123 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72123 The original CVE announcement normally does not include a security-level estimate. In particular, it may not contain a CVSS assessment, an impact level, or enough information to determine whether the reported bug is a practically relevant security issue. One purpose of this parallel CVE list is to provide that missing technical and prioritization information. The original goal of the AL-KERNEL project was to prioritize Linux kernel CVE analysis automatically before manual review. The system can also help identify non-security issues that may be suitable for automatic closure. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. The first analysis stage combines an autonomous classifier with additional LLM-based analysis. The autonomous classifier runs locally on a CPU and is based on a backpropagation neural network. Together, these mechanisms produce a technical vulnerability description, identify likely weakness types, estimate CVSS severity, and provide input for ActionableScore. Two CVSS estimates are retained because incomplete kernel vulnerability information often permits more than one defensible interpretation: Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 7 The conservative vector represents a lower-impact interpretation. The Best/paranoid vector intentionally represents a plausible upper-bound interpretation and should not automatically be treated as demonstrated real-world impact. CVSS may also need to be adjusted for a particular Linux deployment, because actual reachability, privileges, enabled kernel configuration, hardware, namespaces, exposed device nodes, and other environmental conditions can differ significantly between systems. A separate ActionableScore mechanism evaluates practical remediation urgency. Its analysis may include reachability, attack prerequisites, subsystem exposure, memory-corruption characteristics, denial-of-service reliability, and possible confidentiality, integrity, or privilege-escalation impact. Conservative ActionableScore: 5 Paranoid ActionableScore: 6 The final base severity is taken directly from the second tab-separated field of the AL-KERNEL classification result. ActionableScore does not replace or independently override that final AL-KERNEL decision, and if ActionableScore adjusted impact level of ALKERNEL, then you would see self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7. For an AL-KERNEL result of MODERATE, this report uses the following additional presentation split: ActionableScore below 5 -> MODERATE REGULAR ActionableScore 5 or more -> MODERATE 7.0 The distinction between MODERATE REGULAR and MODERATE 7.0 makes it possible to identify Moderate issues that should receive manual analysis and fixes before lower-priority MODERATE REGULAR issues. In many cases, MODERATE REGULAR fixes may wait for a later rebase or routine update. There is one override in which MODERATE REGULAR becomes MODERATE 7.0 even when the ActionableScore is below 5. When the AL-KERNEL result contains the KPANIC flag, a MODERATE result is always presented as MODERATE 7.0. The KPANIC flag selected with few regexps without usage of AI at all, so it helps to detect cases when Kernel Crash happens and similar (to filter False-Negative results from the LLM usage). KPANIC indicates that a reliable kernel crash, kernel panic, or similarly serious kernel availability impact was identified by the classification workflow. AL-KERNEL base severity for this report: IMPORTANT KPANIC detected for this report: YES Published priority for this report (same as in Subject): IMPORTANT These results are intended to support engineering triage. They are machine-generated estimates, and cases marked for manual review should be validated by a human security engineer before final disposition. For more info read docs linked from here: https://kernelcve.org/ (and you can submit you own patch there to generate such a report for non-existant CVE-id yet). Note that in many cases this AI tool selects higher severity, than real is (means you can expect Importants instead of Moderate 7.0 or Moderates 7.0 instead of regular Moderates). If you see such cases, please use reply email interface to add additional manual analyses info to this particular CVE. And please, please, let me know when you see Lows instead of Importants or Important instead of Low (because particular for such cases I need to tune this AI tool to make it better for this one and next similar). My contact email for such notifications is alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).