CVE-2026-98994 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
statmount: read the parent of an unmounted mount under mount_lock
statmount_mnt_basic() reads the ids of the parent through m->mnt_parent with namespace_sem held for reading. That's fine for a mount that is in a namespace. Everything above it is pinned by that namespace.
STATMOUNT_BY_FD also accepts a file descriptor on a mount that has been unmounted on purpose and that only the file descriptor keeps alive. If that mount stayed attached to its parent when it was unmounted the parent is unmounted as well and nothing pins it for the child.
The final mntput() unmounts the children under mount_lock alone and frees the parent afterwards. Concurrently, statmount_mnt_basic() reads m->mnt_parent twice while that can happen and can easily dereference a mount that's already gone.
Read the parent of an unmounted mount under mount_lock. The parent can't be put while it is held and both ids come from the same mount. A mounted mount's parent is pinned by the namespace and a mount can't leave its namespace while namespace_sem is held, so the common case takes no global lock.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel vulnerability identified in the statmount subsystem represents a classic race condition arising from improper synchronization of shared data structures during unmount operations. The core issue resides within the statmount_mnt_basic function, which is responsible for retrieving basic mount information such as parent identifiers. Under normal circumstances when dealing with mounts that remain attached to their respective namespaces, this operation is safe because it acquires namespace_sem in read mode. This semaphore ensures that all hierarchical structures above the target mount are pinned and cannot be freed while the lock is held. However, the vulnerability emerges specifically when handling file descriptors associated with unmounted mounts. In Linux, a file descriptor can maintain a reference to a mount point even after the mount has been explicitly removed from the namespace tree, keeping the structure alive solely through that fd reference rather than its position in the hierarchy.
The technical flaw occurs because statmount_mnt_basic attempts to read the parent pointer via m->mnt_parent while only holding namespace_sem for reading. When a mount is unmounted but kept alive by an open file descriptor, it may no longer be pinned by any active namespace structure above it. The kernel's finalization process involves calling mntput under the protection of mount_lock alone to unmount child mounts and subsequently free the parent mount structures. This creates a critical window where concurrent execution can lead to data corruption or crashes. Specifically, statmount_mnt_basic reads the m->mnt_parent field twice during its operation. If an unmount event occurs between these two read operations, particularly when triggered by the finalization of an unmounted mount kept alive only by a file descriptor, the parent structure may be freed and reclaimed before the second read completes. This results in dereferencing memory that has already been released back to the kernel allocator, leading to use-after-free conditions or null pointer dereferences depending on how quickly the memory is reused.
From an operational impact perspective, this vulnerability allows for potential denial of service through system crashes if triggered by a local user with appropriate file descriptor access. An attacker could open a mount point, unmount it while keeping the fd open, and then repeatedly invoke statmount to trigger the race condition. The resulting kernel panic would disrupt all services on the affected host. While remote exploitation is unlikely given that this requires specific filesystem manipulation capabilities typically restricted to privileged users or container environments, the severity remains high due to the instability introduced in core kernel memory management routines. This type of vulnerability aligns with CWE-362, which describes concurrent execution race conditions leading to resource access errors, and specifically relates to improper locking mechanisms where shared resources are accessed without adequate synchronization primitives protecting against modification by other threads or interrupt contexts.
The mitigation implemented addresses this issue by ensuring that the parent mount pointer is read under the protection of mount_lock rather than relying solely on namespace_sem for unmounted mounts kept alive by file descriptors. By holding mount_lock during the retrieval of both the parent identifier and related data, the kernel guarantees that no concurrent mntput operation can free the parent structure while it is being accessed. This approach maintains performance for the common case where mounted mounts are pinned by namespaces, avoiding unnecessary global locking overhead, but provides robust protection for edge cases involving detached yet referenced mount structures. Security practitioners should ensure systems are updated with patches addressing this specific race condition in the statmount implementation to prevent potential kernel instability and maintain system integrity against local privilege escalation attempts that might leverage such memory corruption vulnerabilities.