CVE-2026-99084 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
readdir: take no inode lock on an immutable directory
iterate_dir() takes the directory's i_rwsem shared and holds it across ->iterate_shared(). For a directory that never has an entry and is never removed the lock keeps nothing still, it only orders every reader and every writer of that inode behind each other.
For the directory of a nullfs instance that matters. The instance of the initial mount namespace is the root of every empty mount namespace and the private instance is the root of every kernel thread, so one inode is shared across users who have nothing else in common. And a reader can hold the lock for as long as it likes: back the getdents() buffer with a mapping of a file on a FUSE mount of your own, let the copy of "." and ".." fault and let the server wait. Queue an exclusive taker behind it, a mkdir() in that directory goes through start_dirop() before the read-only mount is reported, and from then on every lookup that misses the dcache in that directory, every create and every mount on it waits until the server answers. One user of an empty mount namespace stalls all the others.
Add FOP_IMMUTABLE for the file operations of a directory that never changes and is never removed and let iterate_dir() skip the lock for it. The flag never changes for a file, ->f_pos is protected by f_pos_lock since directories are FMODE_ATOMIC_POS, IS_DEADDIR can't be set on such a directory and neither touch_atime() nor fsnotify take i_rwsem. Set it on the nullfs directory. The placeholder directories of libfs never have an entry either but their owners remove them, so they keep the lock.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel vulnerability addressed in this update pertains to a performance and availability issue within the virtual file system layer, specifically affecting how inode locks are managed during directory iteration operations. The core technical flaw lies in the behavior of the iterate_dir function, which acquires the i_rwsem lock on a shared basis for directories and maintains it throughout the execution of the ->iterate_shared callback. While this locking mechanism is necessary to prevent race conditions when directory entries are being added or removed, it becomes counterproductive for specific types of static directories that never change their content structure. In such cases, holding the exclusive read-write semaphore serves no protective purpose regarding data integrity but instead imposes significant serialization overhead on all concurrent access attempts.
This issue has critical operational implications for systems utilizing nullfs mounts, particularly those involving empty mount namespaces and kernel threads. The initial mount namespace root is shared across every empty mount namespace, while a private instance of this directory structure serves as the root for every kernel thread. Consequently, a single inode is accessed by numerous unrelated users who share no other common context. Under normal circumstances, if one user performs a readdir operation on such an immutable directory and triggers page faults that require interaction with a remote FUSE server, they can hold the i_rwsem lock indefinitely while waiting for the external response. This creates a severe availability bottleneck because any subsequent writer attempting to create directories or mount filesystems within this namespace must wait in queue behind the stalled reader. Furthermore, every lookup operation that misses the dcache and every new creation request is blocked until the original read operation completes, effectively allowing one user to stall all other users of the same empty mount namespace.
The resolution involves introducing a new file operation flag designated as FOP_IMMUTABLE for directory operations where the content never changes and entries are never removed. By setting this flag on nullfs directories, the kernel is instructed to skip acquiring the i_rwsem lock during iteration via iterate_dir. This optimization relies on several technical guarantees that ensure safety without locking: the file position pointer f_pos is already protected by a separate atomic lock because these files operate with FMODE_ATOMIC_POS flags; such directories cannot be marked as dead through IS_DEADDIR checks since they are static; and standard operations like touch_atime or fsnotify do not require holding the i_rwsem semaphore. It is important to note that this optimization does not apply to placeholder directories in libfs, which may have entries removed by their owners, thus requiring them to retain the locking mechanism for consistency.
From a security perspective, while this vulnerability primarily manifests as a denial-of-service condition due to resource contention rather than a direct privilege escalation or data leak, it aligns with CWE-400: Uncontrolled Resource Consumption and CWE-613: Insufficient Session Expiration or Timeout in the context of lock holding duration. The attack vector allows an unprivileged user to degrade system performance for other users sharing the same mount namespace by exploiting the lack of timeout mechanisms on directory locks during I/O waits. To mitigate this, administrators should ensure their systems are updated with the kernel patch that implements FOP_IMMUTABLE for nullfs directories. Additionally, monitoring tools can be configured to detect excessive wait times on inode locks in environments heavily utilizing containerized or namespace-isolated workloads, allowing for early detection of such contention issues before they impact broader system stability.