CVE-2026-31482: Linux Kernel Privilege Escalation Flaw

CVE-2026-31482 Overview

A vulnerability has been identified in the Linux kernel affecting the s390 architecture entry handlers. The issue stems from an incomplete register clearing sequence following changes made in commit f33f2d4c7c80 ("s390/bp: remove TIF_ISOLATE_BP"). When the TIF_ISOLATE_BP feature was removed, the associated branch prediction macros (BPENTER/BPEXIT) and the r12 register load were dropped, but the r12 register was not added to the register clearing sequence upon kernel entry. This oversight results in potentially uncleared register data persisting across entry points, which could lead to information disclosure or other security implications.

Critical Impact

Incomplete register scrubbing on s390 kernel entry may allow sensitive data leakage through unsanitized register values across privilege boundaries.

Affected Products

• Linux Kernel (s390 architecture)
• Systems running s390/s390x processors with affected kernel versions

Discovery Timeline

• 2026-04-22 - CVE CVE-2026-31482 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2026-31482

Vulnerability Analysis

This vulnerability exists in the Linux kernel's s390 architecture entry handling code. Prior to commit f33f2d4c7c80, all entry handlers loaded the r12 register with the current task pointer using lg %r12,__LC_CURRENT for use by the BPENTER/BPEXIT macros. When the TIF_ISOLATE_BP branch prediction isolation feature was removed, the corresponding macros and r12 loading were eliminated, but the developers failed to add r12 to the existing register clearing (scrubbing) sequence.

The register scrubbing process is a critical security mechanism that ensures sensitive data from user space or previous contexts does not persist into the kernel context. By omitting r12 from this sequence, the register could potentially contain stale data that persists across entry points, creating inconsistent security boundaries.

Root Cause

The root cause is an incomplete code cleanup during the removal of the TIF_ISOLATE_BP feature. When commit f33f2d4c7c80 removed the branch prediction isolation code, it properly removed the r12 load instruction (lg %r12,__LC_CURRENT) and the associated BPENTER/BPEXIT macros. However, the corresponding addition of xgr %r12,%r12 (XOR to zero the register) to the register clearing sequence was missed, leaving r12 unscrubbed during kernel entry.

Attack Vector

The attack vector for this vulnerability is currently unknown. However, the theoretical exploitation scenario involves leveraging the unscrubbed r12 register to leak information across privilege boundaries. An attacker with local access could potentially:

1. Place sensitive data in the r12 register before triggering a kernel entry
2. Exploit the inconsistent register scrubbing to observe or influence kernel behavior
3. Potentially chain with other vulnerabilities for information disclosure

The fix involves adding the missing xgr %r12,%r12 instruction to zero the r12 register, ensuring consistent register scrubbing across all s390 entry points.

Detection Methods for CVE-2026-31482

Indicators of Compromise

• Unusual kernel behavior on s390/s390x systems following kernel entry operations
• Potential information leakage through register values in debugging or crash dumps
• Anomalous system calls or entry patterns that may indicate exploitation attempts

Detection Strategies

• Monitor s390 systems for kernel updates that include the register scrubbing fix
• Review kernel commit history on s390 systems for the presence of the fix commits
• Implement kernel integrity monitoring to detect unexpected entry handler modifications

Monitoring Recommendations

• Enable comprehensive logging on s390 systems to detect potential exploitation attempts
• Deploy endpoint detection solutions capable of monitoring kernel-level activities
• Monitor for any s390-specific security advisories from Linux kernel maintainers

How to Mitigate CVE-2026-31482

Immediate Actions Required

• Update the Linux kernel to a version containing the fix for CVE-2026-31482
• Prioritize patching on s390/s390x architecture systems
• Review system logs for any suspicious activity prior to patching
• Apply kernel updates during scheduled maintenance windows with proper testing

Patch Information

The vulnerability has been addressed through multiple kernel commits that add the missing xgr %r12,%r12 instruction to the register scrubbing sequence. The fix ensures consistent register clearing across all s390 entry points.

Official patches are available from the Linux kernel stable repositories:

• Kernel Git Commit 0738d39
• Kernel Git Commit 7f4e323
• Kernel Git Commit 95c899c
• Kernel Git Commit 99a8b42
• Kernel Git Commit a58d298

Workarounds

• If immediate patching is not possible, consider restricting access to s390 systems to trusted users
• Implement additional monitoring on affected systems until patches can be applied
• Evaluate the use of live patching mechanisms if available for your kernel version

The recommended approach is to apply the official kernel patches as soon as possible, as no direct workaround can fully address the missing register scrubbing functionality.