CVE-2026-72123 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
can: bcm: defer rx_op deallocation to workqueue to fix thrtimer UAF
Commit f1b4e32aca08 ("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.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described represents a use-after-free condition in the Linux kernel's CAN bus communication subsystem specifically within the broadcast channel manager bcm module. This issue arises from improper handling of timer cleanup operations during concurrent access scenarios, creating a race condition that can lead to system instability and potential security implications. The problem manifests when multiple threads attempt to access and modify shared data structures without proper synchronization mechanisms.
The technical flaw stems from the implementation of the RX_NO_AUTOTIMER flag introduced in commit f1b4e32aca08 which replaced synchronize_rcu() with call_rcu() for performance reasons. However, this change created an inconsistency where the flag check was properly implemented in the main deletion path bcm_delete_rx_op() but omitted from the timer fast-path in bcm_rx_update_and_send(). This omission creates a time-of-check to time-of-use race condition where a concurrent reader can re-arm the timer after call_rcu() has been scheduled but before the grace period completes. The vulnerability is classified as CWE-416 Use After Free, which directly maps to the ATT&CK technique T1059.008 for command and scripting interpreter execution through kernel-level memory corruption.
The operational impact of this vulnerability extends beyond simple system crashes, potentially allowing attackers to exploit the memory corruption for privilege escalation or denial of service attacks. The race condition occurs during BCM RX operation teardown when a timer callback attempts to access freed memory, creating an unpredictable state that can be leveraged for malicious purposes. The timing aspect of this vulnerability makes it particularly challenging to detect and reproduce in controlled environments, as it depends on precise thread scheduling and concurrent access patterns.
The mitigation strategy employs a sophisticated approach using a dedicated unbound workqueue to handle timer cancellation and memory deallocation operations. This solution addresses the fundamental issue by moving timer cleanup from atomic softirq context to a sleepable process context where hrtimer_cancel() can safely execute without triggering scheduling while atomic panics. The implementation uses a reference counting mechanism to ensure socket objects remain valid during timer execution, preventing scenarios where a socket could be freed while still referenced by an active timer callback. This approach aligns with security best practices for kernel memory management and follows the principle of least privilege by isolating potentially problematic operations in dedicated execution contexts. The use of a dedicated workqueue also prevents system-wide workqueue saturation that could occur if such operations were handled through the default global queue, ensuring proper resource management and system stability during module unload operations.