CVE-2026-43109 Overview
CVE-2026-43109 is a Linux kernel vulnerability in the x86 shadow stacks subsystem. The flaw resides in shstk_pop_sigframe(), which fails to check the return value of mmap_read_lock_killable(). The kernel maintainers also identified that related locking functions were not annotated with __must_check, which would have caught the oversight at compile time. The patch addresses both the missing error handling in the signal frame pop path and the absent compiler annotation. Shadow stacks are a control-flow integrity feature on x86 designed to defeat return-oriented programming attacks, making correct error handling in this path important for kernel reliability.
Critical Impact
Improper error handling in the x86 shadow stack signal frame pop path can lead to undefined behavior when an mmap_read_lock_killable() call is interrupted by a fatal signal.
Affected Products
- Linux kernel versions containing the x86 shadow stack shstk_pop_sigframe() implementation prior to the fix
- x86_64 systems with User Shadow Stack (CET-SS) enabled
- Distributions shipping the affected upstream kernel commits
Discovery Timeline
- 2026-05-06 - CVE-2026-43109 published to NVD
- 2026-05-06 - Last updated in NVD database
Technical Details for CVE-2026-43109
Vulnerability Analysis
The vulnerability exists in the x86 shadow stack handling code within the Linux kernel. The function shstk_pop_sigframe() is invoked when restoring shadow stack state during signal frame processing. It calls mmap_read_lock_killable() to acquire the memory map read lock in a killable manner, allowing the lock acquisition to be interrupted by a fatal signal.
The original code did not validate the return value of mmap_read_lock_killable(). When the function returns a non-zero error indicating the wait was interrupted, the caller continued execution as if the lock had been acquired. This produces inconsistent locking state and incorrect signal frame restoration.
The maintainers also corrected a secondary defect: the lock helper functions lacked the __must_check attribute. Without this annotation, the compiler did not warn callers that ignored the return value, allowing the original oversight to go undetected.
Root Cause
The root cause is missing return value validation on a fallible locking primitive [CWE-252]. The _killable variant of an mmap lock acquisition is explicitly designed to fail when interrupted by a fatal signal, and callers must branch on this outcome.
Attack Vector
Triggering the condition requires a process running on a kernel with shadow stacks enabled to receive a fatal signal during signal frame processing while contending for the mmap read lock. The vulnerability is a local kernel reliability issue rather than a remote attack vector. No public exploit is available, and the EPSS data indicates an extremely low predicted exploitation probability.
No verified code examples are available for this CVE. Refer to the upstream kernel patches for technical details: Kernel Patch 262b6d3, Kernel Patch 52f657e, and Kernel Patch c64cebcc.
Detection Methods for CVE-2026-43109
Indicators of Compromise
- No specific indicators of compromise are published for this kernel reliability fix.
- Kernel oops or warning messages referencing shstk_pop_sigframe or shadow stack signal handling may indicate the unpatched code path was hit.
Detection Strategies
- Inventory running kernel versions across Linux fleets and compare against fixed commits 262b6d3, 52f657e, and c64cebcc.
- Audit x86_64 hosts where User Shadow Stack (CET-SS) is enabled, since only those systems exercise the vulnerable path.
- Monitor kernel logs (dmesg, journalctl -k) for stack trace entries involving shstk_pop_sigframe or mmap_read_lock_killable.
Monitoring Recommendations
- Centralize kernel log collection and alert on unexpected shadow stack faults or signal-handling errors.
- Track package update status for distribution kernel images using configuration management tooling.
- Correlate process crashes on shadow-stack-enabled binaries with kernel version data to identify exposure.
How to Mitigate CVE-2026-43109
Immediate Actions Required
- Apply the upstream kernel patches 262b6d3, 52f657e, and c64cebcc or upgrade to a distribution kernel that includes them.
- Identify systems running x86_64 kernels with shadow stacks enabled and prioritize them for patching.
- Validate kernel build processes treat __must_check warnings as errors to prevent similar oversights.
Patch Information
The fix is delivered through three upstream commits to the stable kernel tree. The patches add proper error handling to shstk_pop_sigframe() and add the __must_check attribute to the relevant mmap_read_lock_killable() family of functions. Reference Kernel Patch 262b6d3, Kernel Patch 52f657e, and Kernel Patch c64cebcc.
Workarounds
- Disable User Shadow Stack support at boot using kernel parameters that turn off CET features on affected systems if patching is not immediately possible.
- Avoid running workloads compiled with shadow stack support on unpatched x86_64 kernels in production.
- Restrict local user access on multi-tenant hosts until the fix is deployed.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
