Junglewise Threat Intelligence

CVE-2026-31555: Linux Kernel stale pointer in futex_lock_pi retry path

CVE-2026-31555 · Severity: medium · CVSS 5.5 · Published 2026-04-24

Technologies: Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP, Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP, Linux Kernel, Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP. Vendors: Siemens, Linux.

Executive brief

A vulnerability was identified in the Linux kernel's futex subsystem, which manages how different programs coordinate access to shared resources. Under specific conditions where a process is exiting while another tries to lock a resource, the system can encounter a memory error that causes a kernel warning or potential instability. This could be used by a local attacker to cause a system crash or denial-of-service, impacting the availability of the affected machine or industrial controller.

Technical details

A vulnerability exists in the Linux kernel futex subsystem due to a stale pointer in the futex_lock_pi() retry path. When futex_lock_pi_atomic() detects an exiting owner, it returns -EBUSY and stores a refcounted task pointer in 'exiting'. If wait_for_owner_exiting() consumes the reference but the operation retries and subsequently returns a different error (like -EAGAIN), the 'exiting' pointer remains populated with a stale value. This stale pointer is then passed back into wait_for_owner_exiting(), triggering a kernel warning (WARN_ON_ONCE) and potential memory corruption or instability. The fix involves explicitly resetting the 'exiting' pointer to NULL at the start of the retry loop.

Affected products

  • Linux Linux Kernel All versions prior to the fix (including 5.x, 6.x)
  • Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP V3.1.5, V3.1.6
  • Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP V3.1.5, V3.1.6
  • Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP V3.1.5, V3.1.6

Timeline

  • 2026-03-25: other: Patch authored by Davidlohr Bueso
  • 2026-04-24: disclosed: CVE published

References

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