CVE-2026-90002 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
ftrace: Take trace_array reference before accessing its ftrace_ops
The trace instance files set_ftrace_filter and set_ftrace_notrace was updated to work with specific trace instances (trace_arrays). The issue is that when these files are opened, there is a small race window where it will use the ftrace_ops from the inode->private pointer to get a reference to the trace_array and then take its reference. The problem is that the ftrace_ops itself could be freed. If the rmdir on the instance happens at the same time the set_ftrace_filter file is opened, the rmdir could have also freed the ftrace_ops and referencing it will cause a use-after-free bug and crash the kernel.
Instead, pass in the trace_array as the file private data (NULL for the top level instance), and then pass both the trace_array and the ftrace_ops to the ftrace_regex_open() function. If the trace_array is NULL, then it just uses the ftrace_ops without the need to take its reference (like normal). If the ftrace_ops is NULL, that is only the case for the top level instance and the global_ops can be used.
This allows the trace_array to have its reference incremented before touching the ftrace_ops that could also be freed when the instance is.
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Analysis
by VulDB Data Team • 09/16/2026
The Linux kernel's dynamic tracing subsystem, specifically the function tracer (ftrace), contains a critical race condition vulnerability related to resource management during file operations on trace instances. This flaw affects the handling of set_ftrace_filter and set_ftrace_notrace files, which are used by administrators and debugging tools to control which functions are traced within specific trace instances. The core issue stems from an incorrect ordering of reference counting operations when opening these device-like interface files. When a user space process opens one of these files, the kernel code attempts to retrieve a pointer to the associated ftrace_ops structure via the inode's private data field and subsequently takes a reference count on the parent trace_array object. This sequence creates a narrow but exploitable window where the underlying ftrace_ops structure may be deallocated by another concurrent operation before the reference is safely secured, leading to a use-after-free condition that can result in kernel panic or arbitrary code execution depending on memory layout and exploitation techniques.
The vulnerability manifests specifically when there is concurrency between opening a trace instance file and removing (rmdir) that same trace instance from the system. During the removal process, the kernel frees the ftrace_ops structure associated with the instance to clean up resources. However, if another thread simultaneously attempts to open the set_ftrace_filter or set_ftrace_notrace interface for that specific instance, it may access the now-freed memory address stored in the inode's private pointer. Because the code assumes the ftrace_ops is valid and proceeds to dereference pointers within it to locate the trace_array, this use-after-free scenario triggers undefined behavior. In many cases, this leads to an immediate kernel crash due to invalid memory access, but sophisticated attackers could potentially leverage this race condition to execute arbitrary code with root privileges by controlling the contents of the freed memory region before it is reallocated for other purposes.
From a technical standards perspective, this vulnerability aligns closely with CWE-416, Use After Free, as the system accesses memory after it has been returned to the free pool without ensuring its validity or re-fetching a safe reference. Furthermore, in the context of attack patterns, this flaw relates to ATT&CK technique T1059, Command and Scripting Interpreter, specifically through mechanisms that allow attackers to manipulate kernel state via file system interactions if they can achieve local code execution. The root cause is identified as CWE-362, Concurrent Execution using Shared Resource with Improper Synchronization of Reference Counts, where the atomicity of checking for existence and taking a reference was not properly maintained against concurrent modification by instance teardown routines.
The resolution implemented in this patch addresses the race condition by fundamentally changing how file private data is managed for trace instances. Instead of relying on the inode's private pointer to hold an ftrace_ops that might be freed, the kernel now passes the trace_array directly as the file private data during the open operation. This ensures that the reference count for the trace_array itself is incremented before any operations are performed on its associated structures. The updated logic in ftrace_regex_open() handles both scenarios: if a specific trace instance exists, it uses the provided trace_array and ftrace_ops together; if no specific instance is targeted (the top-level global case), it defaults to using global_ops without requiring complex reference management for individual ops. This approach decouples the lifetime of the file handle from the potentially transient state of intermediate structures like ftrace_ops during initialization, thereby eliminating the window where a freed pointer could be dereferenced.
To mitigate this vulnerability in environments running affected kernel versions, immediate patching is required as no reliable workaround exists that does not involve disabling dynamic tracing features entirely. Administrators should apply the latest stable Linux kernel updates that include this fix for ftrace reference counting. For systems requiring continuous availability of debugging capabilities, it is advisable to restrict access to trace instance files using strict file system permissions and SELinux or AppArmor policies to limit which users can open these interfaces. Additionally, monitoring for unusual patterns in process creation around the time of trace instance manipulation may help detect attempted exploitation activities, although the primary defense remains timely application of vendor-provided security patches that correct the synchronization logic within the kernel's tracing subsystem.