CVE-2026-89902 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
LoongArch: Avoid preempt count underflow without probe
LoongArch uses break 11 for the breakpoint placed after an instruction that Kprobes executes out of line. Since userspace can issue the same break instruction, do_bp() can reach kprobe_singlestep_handler() when there is no current probe.
The handler actually returns false in this case, but it first calls preempt_enable_no_resched(). The corresponding preempt_disable() is done by kprobe_breakpoint_handler() on a real Kprobe hit, so it has not run here. As a result, an ordinary userspace breakpoint (code 11) underflows the current task's preempt count.
This also makes in_interrupt() return true until the task schedules. One visible consequence is the socket cgroup attribution: cgroup_sk_alloc() treats the allocation as interrupt context and assigns the socket to the root cgroup. A socket opened from the SIGTRAP handler can then avoid a BPF_CGROUP_INET_SOCK_CREATE policy attached to the task's own cgroup.
Return as soon as kprobe_running() reports no active probe.
The same check has appeared in [PATCH v10 2/4] of the original LoongArch
Kprobes series, but was dropped before the feature reached mainline.
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Analysis
by VulDB Data Team • 09/16/2026
A critical vulnerability exists within the Linux kernel's handling of breakpoint exceptions on the LoongArch architecture, specifically involving the interaction between user-space generated signals and the Kprobes subsystem. The flaw arises because the instruction used to trigger a software breakpoint for out-of-line execution by Kprobes is identical to the break 11 instruction that userspace applications can legitimately issue via system calls or direct assembly instructions. When such an instruction is executed, it triggers a trap exception handled by the do_bp function in the kernel. This handler attempts to determine whether the fault was caused by a legitimate Kprobe insertion or merely a user-space breakpoint. While the logic correctly identifies non-Kprobe cases and returns false from kprobe_singlestep_handler, it fails to account for the state of the preempt count before returning.
The technical root cause lies in an asymmetry of preemption control operations within the exception handling path. In scenarios where a genuine Kprobe is hit, the kernel first disables preemption via preempt_disable() and later re-enables it after processing. However, when a user-space breakpoint triggers the same handler without an active probe being present, the initial preempt_disable() call never occurs because the code path for actual probes was not taken. Despite this, kprobe_singlestep_handler proceeds to invoke preempt_enable_no_resched(), which increments the task's preemption counter. Since there was no corresponding decrement from a prior disable operation, this results in an underflow of the current task's preempt count field within the thread_info structure. This integer underflow corrupts internal kernel state regarding scheduling and interrupt context tracking for that specific process.
The operational impact of this vulnerability extends beyond simple arithmetic errors to significant security implications related to control group attribution and policy enforcement. The corrupted preemption counter causes the in_interrupt() function, which checks whether the current execution context is an interrupt or softirq, to incorrectly return true until the task next schedules out. This misclassification has direct consequences for socket creation events managed by cgroup subsystems. Specifically, when a new socket is allocated during the handling of this SIGTRAP signal, the kernel's cgroup_sk_alloc function perceives the context as an interrupt context rather than process context. Consequently, it assigns the newly created socket to the root cgroup instead of the task's own cgroup. This bypasses BPF_CGROUP_INET_SOCK_CREATE policies that are attached to the specific user or service cgroup, allowing processes to create network connections that evade intended access controls and isolation boundaries defined by containerization or security modules like SELinux or AppArmor when they rely on these cgroup attributes for enforcement decisions.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The primary fix involves modifying the kprobe_singlestep_handler to check if a probe is actually running using kprobe_running() before attempting any preemption state adjustments, ensuring that preempt_enable_no_resched is only called when preceded by its counterpart disable operation. For systems unable to immediately patch the kernel, administrators should consider restricting access to ptrace capabilities and monitoring for unusual patterns of SIGTRAP generation from untrusted user-space applications. Additionally, deploying BPF programs with strict validation logic can help detect anomalies in socket creation contexts, although this is a compensatory measure rather than a fix for the underlying race condition. The vulnerability highlights the importance of rigorous state management in exception handlers that bridge user-mode and kernel-mode execution environments, particularly on architectures where breakpoint instructions are not reserved exclusively for debugging tools but remain accessible to general application code.