CVE-2026-31525: Linux Kernel Privilege Escalation Flaw

CVE-2026-31525 Overview

A critical undefined behavior vulnerability has been identified in the Linux kernel's BPF (Berkeley Packet Filter) interpreter. The vulnerability exists in the signed 32-bit division (sdiv) and modulo (smod) handlers, where the kernel's abs() macro is improperly used with s32 operands. When the destination register contains S32_MIN (0x80000000), the abs() function triggers undefined behavior, returning S32_MIN unchanged on arm64/x86 architectures. This incorrect value is then sign-extended to u64 as 0xFFFFFFFF80000000, causing do_div() to compute incorrect results.

The critical security concern is that this creates a verifier/interpreter mismatch. The BPF verifier's abstract interpretation (scalar32_min_max_sdiv) correctly computes the mathematically accurate result for range tracking, but the interpreter produces a different result. This discrepancy can be exploited to achieve out-of-bounds map value access, potentially leading to information disclosure or arbitrary memory manipulation.

Critical Impact

Attackers can exploit the verifier/interpreter mismatch to bypass BPF safety checks and achieve out-of-bounds memory access, potentially compromising system integrity on affected Linux systems.

Affected Products

• Linux kernel (versions with BPF interpreter sdiv32/smod32 handlers)
• Systems running arm64 architecture with affected kernel versions
• Systems running x86 architecture with affected kernel versions

Discovery Timeline

• 2026-04-22 - CVE CVE-2026-31525 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2026-31525

Vulnerability Analysis

The vulnerability stems from improper handling of the signed integer minimum value in the BPF interpreter's arithmetic operations. The abs() macro, as documented in include/linux/math.h, explicitly states that the result is undefined when the input is the type minimum. For signed 32-bit integers (s32), this minimum value is S32_MIN (0x80000000 or -2147483648).

When a BPF program performs a signed division or modulo operation where the dividend is S32_MIN, the interpreter calls abs((s32)DST) to handle the sign. Due to the mathematical property that the absolute value of the minimum signed integer cannot be represented in the same signed type (since |S32_MIN| would be 2147483648, which exceeds S32_MAX of 2147483647), undefined behavior occurs.

On arm64 and x86 architectures, this undefined behavior manifests as abs() returning S32_MIN unchanged. When this value is subsequently sign-extended to a 64-bit unsigned integer (u64), it becomes 0xFFFFFFFF80000000. The do_div() function then operates on this incorrect value, producing a result that differs from what the verifier predicted.

The BPF verifier performs abstract interpretation to track value ranges and ensure memory safety. Since the verifier correctly handles this edge case mathematically while the interpreter does not, an attacker can craft BPF programs that pass verification but execute with unexpected values at runtime, enabling out-of-bounds access to BPF map values.

Root Cause

The root cause is the use of the kernel abs() macro on s32 operands without accounting for the documented undefined behavior when the input is S32_MIN. The abs() macro cannot safely handle the type minimum value because negating S32_MIN in signed arithmetic causes signed overflow, which is undefined behavior in C.

The fix introduces a new abs_s32() function that handles S32_MIN correctly by casting to u32 before negating, avoiding signed overflow entirely. This replacement is applied to all 8 call sites in the interpreter's sdiv32 and smod32 handlers where abs((s32)...) was used.

Attack Vector

An attacker with the ability to load BPF programs (either via CAP_BPF capability or unprivileged BPF if enabled) can craft a malicious BPF program that exploits the verifier/interpreter mismatch. The attack proceeds as follows:

1. The attacker creates a BPF program that performs signed division or modulo operations with S32_MIN as the dividend
2. The verifier tracks the expected range of results based on correct mathematical computation
3. At runtime, the interpreter produces a different result due to the undefined behavior
4. This allows the attacker to access memory outside the bounds that the verifier believed were safe
5. The out-of-bounds access can be used for information disclosure or memory corruption

The vulnerability affects the s32 (signed 32-bit integer) case specifically. The s64 division and modulo handlers do not use the problematic abs() macro and are therefore not affected.

Detection Methods for CVE-2026-31525

Indicators of Compromise

• Unexpected BPF program behavior involving signed 32-bit division or modulo operations
• Anomalous memory access patterns from BPF programs, particularly accessing map values outside expected bounds
• Kernel log messages related to BPF verification failures that subsequently execute successfully
• Unusual BPF program loads that specifically use S32_MIN (0x80000000) as dividend values

Detection Strategies

• Monitor BPF program loading events and inspect bytecode for suspicious sdiv32/smod32 operations with hardcoded S32_MIN values
• Implement runtime BPF program auditing to detect arithmetic operations that may trigger the undefined behavior condition
• Deploy kernel integrity monitoring to detect unauthorized memory access from BPF contexts
• Use SentinelOne's behavioral AI to identify anomalous BPF program execution patterns

Monitoring Recommendations

• Enable BPF program audit logging to track all program loads and their bytecode
• Monitor for privilege escalation attempts that may follow successful exploitation of this vulnerability
• Implement kernel-level memory access monitoring for BPF map operations
• Review system logs for any signs of BPF-related crashes or unexpected behavior

How to Mitigate CVE-2026-31525

Immediate Actions Required

• Apply the kernel patches from the official Linux kernel git repository immediately
• If patching is not immediately possible, consider disabling unprivileged BPF (sysctl kernel.unprivileged_bpf_disabled=1)
• Audit existing BPF programs for suspicious patterns involving S32_MIN division operations
• Review and restrict BPF program loading permissions to trusted users only

Patch Information

The Linux kernel development team has released fixes through multiple commits addressing this vulnerability. The fix introduces a new abs_s32() function that correctly handles S32_MIN by casting to u32 before negating, avoiding signed overflow. Key patch commits include:

• Linux Kernel Commit 0d5d8c3
• Linux Kernel Commit 694ea55
• Linux Kernel Commit 9ab1227
• Linux Kernel Commit c77b30b
• Linux Kernel Commit f14ca60

System administrators should update to kernel versions containing these commits or apply the patches to their current kernel builds.

Workarounds

• Disable unprivileged BPF to prevent untrusted users from loading BPF programs that could exploit this vulnerability
• Restrict CAP_BPF capability to only essential system services and trusted administrators
• Implement additional access controls around BPF program loading using seccomp or AppArmor/SELinux policies
• Consider using BPF program signing where available to ensure only authorized programs are loaded

# Disable unprivileged BPF as a temporary workaround
echo 1 > /proc/sys/kernel/unprivileged_bpf_disabled

# Or persistently via sysctl configuration
echo "kernel.unprivileged_bpf_disabled = 1" >> /etc/sysctl.d/99-bpf-security.conf
sysctl -p /etc/sysctl.d/99-bpf-security.conf