CVE-2026-99040 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
KEYS: Fix add_key() race with keyring restriction
__key_create_or_update() snapshots keyring->restrict_link before taking the destination keyring's semaphore. keyring_restrict() installs a restriction while holding that semaphore.
This allows a writer to observe no restriction, wait for the keyring owner to install a reject-all restriction and return successfully, and then link a key using the stale NULL snapshot. The writer only needs write permission on the destination keyring.
Move the restrict_link read after __key_link_lock() and __key_link_begin(). The read and the subsequent restriction check are then serialized with restriction installation by keyring->sem.
The race was reproduced on v7.2.8 in 19 executions where restriction installation returned before the link completed. All 19 linked the key despite the reject-all restriction. With this change, 312 executions reached the same ordering and every add_key() call failed with -EPERM.
The issue was found by manual concurrency analysis assisted by AI-based analysis and independently verified with a QEMU reproducer and kernel instrumentation.
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Analysis
by VulDB Data Team • 10/24/2026
A critical race condition has been identified within the Linux kernel's key management subsystem, specifically affecting the add_key() system call when interacting with restricted keyrings. This vulnerability arises from an improper synchronization mechanism during the creation or update of keys linked to a destination keyring that enforces restrictions on which types of keys can be added. The core technical flaw lies in how __key_create_or_update() handles the evaluation of keyring_restrict_link(). In the vulnerable implementation, this function snapshots the value of keyring->restrict_link before acquiring the semaphore associated with the destination keyring. This ordering creates a window where a writer process holding write permissions on the destination keyring can observe that no restriction is currently active or that the existing restrictions are permissive. The attacker then waits for the legitimate owner of the keyring to install a restrictive policy, such as a reject-all rule, which occurs while holding the same semaphore. Because the initial snapshot was taken prior to acquiring this lock, the writer proceeds with stale data indicating no restriction exists. Consequently, when the write operation completes and links the key, it bypasses the newly installed security controls, allowing unauthorized keys to be added despite explicit restrictions designed to prevent such actions.
The operational impact of this vulnerability is significant for systems relying on kernel-level access control mechanisms provided by Linux Keyrings. These structures are frequently used in conjunction with secure boot processes, container runtimes like Docker or Kubernetes, and other security frameworks that depend on strict isolation between different trust levels. By exploiting this race condition, a local attacker can bypass keyring restrictions to inject malicious keys into the kernel's credential store. This could potentially lead to privilege escalation if those keys are used for authentication or encryption operations controlled by higher-privilege processes. The vulnerability was reproduced with high reliability on Linux version 7.2.8, where testing demonstrated that in nineteen consecutive executions involving specific timing conditions, all attempts successfully linked the key despite the presence of a reject-all restriction. This consistency highlights the deterministic nature of the race condition under certain scheduling scenarios, making it a reliable vector for exploitation by any user with write access to the target keyring.
The resolution involves correcting the synchronization order within __key_create_or_update(). The fix moves the read operation of restrict_link to occur after acquiring both __key_link_lock() and __key_link_begin(). This adjustment ensures that the evaluation of restrictions is serialized with the installation of new restrictions via keyring->sem, thereby eliminating the window where stale data could be used. Testing following this patch confirmed its effectiveness; in three hundred twelve executions reaching the same race condition ordering as before, every add_key() call correctly failed with an -EPERM error code, demonstrating that the restriction enforcement is now robust against timing attacks. The issue was discovered through manual concurrency analysis augmented by AI-based static analysis tools and independently verified using a QEMU emulator reproducer combined with kernel instrumentation to capture precise execution states during the race window.
From a classification perspective, this vulnerability aligns closely with CWE-362, which describes concurrent execution error or race conditions where shared resources are accessed without proper synchronization. It also relates to CWE-841, improper enforcement of behavioral workflow, as the system fails to enforce the intended sequence of security checks before allowing an action that violates policy. In terms of adversarial tactics, this exploit technique maps to ATT&CK T1059, Command and Scripting Interpreter, specifically in how local scripts or programs manipulate kernel interfaces for privilege escalation, and potentially T1499, Endpoint Denial of Service if the goal is to disrupt secure boot processes by corrupting keyring states. Mitigation strategies primarily involve applying the upstream Linux kernel patch that corrects the locking order. For environments where immediate patching is not feasible, administrators should restrict write permissions on sensitive keyrings to only trusted root-level services and monitor for unusual patterns of add_key() system calls originating from unprivileged user spaces. Regular auditing of kernel updates and adherence to strict least-privilege principles regarding credential management interfaces are essential defensive measures against this class of concurrency-based vulnerabilities.