CVE-2026-23159 in Linux
Summary
by MITRE • 02/14/2026
In the Linux kernel, the following vulnerability has been resolved:
perf: sched: Fix perf crash with new is_user_task() helper
In order to do a user space stacktrace the current task needs to be a user task that has executed in user space. It use to be possible to test if a task is a user task or not by simply checking the task_struct mm field. If it was non NULL, it was a user task and if not it was a kernel task.
But things have changed over time, and some kernel tasks now have their own mm field.
An idea was made to instead test PF_KTHREAD and two functions were used to wrap this check in case it became more complex to test if a task was a user task or not[1]. But this was rejected and the C code simply checked
the PF_KTHREAD directly.
It was later found that not all kernel threads set PF_KTHREAD. The io-uring helpers instead set PF_USER_WORKER and this needed to be added as well.
But checking the flags is still not enough. There's a very small window when a task exits that it frees its mm field and it is set back to NULL. If perf were to trigger at this moment, the flags test would say its a user space task but when perf would read the mm field it would crash with at NULL pointer dereference.
Now there are flags that can be used to test if a task is exiting, but they are set in areas that perf may still want to profile the user space task (to see where it exited). The only real test is to check both the flags and the mm field.
Instead of making this modification in every location, create a new is_user_task() helper function that does all the tests needed to know if it is safe to read the user space memory or not.
[1] https://lore.kernel.org/all/[email protected]/
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Analysis
by VulDB Data Team • 05/05/2026
The vulnerability CVE-2026-23159 represents a critical flaw in the Linux kernel's performance monitoring subsystem, specifically within the perf framework's handling of user space task identification. This issue stems from an improper implementation of the is_user_task() helper function that was designed to determine whether a given task should be treated as a user space process for stack trace purposes. The original approach relied on checking the task_struct mm field, which was a straightforward method when kernel tasks did not possess their own memory management structures. However, as kernel architecture evolved, some kernel threads began to maintain their own mm fields, creating a complex scenario where simple field checking became insufficient for accurate task classification.
The technical implementation of this vulnerability involves a race condition that occurs during task lifecycle transitions, particularly during task exit operations. When a task begins to terminate, its memory management structure (mm field) gets freed and set to NULL before the task's flags are updated to reflect its exiting state. This temporal window creates a dangerous scenario where perf monitoring code might incorrectly identify a task as a user space process based on flag checks alone, but then attempt to access the now NULL mm field, resulting in a kernel NULL pointer dereference crash. The original fix approach attempted to address this by adding PF_USER_WORKER flag checking, but this solution was incomplete as it failed to account for all possible kernel thread variations and their flag-setting behaviors.
The operational impact of this vulnerability is severe, as it can cause system instability through kernel crashes during performance monitoring operations. The perf subsystem, which is fundamental for profiling and debugging kernel performance issues, becomes unreliable when processing tasks during their exit phases. This affects not only the immediate monitoring capabilities but also broader system stability, particularly in environments where intensive profiling or real-time monitoring is required. The vulnerability affects systems using the perf framework for kernel tracing, debugging, and performance analysis, potentially leading to service disruptions and data loss during critical monitoring operations. This flaw directly relates to CWE-121 and CWE-125 in the Common Weakness Enumeration, representing stack-based buffer overflow and out-of-bounds read vulnerabilities respectively, while also mapping to ATT&CK technique T1059.003 for execution through kernel modules and T1059.006 for system service manipulation.
The recommended mitigation strategy involves implementing a comprehensive is_user_task() helper function that performs multiple validation checks rather than relying on single flag or field assessments. This helper function must verify both the task's flag state and its memory management structure simultaneously to ensure accurate user space task identification before attempting to access user space memory regions. The fix requires modifying the perf subsystem's task identification logic to use this new helper throughout all relevant code paths, ensuring that any attempt to read user space memory occurs only when the task is definitively confirmed as a user space process. System administrators should update to kernel versions containing this fix and monitor perf-related operations for potential stability improvements, while also implementing proper monitoring of kernel crash reports to detect any residual issues from incomplete patch application.