CVE-2026-98888 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
arm64: kprobes: Allow reentering kprobes while single-stepping
A kprobe can be hit while another kprobe is in KPROBE_HIT_SS state. This can happen when tracing or perf code runs from the debug exception path while the first kprobe is preparing or executing its out-of-line single-step instruction.
Currently arm64 treats a kprobe hit in KPROBE_HIT_SS as unrecoverable, the same as a hit in KPROBE_REENTER. This is too strict. A hit in KPROBE_HIT_SS is still a one-level reentry and can be handled by saving the current kprobe state and setting up single-step for the new probe, just like reentry from KPROBE_HIT_ACTIVE or KPROBE_HIT_SSDONE.
The truly unrecoverable case is hitting another kprobe while already in KPROBE_REENTER, because the reentry save area has already been consumed.
Move KPROBE_HIT_SS to the recoverable reentry cases and leave KPROBE_REENTER as the unrecoverable nested reentry case.
This change also requires saving saved_irqflag in struct prev_kprobe. When a nested kprobe calls kprobes_save_local_irqflag(), it overwrites kcb->saved_irqflag with the currently masked DAIF value, losing the outer kprobe's original DAIF state. Without this fix, when the outer kprobe's single-step finishes, kprobes_restore_local_irqflag() applies the wrong DAIF mask and leaves interrupts permanently disabled.
Extend struct prev_kprobe with a saved_irqflag field and save/restore it alongside kp and status. This ensures the outer kprobe's original interrupt state is preserved across reentry.
This mirrors the x86 fix in commit 6a5022a56ac3 ("kprobes/x86: Allow to handle reentered kprobe on single-stepping").
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel for ARM64 architecture contained a critical flaw within its dynamic tracing subsystem, specifically affecting the handling of nested kprobe events during instruction single-stepping. Kprobes allow developers and system administrators to insert breakpoints at arbitrary kernel code locations for debugging or performance analysis. When a probe is hit, the kernel often replaces the original instruction with an out-of-line breakpoint handler and uses single-stepping to execute the original instruction safely in a separate memory area. The vulnerability arose because the ARM64 implementation incorrectly classified certain nested kprobe hits as unrecoverable reentries when they were actually safe to handle. Specifically, if a second kprobe was triggered while the first one was already in the KPROBE_HIT_SS state—meaning it was preparing or executing its single-stepped instruction—the kernel treated this scenario identically to a deeply nested reentry that had exhausted its save area. This overly strict classification caused the system to fail gracefully by panicking, effectively turning a manageable tracing event into a denial of service condition for any workload relying on these debugging features.
From a technical perspective, the root cause lies in the mismanagement of processor state during exception handling paths used by tracing tools like perf or kernel self-tracing code. When an interrupt or debug exception occurs while a kprobe is active, the architecture must preserve the current CPU context to ensure that execution can resume correctly after the probe handler completes. The ARM64 implementation failed to distinguish between recoverable single-level reentries and truly unrecoverable deep nesting. In the flawed logic, any hit occurring during KPROBE_HIT_SS was deemed fatal because it did not follow the standard recovery path used for other states like KPROBE_HIT_ACTIVE or KPROBE_HIT_SSDONE. Furthermore, there was a specific register state corruption issue involving interrupt flags. The kernel uses a structure called prev_kprobe to save context across probe executions. However, when nested probes invoked functions to save local interrupt flag states, they would overwrite the saved_irqflag field in the control block without preserving the original value from the outermost probe. This resulted in the loss of the initial DAIF (Data Abort, Interrupts, Fast IRQ, and Secure Fault) mask state.
The operational impact of this vulnerability is significant for systems utilizing dynamic kernel tracing. If an administrator or automated tool attempts to trace code paths that trigger nested kprobes during single-stepping, the system will crash with a kernel panic rather than handling the event gracefully. This creates a reliable denial-of-service vector against any production environment where such debugging tools are active. Additionally, even in scenarios where the immediate crash is avoided through other means, the improper restoration of interrupt flags can lead to interrupts being permanently disabled after the probe completes. A system with interrupts disabled cannot process hardware events, network packets, or timer ticks, leading to a complete hang of the operating environment until a hard reset occurs. This behavior undermines the reliability and stability of kernel debugging infrastructure, making it unsafe for use in production environments where continuous availability is required.
To mitigate this vulnerability, the Linux kernel developers implemented changes that align ARM64 kprobe handling with established practices from other architectures like x86. The primary fix involves reclassifying KPROBE_HIT_SS as a recoverable reentry case rather than an unrecoverable one. This allows the kernel to save the current probe state and set up single-stepping for the new nested probe, mirroring the behavior already present for other kprobe states. Crucially, the data structure prev_kprobe was extended with a saved_irqflag field. The implementation now ensures that when a nested kprobe saves its local interrupt flag status, it does not overwrite the original DAIF state of the outermost probe without preserving it. Upon completion of all nested probes and their single-stepped instructions, the kernel correctly restores the initial interrupt mask, ensuring that normal system operation resumes with interrupts enabled as expected. This change effectively eliminates both the denial-of-service crash condition and the potential for permanent interrupt disabling bugs associated with dynamic tracing on ARM64 platforms.
This vulnerability is categorized under CWE-20: Improper Input Validation, specifically regarding the validation of execution state during nested exception handling. It also relates to CWE-787: Out-of-bounds Write if considering the improper overwriting of context structures without preservation logic, though primarily it represents a logical error in control flow management within kernel security mechanisms. In terms of MITRE ATT&CK mapping, this issue falls under T1059: Command and Scripting Interpreter or more accurately T1204: User Execution if exploited via malicious tracing tools, but fundamentally it is an internal reliability flaw rather than a direct exploitation vector for external attackers unless combined with other vulnerabilities. The fix ensures that the kernel maintains integrity during complex debugging operations, preventing local denial of service through legitimate administrative actions involving kprobes.