CVE-2026-45876: Linux Kernel GCS Error Handling Flaw

CVE-2026-45876 Overview

CVE-2026-45876 is a Linux kernel vulnerability affecting the arm64 Guarded Control Stack (GCS) subsystem. The flaw resides in arch_set_shadow_stack_status(), where error handling for alloc_gcs() fails to detect failures from do_mmap(). The function returns an error-encoded pointer on failure, not NULL, so the existing NULL check is ineffective. This can result in the kernel using an invalid GCS address derived from an error-encoded pointer value.

Critical Impact
Improper error handling in the arm64 shadow stack code path can cause the kernel to operate on an invalid GCS address, leading to undefined behavior or memory corruption on affected arm64 systems.

Affected Products

Linux kernel (arm64 architecture builds with Guarded Control Stack support)
Distributions shipping affected upstream Linux kernel versions
Systems leveraging arm64 GCS shadow stack functionality

Discovery Timeline

2026-05-27 - CVE-2026-45876 published to NVD
2026-05-27 - Last updated in NVD database

Technical Details for CVE-2026-45876

Vulnerability Analysis

The vulnerability is a [Null Pointer Dereference] class issue rooted in incorrect error-pointer handling within the arm64 Guarded Control Stack code. The function arch_set_shadow_stack_status() calls alloc_gcs() to allocate a shadow stack region. Internally, alloc_gcs() relies on do_mmap(), which signals failure by returning an error-encoded pointer using the ERR_PTR() convention rather than NULL.

The existing check in arch_set_shadow_stack_status() only tests for NULL. Error-encoded pointers bypass this check and are treated as a valid GCS base address. Subsequent code paths then operate on a pointer value that encodes a negative errno, producing undefined kernel behavior.

The upstream fix replaces the NULL test with IS_ERR_VALUE(), aligning the validation with the same pattern used in gcs_alloc_thread_stack().

Root Cause

The root cause is an incorrect failure check on the return value of alloc_gcs(). The function returns an ERR_PTR()-encoded value on failure, but the caller treats only NULL as an error. This mismatch in error semantics allows error-encoded pointers to flow into code that assumes a valid mapped address.

Attack Vector

The attack vector requires a local arm64 environment where shadow stack status can be configured through the GCS interface. An attacker would need to induce failure conditions in do_mmap() during shadow stack allocation to reach the unchecked error pointer path. No public exploit is referenced for this CVE, and the EPSS score reflects very low predicted exploitation activity.

The vulnerability is described in prose because no verified proof-of-concept code is available. Refer to the upstream commits for the exact source-level change: Linux Kernel Commit 53c9985, Linux Kernel Commit a474111, and Linux Kernel Commit c787a23.

Detection Methods for CVE-2026-45876

Indicators of Compromise

No file-based or network IOCs are associated with this kernel error-handling fix.
Unexpected kernel oops, panic, or invalid pointer dereference messages referencing arm64 GCS or arch_set_shadow_stack_status in dmesg may indicate triggering of the flaw.
Processes failing prctl() operations related to shadow stack status on arm64 hosts under memory pressure.

Detection Strategies

Inventory arm64 hosts and compare running kernel versions against the patched commits listed in upstream stable trees.
Monitor kernel logs for crash signatures involving GCS allocation paths, do_mmap failures, or shadow stack syscalls.
Use configuration management tooling to flag systems with vulnerable kernels that also expose GCS to userspace.

Monitoring Recommendations

Centralize dmesg and kmsg collection from arm64 fleets to surface kernel anomalies tied to shadow stack initialization.
Track prctl(PR_SET_SHADOW_STACK_STATUS) usage patterns and associated error returns from monitored workloads.
Alert on kernel panics or unexpected reboots on arm64 systems running unpatched kernel builds.

How to Mitigate CVE-2026-45876

Immediate Actions Required

Apply the upstream Linux stable kernel updates that include the IS_ERR_VALUE() fix in arch_set_shadow_stack_status().
Identify arm64 systems with GCS enabled and prioritize them for patch deployment.
Validate vendor kernel packages (distribution kernels) for backported fixes before scheduling reboots.

Patch Information

The fix replaces the incorrect NULL check with IS_ERR_VALUE() to correctly detect error-encoded pointers returned by alloc_gcs(). Patched commits are available upstream: 53c998527ffa, a4741114c962, and c787a235deb3. Pull the corresponding stable kernel release or your distribution's backport.

Workarounds

Where patching is delayed, disable Guarded Control Stack support on arm64 builds via kernel configuration if operationally acceptable.
Restrict untrusted local code execution on arm64 hosts to reduce the chance of reaching the vulnerable allocation path.
Apply standard kernel hardening (KASLR, lockdown, restricted prctl surfaces through seccomp filters) to limit exposure.

bash

# Verify the running kernel version on arm64 hosts
uname -r

# Check whether GCS-related symbols are present in the running kernel
grep -i gcs /proc/config.gz 2>/dev/null | zcat 2>/dev/null || \
  grep -i gcs /boot/config-$(uname -r)

# Schedule a patched kernel install (Debian/Ubuntu example)
sudo apt-get update && sudo apt-get install --only-upgrade linux-image-arm64
sudo reboot