CVE-2026-98856 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
kprobes: Skip disarmed probes when checking optkprobe overlap
On x86, an optkprobe at A replaces five bytes with a jump. If a disabled probe B is at A+2, get_optimized_kprobe() stops at B when arming a new probe C at A+4. It leaves A optimized:
A A+1 A+2 A+3 A+4 A's jump | e9 | d0 | d1 | d2 | d3 | after C | e9 | d0 | d1 | d2 | cc |
The INT3 for C overwrites the last byte of A's jump displacement, so execution can jump to the wrong address. B can have prepared optinsns while disarmed, but has no jump to unoptimize.
Continue past disarmed and unprepared probes to find the active optimized probe before arming a probe in its jump.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel's kprobes subsystem facilitates dynamic tracing by allowing developers to insert breakpoints at arbitrary instruction addresses within running kernels or modules. A critical flaw was identified in how the system handles overlapping optimized probes, specifically on x86 architectures where optimization replaces original instructions with a jump opcode followed by displacement bytes. When an optkprobe is placed at address A, it typically overwrites five bytes of code to install a relative jump instruction that redirects execution to the probe handler. The vulnerability arises when this active optimized probe overlaps in memory space with another probe location B that has been disarmed or unprepared but still retains residual optimization instructions such as prepared optinsns without an associated jump mechanism for cleanup.
During the process of arming a new probe C at address A+4, the function get_optimized_kprobe() is invoked to check for existing optimized probes in the vicinity. Due to a logic error, this function incorrectly stops its search upon encountering the disarmed probe B located at A+2 rather than continuing past it to identify any active optimized probes further down the instruction stream. This premature termination means that the system fails to recognize that address A is already occupied by an active optimization structure. Consequently, when probe C is armed, it proceeds without detecting the conflict with probe A's jump instruction.
The operational impact of this oversight is severe and leads to memory corruption and potential kernel instability or privilege escalation depending on context. Specifically, arming probe C at A+4 results in writing a breakpoint interrupt instruction (INT3) that overwrites the final byte of the displacement field belonging to probe A's jump instruction. Since x86 relative jumps rely on accurate four-byte displacements to calculate target addresses correctly, altering even one byte can drastically change where execution is redirected. Instead of jumping back or forward as intended by probe A's handler setup, the CPU may execute a jump to an arbitrary and incorrect memory address upon hitting breakpoint A. This corruption breaks control flow integrity and can cause immediate kernel panics or allow attackers who have gained some level of code execution capability to redirect program flow maliciously.
From a technical classification perspective, this vulnerability aligns with CWE-824 Access of Uninitialized Pointer if the disarmed probe's state is not properly cleared before overlap checks, though more accurately it reflects CWE-367 Time-of-check Time-of-use (TOCTOU) race condition or logic error in resource management where the system fails to validate current state against intended action. In terms of MITRE ATT&CK mapping, this could be leveraged under techniques related to Execution Guarding such as Hooking or API Hooking if an attacker can trigger these probe insertions through privileged interfaces like kprobes registration APIs available to root users. The core issue lies in the failure to maintain accurate state tracking for all potential overlap points including those marked as inactive but structurally present in memory maps used by optimization routines.
To mitigate this vulnerability, kernel developers implemented a fix that modifies get_optimized_kprobe() to continue scanning past disarmed and unprepared probes when determining whether an active optimized probe exists within the range of a new probe's jump instruction. By ensuring that only truly active optimizations block or influence the arming process, the system prevents the accidental overwriting of displacement bytes belonging to other live probes. Administrators should apply kernel updates containing this patch immediately on systems relying heavily on dynamic tracing features like perf events and ftrace which utilize kprobes under the hood. Additionally, restricting access to probe registration mechanisms via strict SELinux policies or capabilities can reduce exposure surfaces until all affected instances are patched across distributed environments.