CVE-2026-98176 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
drm/amdkfd: Avoid integer underflow with ffs in EOP ring size calc
The low 6 bits of cp_hqd_eop_control store the base-2 logarithm of the EOP ring size. This was calculated as
ffs(q->eop_ring_buffer_size / sizeof(unsigned int)) - 1 - 1
But ffs can in theory return 1 or 0, so this could underflow (although in practice the ring buffer size cannot be less than 4096).
Change this to
ffs(q->eop_ring_buffer_size / sizeof(unsigned int) / 4)
using properties of logarithms.
(cherry picked from commit 4f18c56630383c14bfc6b2d65f88f2f895d2121a)
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel's Direct Rendering Manager subsystem, specifically the AMD Kernel Fusion Driver component for graphics processing units, contained a logic error in the calculation of the Event Output Processor ring buffer size. This vulnerability stems from an incorrect implementation of integer arithmetic when determining the base-2 logarithm of the EOP ring size based on hardware register constraints. The cp_hqd_eop_control register stores the low six bits representing this logarithmic value, which is critical for configuring the GPU's event handling mechanisms correctly during context initialization and management operations within the AMDGPU driver stack.
The original code attempted to compute this value using the formula ffs(q->eop_ring_buffer_size / sizeof(unsigned int)) - 1 - 1. The function ffs returns the position of the least significant bit set, which effectively calculates one plus the base-2 logarithm for positive integers. Consequently, subtracting two from this result was intended to yield the correct exponent value required by the hardware specification. However, this approach introduces a risk of integer underflow if the input value results in an ffs return value of 1 or 0. While practical constraints on ring buffer allocation typically ensure that sizes remain above four kilobytes, preventing actual underflow in standard operational scenarios, the logical flaw remains present and violates safe coding practices for kernel-level memory management routines.
This type of vulnerability is classified as CWE-192 Integer Coercion Error or more specifically CWE-190 Integer Overflow or Wraparound depending on how the compiler handles negative results from unsigned arithmetic underflow. In terms of attack surface, this falls under CWE-682 Incorrect Calculation due to improper handling of edge cases in mathematical operations within system drivers. The potential impact includes undefined behavior during GPU context setup, which could lead to driver crashes, kernel panics, or potentially exploitable conditions if an attacker can influence the ring buffer size parameters through privileged API calls or malicious device interactions.
The remediation involves refactoring the calculation to leverage logarithmic properties directly within integer arithmetic constraints. By changing the expression to ffs(q->eop_ring_buffer_size / sizeof(unsigned int) / 4), the code effectively divides the input by four before applying the bit scan operation. Since dividing by four is equivalent to subtracting two from the exponent in base-2 logarithms, this approach avoids the subtraction of constants that could lead to underflow while maintaining mathematical equivalence for valid inputs. This change aligns with ATT&CK technique T1059 Command and Scripting Interpreter if considering how driver bugs can be chained into broader system compromise vectors via local privilege escalation paths through kernel memory corruption or state manipulation.
Mitigation strategies include applying the upstream kernel patch that corrects this arithmetic logic, ensuring that all AMDGPU drivers are updated to versions containing this fix. Administrators should monitor for stability issues related to GPU context creation and ensure that security updates are applied promptly to mitigate risks associated with driver-level vulnerabilities. Additionally, static analysis tools configured to detect integer underflow patterns in C code can help identify similar logical errors in other parts of the kernel or third-party drivers before they reach production environments.