CVE-2026-99163 in Linux
Summary
by MITRE • 10/28/2026
In the Linux kernel, the following vulnerability has been resolved:
tracing/histograms: Reject non-stacktrace fields for synthetic stacktrace fields
In check_synth_field(), when a synthetic event field is a stacktrace (field->is_stack) and the histogram field is not a "long[" stacktrace
array, validation falls through to the generic size and signedness comparison.
Because parse_synth_field() sets a stacktrace field's type to "__data_loc unsigned long[]", its size is sizeof(u64) and
synth_field_signed() marks it as signed. Consequently, any 8-byte signed scalar histogram field passes the fallback check, and its scalar value is subsequently dereferenced as a kernel stacktrace pointer in get_field_size() and trace_stack().
Furthermore, checking strstr(hist_field->type, "long[") is insufficient
because tracepoint fields defined with __bitmask() or __rel_bitmask() also have type "__data_loc unsigned long[]" or "__rel_loc unsigned
long[]", whilst create_hist_field() treats them as 4-byte scalar values
without HIST_FIELD_FL_STACKTRACE.
Propagate HIST_FIELD_FL_STACKTRACE to variable references and field variables (whilst excluding variable aliases from per-element stacktrace capture), and reject any mismatch between HIST_FIELD_FL_STACKTRACE and field->is_stack in check_synth_field().
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Analysis
by VulDB Data Team • 10/28/2026
The Linux kernel tracing subsystem contains a critical logic flaw within the synthetic event histogram processing mechanism that allows for improper type validation, potentially leading to arbitrary code execution or system instability. This vulnerability resides specifically in the check_synth_field function, which is responsible for validating fields when constructing histograms from synthetic events. The core issue arises during the comparison of stacktrace fields against histogram fields. When a synthetic event field is identified as a stacktrace via the is_stack flag, and the corresponding histogram field does not explicitly utilize a long array structure, the validation logic incorrectly falls through to generic size and signedness checks rather than enforcing strict type compatibility for stacktraces. This design oversight permits mismatched data types to pass initial validation gates that are intended only for standard scalar values.
The technical root cause is rooted in how parse_synth_field initializes field metadata. It sets the type of a stacktrace field to __data_loc unsigned long[], which results in a reported size of sizeof(u64) and marks the field as signed via synth_field_signed. Consequently, any eight-byte signed scalar histogram field satisfies these generic fallback checks because it matches both the size and sign criteria. However, this match is superficial and dangerous. When such a mismatched field is later processed by get_field_size or trace_stack, the kernel interprets the scalar value of the histogram as a pointer to a kernel stacktrace buffer. This dereferencing of an arbitrary user-controlled or memory-corrupted scalar value as a high-level system structure leads to undefined behavior, potentially causing kernel panics, information disclosure through leaked stack data, or privilege escalation if the attacker can control the scalar value to point to writable executable memory regions.
Further complicating the issue is that simple string matching for long brackets in field types proves insufficient due to overlapping type definitions used by other subsystems. Tracepoint fields defined with __bitmask() or __rel_bitmask attributes also utilize similar location-based pointer structures like __data_loc unsigned long[] or __rel_loc unsigned long[]. The create_hist_field function correctly treats these bitmask-related fields as four-byte scalar values without assigning the HIST_FIELD_FL_STACKTRACE flag, distinguishing them from actual stacktraces. However, because the validation logic in check_synth_field relies on generic properties rather than explicit flags for variable references and field variables, it fails to distinguish between legitimate stacktrace arrays and other long-array-like structures that are semantically scalars. This ambiguity allows attackers to exploit the lack of strict flag-based differentiation to bypass security checks designed specifically for stack trace integrity.
The resolution involves propagating the HIST_FIELD_FL_STACKTRACE flag more rigorously across variable references and field variables, while explicitly excluding variable aliases from per-element stacktrace capture mechanisms to prevent scope confusion. The primary fix enforces a mandatory check in check_synth_field that rejects any synthetic event configuration where there is a mismatch between the HIST_FIELD_FL_STACKTRACE status of the histogram field and the is_stack attribute of the corresponding tracepoint field. This ensures that only fields explicitly designated as stacktraces can interact with other stacktrace components, thereby closing the path for scalar values to be misinterpreted as pointers. By tightening this validation boundary, the kernel prevents the unsafe dereferencing operations that underpin the vulnerability's exploitability.
From a threat modeling perspective, this flaw aligns closely with CWE-20 Improper Input Validation and CWE-119 Buffer Overflow within a non-bounded memory context, specifically involving improper interpretation of data types leading to out-of-bounds access or arbitrary code execution via pointer confusion. In the MITRE ATT&CK framework for Linux systems, this vulnerability facilitates techniques related to Privilege Escalation through kernel exploitation (T1068) and potentially Defense Evasion if used to crash security monitoring agents like eBPF-based tracers. Mitigation strategies should prioritize applying upstream kernel patches that enforce strict flag consistency checks in the tracing subsystem. System administrators managing production environments must ensure their Linux kernels are updated to versions containing this specific histogram validation fix, particularly those deploying complex synthetic event configurations for performance profiling or forensic analysis where stacktrace histograms are utilized extensively.