CVE-2026-68179 in Linux
Summary
by MITRE • 08/10/2026
In the Linux kernel, the following vulnerability has been resolved:
misc: nsm: only unlock nsm_dev on post-lock error paths
nsm_dev_ioctl() jumps to the common out label even when the initial copy_from_user() fails before nsm->lock has been taken. The error path then blindly unlocks a mutex that was never acquired.
This issue was found by our static analysis tool and then manually reviewed against the current tree.
The grounded PoC kept the miscdevice ioctl entry and the pre-lock copy_from_user(&raw, argp, _IOC_SIZE(cmd)) failure path by issuing NSM_IOCTL_RAW with an invalid user pointer. That failure reaches the shared out label before mutex_lock(&nsm->lock). Lockdep reported:
WARNING: bad unlock balance detected! exploit/193 is trying to release lock (&global_nsm.lock) at: nsm_dev_ioctl+0x5f/0xcf [vuln_msv]
but there are no more locks to release! no locks held by exploit/193.
Return immediately on the pre-lock copy_from_user() failure and keep the common unlock label for the post-lock paths only.
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Analysis
by VulDB Data Team • 08/10/2026
This vulnerability resides in the Linux kernel's network service management subsystem, specifically within the nsm_dev_ioctl function that handles miscellaneous device operations. The issue represents a classic mutex imbalance problem where the kernel's locking mechanism fails to properly track acquisition and release operations. When the initial copy_from_user() operation fails before acquiring the nsm->lock mutex, the code path incorrectly attempts to unlock a mutex that was never actually locked, creating a dangerous state in the kernel's locking subsystem.
The technical flaw manifests as improper error handling logic within the ioctl handler where the function transitions to a common cleanup label without first verifying whether the lock was successfully acquired. This static analysis identified by the security team reveals a fundamental race condition and resource management issue in the device driver's control flow. The vulnerability occurs because the error path assumes that a mutex lock was obtained when it actually wasn't, leading to kernel lockdep subsystem warnings and potential system instability.
The operational impact of this vulnerability extends beyond simple resource management issues to potentially compromise the integrity of the kernel's locking infrastructure. When the copy_from_user() operation fails with an invalid user pointer, typically through the NSM_IOCTL_RAW command with a malformed argument, the code follows an incorrect execution path that attempts to release an unlocked mutex. This can trigger kernel lockdep warnings as demonstrated by the specific output showing "bad unlock balance detected" where the kernel thread tries to release a lock that was never acquired. Such behavior creates conditions that may lead to more severe system instability or potential privilege escalation scenarios.
The fix implemented addresses this through proper conditional logic that returns immediately upon pre-lock copy_from_user() failure, ensuring that only post-lock error paths execute the mutex unlock operation. This approach follows secure coding practices by maintaining strict lock acquisition and release semantics throughout all code execution paths. The solution aligns with CWE-1217 which addresses improper locking mechanisms and represents a fundamental defensive programming principle against mutex imbalance conditions. This vulnerability demonstrates how seemingly minor control flow issues in kernel drivers can lead to serious security implications, particularly when dealing with device ioctls that handle user-space memory operations.
The referenced ATT&CK technique T1068 covers privilege escalation through local attack vectors, where such kernel-level mutex errors could potentially be exploited by malicious actors to gain elevated privileges. The static analysis approach used to identify this vulnerability reflects modern security practices for kernel code review, emphasizing automated tools combined with manual verification. This particular issue highlights the importance of proper error path management in kernel drivers and demonstrates how improper handling of memory copy operations can lead to fundamental system integrity issues that affect the entire kernel's locking subsystem.
The mitigation strategy ensures that all execution paths maintain proper lock state tracking by separating pre-lock and post-lock error handling, preventing the unlock operation from executing on mutexes that were never acquired. This fix maintains the existing functionality while eliminating the dangerous race condition that could potentially be exploited to compromise system stability or security. The solution represents a standard defensive programming pattern that should be applied to all kernel driver ioctl handlers where mutex locking is involved, particularly those handling user-space memory operations that could fail before lock acquisition occurs.