CVE-2026-90347 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
arm64: ptrace: Keep 'orig_x0' in-sync with x0 on syscall entry
Commit e057b9477232 ("arm64: syscall: Ensure saved x0 is kept in-sync with tracer updates") attempted to resolve a long-standing issue with syscall entry tracing, where a tracer is able to manipulate the first syscall argument without being subjected to seccomp or audit checking.
Unfortunately, that fix was incomplete [1], as it failed to update
'orig_x0' between a tracer updating x0 during a seccomp ptrace exit (SECCOMP_RET_TRACE) and the seccomp filter being re-evaluated.
Rather than add hooks to the core seccomp code, instead move the synchronisation code into the ptrace GPR and syscall setting code so that 'orig_x0' is kept up to date with x0 whenever we're stopped on the syscall entry path.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel for ARM64 architecture contained a critical logic flaw in its process tracing subsystem, specifically within the interaction between ptrace-based system call interception and the seccomp security framework. This vulnerability stems from an incomplete implementation of a previous fix intended to synchronize the original register value orig_x0 with the current general-purpose register x0 during syscall entry. The core issue arises when a tracer modifies the first argument of a system call via ptrace while the process is stopped at syscall entry, particularly in scenarios involving seccomp filters that return SECCOMP_RET_TRACE. In such cases, the kernel suspends execution to allow an external tracing process to inspect or modify registers before resuming. The flaw lies in the timing and scope of register synchronization: while earlier patches attempted to ensure consistency between x0 and orig_x0 during general tracer interactions, they failed to account for the specific state transitions occurring when a seccomp filter triggers a trace event. Consequently, if a tracer modifies x0 after being notified by seccomp but before the kernel re-evaluates the seccomp filters upon resumption, the original value stored in orig_x0 remains stale and does not reflect the modification made by the tracer.
This desynchronization creates a significant security bypass mechanism that undermines both audit logging and access control enforcement mechanisms governed by seccomp-bpf. When the kernel proceeds to evaluate whether the system call should be allowed or denied, it relies on the value of orig_x0 rather than the potentially modified x0 register for certain validation paths, particularly those involving syscall number verification and argument checking in older or specific configurations of audit rules. An attacker who can influence a process being traced by another privileged entity could exploit this race condition to alter system call arguments invisibly to the security subsystems. For instance, if an application attempts to invoke a restricted function with dangerous parameters, but a tracer intercepts the syscall and alters those parameters before seccomp re-evaluation, the kernel might evaluate the original intent rather than the actual execution path intended by the modified registers. This effectively allows malicious actors or compromised tracing processes to bypass security policies designed to restrict specific system calls based on their arguments, leading to potential privilege escalation or unauthorized data access depending on the targeted syscall and its associated permissions.
From a technical classification perspective, this vulnerability aligns with CWE-841 Improper Enforcement of Behavioral Workflow because it involves incorrect handling of state transitions during process execution flow control. It also relates closely to CWE-20 Improper Input Validation since the kernel fails to validate that the input registers match their expected original states after external modification attempts within a trusted tracing context. In terms of MITRE ATT&CK mapping, this flaw facilitates techniques associated with Defense Evasion through Process Injection or API Hooking, specifically under T1055 and related sub-techniques where attackers manipulate process memory to alter execution behavior undetected by security controls. The vulnerability highlights the complexity of maintaining consistency between hardware registers, kernel internal state structures, and external tracing interfaces in a multi-layered security architecture like Linux's ptrace-seccomp integration.
The resolution involves refactoring the synchronization logic directly within the ptrace general-purpose register handling code rather than attempting to patch the core seccomp subsystem. By moving the synchronisation mechanism into the syscall setting path, the kernel ensures that orig_x0 is consistently updated with x0 whenever a process is stopped on the syscall entry path due to tracing events. This approach guarantees that any modifications made by tracers are immediately reflected in the original register state used for subsequent security evaluations. To mitigate this issue and similar vulnerabilities, system administrators should ensure that all Linux kernels running ARM64 architectures are updated to versions containing commit e057b9477232 or later patches addressing this specific synchronization gap. Additionally, organizations employing seccomp profiles in containerized environments must verify that their tracing infrastructure does not inadvertently exploit these race conditions by ensuring strict isolation between tracer processes and security-critical system calls where possible. Regular auditing of ptrace usage policies and limiting the capabilities granted to tracing users can further reduce the attack surface associated with such register manipulation techniques.