CVE-2026-98860 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
HID: bpf: cast size to ssize_t when checking hid_bpf_hw_request
As reported by Sashiko: If a transport driver encounters a hardware error and returns a negative error code such as -EPIPE, ret is implicitly promoted to size_t when compared against size. This causes the negative error code to evaluate as a large positive number, making the (ret > size) condition true.
This silently converts the hardware error into a success return value and copies the unmodified buffer back, which could leave BPF programs operating on uninitialized or stale data.
Fix this by casting size into ssize_t to return the actual negative error code.
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Analysis
by VulDB Data Team • 10/18/2026
The vulnerability identified in the Linux kernel's HID subsystem involves a critical type conversion flaw within the hid_bpf_hw_request function. This issue arises during the interaction between BPF programs and hardware transport drivers, specifically when handling return codes from hardware operations. The core technical flaw lies in an implicit integer promotion that occurs during a comparison operation. When a transport driver encounters a hardware error, it typically returns a negative signed integer value representing standard POSIX error codes, such as -EPIPE for broken pipe errors or other I/O failure indicators. However, the code compares this return value against a size variable which is of type size_t, an unsigned integer type designed to represent non-negative quantities like buffer lengths.
In C programming language semantics, when a signed integer and an unsigned integer are compared in an expression, the signed operand undergoes implicit conversion to match the unsigned type if the unsigned type has equal or greater rank. Consequently, negative error codes such as -1 or -EPIPE are reinterpreted as large positive values due to two's complement representation wrapping around the maximum limit of size_t. For instance, a return value of -1 becomes 4294967295 on a 32-bit system or significantly larger on 64-bit systems depending on the specific architecture and data width. This mathematical transformation fundamentally alters the logic flow because the subsequent conditional check evaluates whether this erroneously large positive number exceeds the buffer size, which it almost invariably does given that hardware error codes are small negative integers while buffer sizes are typically much smaller positive values.
The operational impact of this flaw is severe as it results in a silent failure mode where actual hardware errors are misinterpreted as successful operations. Instead of propagating the error code up the call stack to alert higher-level components or BPF programs that the request failed, the function proceeds under the false assumption that the operation succeeded. This leads to the copying of unmodified buffer data back to the caller. For eBPF programs attached to HID devices, this means they may process stale, uninitialized, or garbage memory contents rather than valid device input data. Such behavior can lead to unpredictable application states, potential information disclosure if sensitive kernel memory is exposed through these buffers, and a complete breakdown of security policies enforced by BPF programs that rely on accurate hardware status reporting.
From a vulnerability classification perspective, this issue aligns with CWE-190 Integer Overflow or Wraparound, specifically involving the improper handling of signed-to-unsigned conversion during comparison operations. It also relates to CWE-253 Incorrect Check of Function Return Value, as the code fails to properly validate and handle negative return codes from lower-level drivers. In terms of MITRE ATT&CK mapping, this flaw could facilitate lateral movement or privilege escalation if an attacker can trigger specific hardware error conditions that result in predictable memory states being exposed to user-space applications via BPF programs. The lack of proper error propagation allows malicious actors potentially exploit the resulting undefined behavior for data exfiltration or denial of service by causing consistent processing of corrupted data streams.
The resolution involves explicitly casting the size variable to ssize_t before performing the comparison, ensuring that both operands are signed integers and preserving the semantic meaning of negative return codes. This simple type correction prevents the implicit unsigned conversion and allows the function to correctly identify hardware errors as failures rather than successes. By returning the actual negative error code instead of silently proceeding with invalid data, the kernel maintains integrity in its interaction between BPF programs and HID subsystems. System administrators should ensure their kernels are updated to include this patch to prevent potential exploitation scenarios stemming from improper error handling in high-performance computing environments that utilize eBPF for network or device monitoring and security enforcement.