CVE-2026-98648: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-98648 Overview

CVE-2026-98648 is an out-of-bounds read vulnerability in the Linux kernel's s390 Ultravisor (UV) subsystem. The flaw resides in the find_secret_in_page function within the secret management code for IBM Z architecture. When the system stores more than 85 secrets, the uv_secret_list struct array only holds up to 85 items per page. The function iterates based on the overall secret count rather than the per sub-list count, causing reads past the page boundary when the target secret resides on a subsequent page or is not stored at all. The issue has been resolved upstream in the Linux kernel source tree.

Critical Impact
An out-of-bounds read in kernel space on s390 Ultravisor-enabled systems can leak adjacent memory contents or trigger kernel instability when more than 85 secrets are provisioned.

Affected Products

Linux kernel on IBM Z (s390) architecture with Ultravisor support
Secure Execution guests leveraging the uv_secret_list interface
Kernel builds prior to the commits referenced in the stable tree

Discovery Timeline

2026-10-18 - CVE-2026-98648 published to NVD
2026-10-19 - Last updated in NVD database

Technical Details for CVE-2026-98648

Vulnerability Analysis

The s390 Ultravisor (UV) subsystem manages secrets for Secure Execution guests on IBM Z hardware. Secrets are stored in a paged list structure where each page accommodates up to 85 uv_secret_list entries. The find_secret_in_page helper walks a single page to locate a requested secret.

The defect occurs because the loop in find_secret_in_page uses the overall count of stored secrets instead of the number of entries actually present in the current sub-list. When the system holds more than 85 secrets, the function reads memory beyond the 85-entry page capacity. The read can traverse unrelated kernel memory, producing undefined behavior or information disclosure. The condition also triggers when the targeted secret is not stored at all, because the loop continues past valid entries.

Root Cause

The root cause is incorrect loop boundary accounting [CWE-125]. The iteration variable is compared against the aggregate secrets count rather than the per-page count. The fix changes the loop to iterate over the per sub-list count of stored secrets, aligning the walk with the actual page capacity.

Attack Vector

Triggering the condition requires a configuration where more than 85 secrets exist, or where a lookup is performed for a secret that is not provisioned. The attack surface is local to the s390 architecture and bounded by the kernel interfaces exposed to callers of the UV secret lookup. No remote network vector is involved. See the stable tree commits 04bb5b2e, 116bba75, and f47190b0 for implementation details.

Detection Methods for CVE-2026-98648

Indicators of Compromise

Kernel log entries referencing find_secret_in_page with KASAN or slab debug warnings on s390 systems
Unexpected kernel oops or panic traces in the arch/s390/kernel/uv.c call path
Secret lookup failures correlated with systems provisioned with more than 85 secrets

Detection Strategies

Enable CONFIG_KASAN on test kernels to surface out-of-bounds reads in the UV secret path
Inventory s390 Secure Execution guests and record kernel versions against the stable tree commits that carry the fix
Audit the number of secrets provisioned per guest and flag any guest exceeding the 85-entry page capacity on unpatched kernels

Monitoring Recommendations

Forward kernel ring buffer messages from s390 hosts to a central log aggregator and alert on UV subsystem faults
Track package versions and kernel build hashes across the IBM Z fleet to confirm patch coverage
Monitor for abnormal process termination or guest reboots correlated with secret retrieval operations

How to Mitigate CVE-2026-98648

Immediate Actions Required

Apply the upstream Linux kernel stable patches referenced in the NVD entry for CVE-2026-98648
Rebuild and redeploy affected s390 kernels across Secure Execution hosts and guests
Limit secret provisioning to fewer than 86 entries on unpatched systems to avoid crossing page boundaries

Patch Information

The fix is available in the Linux stable tree. Review the commits 04bb5b2eb9f1, 116bba758533, and f47190b08b71 and backport them into vendor kernels where upstream builds are not consumed directly. Distribution maintainers for s390 kernels should publish updated packages tracking these commits.

Workarounds

Keep the total number of UV secrets at or below 85 to confine lookups to a single page
Restrict access to kernel interfaces that trigger find_secret_in_page to trusted administrators until patching is complete
Enforce strict change control on secret provisioning workflows on IBM Z until the patched kernel is deployed

bash

# Verify the running s390 kernel version and compare against patched builds
uname -r
# List installed kernel packages for audit purposes
rpm -qa | grep kernel     # RHEL-based distributions
dpkg -l | grep linux-image # Debian-based distributions