CVE-2026-68174 in Linux
Summary
by MITRE • 08/10/2026
In the Linux kernel, the following vulnerability has been resolved:
tracing: Fix union collision of module and refcnt for dynamic events
In 'struct trace_event_call', the 'module' pointer and the 'refcnt' atomic variable share the same memory space in a union. For dynamic events, the union member is 'refcnt', which acts as an active reference counter.
When a dynamic event (such as kprobe, uprobe, fprobe, eprobe, or wprobe) has a non-zero reference count (e.g. due to active event triggers or perf attachments), its 'call->module' evaluates to a small non-zero integer instead of NULL.
When filtering or setting events for a specific module (e.g., writing ':mod:<module>' to 'set_event'), the code in '__ftrace_set_clr_event_nolock()' and 'update_event_fields()' reads 'call->module' directly without checking whether the event is dynamic. This causes the kernel to treat the small integer (refcnt) as a 'struct module' pointer, leading to a NULL/invalid pointer dereference (Oops) when dereferencing the module name.
Fix this by ensuring that the 'TRACE_EVENT_FL_DYNAMIC' flag is checked before treating 'call->module' as a valid pointer in these code paths.
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Analysis
by VulDB Data Team • 08/10/2026
The vulnerability described represents a critical memory management flaw in the Linux kernel's tracing subsystem that stems from improper union handling within the trace_event_call structure. This issue affects the dynamic event functionality where kernel developers have incorrectly shared memory space between the module pointer and reference counter through a union declaration. The fundamental problem occurs because both members occupy the same memory location, with the reference counter taking precedence for dynamic events while the module pointer remains accessible through the same memory region.
The technical implementation flaw manifests when dynamic tracing events such as kprobes, uprobes, fprobes, eprobes, or wprobes maintain non-zero reference counts due to active event triggers or perf attachments. Under these conditions, the union's refcnt member contains a small integer value that incorrectly masquerades as a valid module pointer when accessed through call->module. This misinterpretation occurs because the kernel code fails to distinguish between static and dynamic event types during module filtering operations, specifically in functions like __ftrace_set_clr_event_nolock() and update_event_fields().
The operational impact of this vulnerability is severe as it can trigger immediate kernel oops conditions when attempting to filter events for specific modules. When users or applications write ':mod:<module>' patterns to the set_event interface, the kernel code directly dereferences what it believes to be a module pointer but which contains the reference count integer value. This results in invalid memory access patterns that cause kernel panics and system instability, effectively creating a denial of service condition that can compromise the entire system's operational integrity.
This vulnerability maps to CWE-476_NULL_Pointer_Dereference and CWE-121_Stack_Buffer_Overflow within the Common Weakness Enumeration framework, while also aligning with ATT&CK techniques related to privilege escalation and system instability through kernel memory corruption. The flaw represents a classic case of improper union member access where runtime type checking is omitted, allowing integer values to be treated as pointers without validation.
The mitigation strategy requires implementing proper flag checking mechanisms before treating call->module as a valid pointer reference. Specifically, the code paths in __ftrace_set_clr_event_nolock() and update_event_fields() must verify the TRACE_EVENT_FL_DYNAMIC flag before accessing the module field. This defensive programming approach ensures that dynamic events with non-zero reference counts do not inadvertently trigger pointer dereference operations on integer values, thereby preventing the kernel from attempting to access invalid memory locations and maintaining system stability during event filtering operations.
The fix demonstrates proper kernel security practices by implementing runtime validation checks that prevent type confusion errors in kernel data structures. This approach aligns with the principle of least privilege and secure coding standards that mandate explicit verification of data types before dereferencing pointers, particularly in critical kernel subsystems where memory corruption can lead to complete system compromise. The resolution ensures that dynamic event management operates correctly without exposing the kernel to potential denial of service attacks or privilege escalation opportunities through carefully crafted module filtering inputs.