CVE-2026-98861 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching()
unregister_fprobe() and unregister_fprobe_async() (used by BPF kprobe-multi) rely on standard RCU grace periods (synchronize_rcu() and call_rcu()) to wait until in-flight fprobe handlers complete before freeing the fprobe.
However, if an fprobe handler executes while RCU is not watching (such as in the idle loop or nohz_full extended quiescent states), standard RCU does not track preemption-disabled sections. Consequently, synchronize_rcu() does not wait for those executions, which can lead to a use-after-free if the fprobe is freed immediately after unregistration. Ensure handlers exit early when !rcu_is_watching().
Furthermore, fprobe_fgraph_entry() and fprobe_ftrace_entry() previously used guard(rcu)() and rcu_read_lock(), which invoke lockdep on every hit under CONFIG_PROVE_LOCKING. This adds overhead and can cause lockdep recursion if probed functions interact with lockdep.
Since rhltable_lookup() and rhl_for_each_entry_rcu() use rcu_dereference_all_check() (which checks rcu_read_lock_any_held()), holding preemption disabled via rcu_read_lock_sched_notrace() is fully valid and sufficient so long as rcu_is_watching() is true.
Define and use guard(rcu_sched_notrace)() across fprobe_ftrace_entry(), fprobe_fgraph_entry(), and fprobe_return(). This eliminates fast-path rcu_read_lock() and lockdep overhead while guaranteeing safe grace period synchronization.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel vulnerability addressed in this update centers on a critical race condition within the function probe (fprobe) subsystem, specifically affecting how memory is managed during unregistration when RCU read-side critical sections are not being tracked. The core issue arises from the reliance of unregister_fprobe and its asynchronous counterpart on standard RCU grace periods to ensure that all in-flight fprobe handlers have completed before freeing associated data structures. Standard RCU mechanisms like synchronize_rcu operate under the assumption that they can track preemption-disabled sections, but this tracking fails when an fprobe handler executes during intervals where RCU is not watching, such as within the idle loop or extended quiescent states in nohz_full configurations. In these scenarios, standard RCU does account for the execution of handlers running outside its watchful eye, leading to a situation where synchronize_rcu may return prematurely while an fprobe handler is still actively referencing freed memory. This creates a direct use-after-free vulnerability that can be exploited by local attackers or malicious BPF programs to cause kernel crashes, gain unauthorized access to sensitive data, or potentially achieve arbitrary code execution depending on the specific context of the dereferenced pointer and subsequent operations performed with it.
To mitigate this risk, the fix introduces a check for rcu_is_watching() within the fprobe handler logic, ensuring that handlers exit early if RCU is not currently tracking their state. This prevents the race condition by avoiding execution paths that would otherwise proceed without proper synchronization guarantees. Additionally, the update addresses performance and stability concerns related to lock dependency checking (lockdep). Previously, functions such as fprobe_fgraph_entry and fprobe_ftrace_entry utilized guard(rcu) and rcu_read_lock primitives which invoke lockdep on every hit when CONFIG_PROVE_LOCKING is enabled. This not only introduced significant overhead in high-frequency probing scenarios but also risked causing lockdep recursion if the probed functions themselves interact with locking mechanisms monitored by lockdep. The solution replaces these standard RCU read locks with guard(rcu_sched_notrace), which leverages rcu_read_lock_sched_notrace to hold preemption disabled without triggering unnecessary lockdep checks, provided that rcu_is_watching is true. This optimization eliminates fast-path overhead and prevents potential deadlocks or false positives in debugging builds while maintaining the necessary safety guarantees for RCU synchronization.
From a classification perspective, this vulnerability aligns with CWE-416 Use After Free, as the primary flaw involves accessing memory after it has been freed due to incorrect grace period handling. It also relates to CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, given that the race condition stems from improper synchronization between unregistering probes and their execution in specific CPU states. In terms of the MITRE ATT&CK framework, this issue falls under T1059 Command Line Interface or potentially T1068 Exploitation for Privilege Escalation if an attacker can leverage the use-after-free to escalate privileges within the kernel space. The vulnerability is particularly relevant in environments utilizing BPF kprobe-multi features where dynamic instrumentation of kernel functions is common, making it a significant concern for systems relying on eBPF for observability or security monitoring.
Mitigation strategies primarily involve applying the upstream Linux kernel patch that implements these RCU and locking improvements. System administrators should ensure their kernels are updated to versions containing this fix, particularly if they utilize fprobe-based BPF programs. For organizations unable to immediately update, mitigating factors include restricting unprivileged users from loading eBPF programs or disabling specific high-frequency probing features in production environments where the risk is deemed unacceptable. Regular auditing of kernel configurations and monitoring for unusual system behavior related to memory corruption can also help detect potential exploitation attempts before they lead to full compromise. The technical resolution ensures that fprobe handlers respect RCU state boundaries, thereby preserving data integrity and preventing the dangerous race conditions associated with premature memory reclamation in complex concurrent execution environments.