CVE-2026-98981 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
namespace: unhash a dentry before detaching the mounts on it
unlink(), rmdir() and rename() remove the entry from the filesystem, call detach_mounts() on the entry with the inode locked and then call d_delete() once the inode is unlocked.
The thing is that between detach_mounts() and d_delete() the dentry is still hashed and positive. But after detach_mounts() nothing covers the dentry anymore. A lookup that finds this dentry in the dcache can uncover what the mounts hid.
That's a problem when unlinking files or directories that are mountpoints in other mount namespaces. Everybody who had the underlying entry covered can race the detach_mount() call until d_delete() has run.
The race window isn't all that small. It encompasses namespace_unlock() with a full synchronize_rcu_expedited() grace period and the inode_unlock() of the entry.
In my experiments three walkers that kept trying read an overmounted file in 11938 of the 16140 unlinks that removed its mountpoint from a bind mount of the filesystem within a minute.
Fun fact, d_invalidate() has the same ordering problem but gets it right. It unhashes the dentry first and detaches the mounts afterwards. Let's do the same in __detach_mounts():
- A lookup that hasn't found the dentry yet misses it in the dcache and waits for the directory lock that the caller holds until the name is gone for good.
- A lockless lookup that found it already fails the mount_lock check while the dentry still counts as a mountpoint and the d_seq check once it doesn't, and retries.
This fixes the lockless path. We still need to fix the reference count lookup in a follow-up patch.
So d_drop() the dentry. The dentry stays positive and held but can't be found anymore. Then proceed with the detach and unlink.
Reproducer:
The reads were counted with the program below, three walkers for 60 s in a VM with 4 CPUs. The race window is stretched with the debug patch pasted here. fs.detach_race_walk_us sleeps in lookup_fast() once it found a mountpoint dentry and before its mounts are crossed. fs.detach_race_unlink_us sleeps in vfs_unlink() between detach_mounts() and d_delete().
// SPDX-License-Identifier: GPL-2.0 /* * unlink_covered: unlink a file that is a mountpoint in a detached copy of * its mount, against lookups that open it through that copy. * * A is a tmpfs with the file f1 (content SECRET_F) and the plain file * MARK_A. A' is an open_tree(OPEN_TREE_CLONE) copy of A with MARK_A bound * on A'/f1, held through an O_PATH fd on its root once the tree fd is * closed. Walkers open f1 through that fd while the driver unlinks A/f1, * where nothing is mounted on it. A walker may read MARK_A or get ENOENT. * A read of SECRET_F is a hit: the name was found after its mount was * gone. The lockless walks use openat2(RESOLVE_CACHED). * * usage: unlink_covered [-t seconds] [-w walkers]
*/ #ifndef _GNU_SOURCE #define _GNU_SOURCE #endif #include <errno.h> #include <fcntl.h> #include <pthread.h> #include <stdatomic.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/mount.h> #include <sys/stat.h> #include <sys/syscall.h> #include <linux/openat2.h>
#ifndef __NR_open_tree #define __NR_open_tree 428 #endif #ifndef __NR_move_mount #define __NR_move_mount 429 #endif #ifndef __NR_openat2 #define __NR_openat2 437 #endif #ifndef OPEN_TREE_CLONE #define OPEN_TREE_CLONE 1 #endif #ifndef OPEN_TREE_CLOEXEC #define OPEN_TREE_CLOEXEC O_CLOEXEC #endif #ifndef MOVE_MOUNT_F_EMPTY_PATH #define MOVE_MOUNT_F_EMPTY_PATH 0x00000004 #endif
#define WORK "/tmp/uc" #define ADIR WORK "/A"
static int duration = 60, nwalkers = 3; static atomic_int stop, writer_waiting; static pthread_rwlock_t cur_lock = PTHREAD_RWLOCK_INITIALIZER; static int cur_fd = -1; /* the root of A', -1 wh ---truncated---
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel contained a race condition vulnerability within the Virtual File System (VFS) layer, specifically affecting operations that remove directory entries such as unlink, rmdir, and rename. The core technical flaw lies in the ordering of operations when detaching mounts from a dentry. During these removal processes, the kernel calls detach_mounts while holding the inode lock, followed by calling d_delete after unlocking the inode. However, between the completion of mount detachment and the final deletion of the dentry reference, the dentry remains hashed and positive within the directory cache. This creates a critical window where the dentry is no longer protected by its associated mounts but is still visible to lookups. Consequently, concurrent lookup operations can discover this unmasked dentry before it is fully removed from the namespace hierarchy, allowing access to underlying filesystem entries that were previously hidden behind mount points.
This vulnerability poses significant risks in environments utilizing multiple mount namespaces or bind mounts. An attacker with sufficient privileges could exploit this race condition by performing rapid unlink operations on files or directories that serve as mountpoints for other mounted file systems within different namespaces. The operational impact involves the potential exposure of sensitive data, such as secret files located beneath a bound mount point, to processes that should not have access. In practical scenarios, multiple concurrent walkers attempting to read overmounted files can successfully bypass security boundaries in a substantial percentage of attempts due to this timing window. The race is particularly dangerous because it affects both locked and lockless lookup paths, although the immediate fix primarily addresses the lockless path where lookups might fail mount checks but still retrieve stale or incorrect data before sequence numbers are updated correctly.
The vulnerability aligns with CWE-362, Concurrent Execution using Shared Resource with Improper Synchronization of Reference Counters, as it involves a race condition in managing shared kernel resources (dentries and mounts). It also relates to CWE-829, Inclusion of Functionality from Untrusted Control Sphere, because the improper ordering allows external or unprivileged contexts to access internal filesystem structures they should not. From an ATT&CK perspective, this could be leveraged for Defense Evasion by bypassing file system permissions and isolation mechanisms provided by mount namespaces, potentially leading to Information Discovery if sensitive files are exposed. The fix involves reordering the operations within _detach_mounts_ to unhash the dentry using d_drop before detaching the mounts. This ensures that once the detachment process begins, no new lookups can find the dentry in the cache, thereby eliminating the window where a lookup could uncover hidden underlying entries.
Mitigation strategies require applying kernel patches that enforce this corrected ordering of operations during mount detachment and directory entry removal. System administrators should ensure their Linux kernels are updated to versions containing these VFS fixes. For environments running older or unpatched kernels, restricting access to namespace manipulation syscalls such as open_tree, move_mount, and unlink can reduce the attack surface. Additionally, monitoring for unusual patterns of rapid file deletions combined with high-frequency lookups in sensitive directories may help detect exploitation attempts. The fix also highlights the importance of consistent ordering in kernel synchronization primitives, noting that while d_invalidate handled this correctly by unhashing first, other paths like __detach_mounts did not until this correction was applied. Future updates must continue to address reference counting issues in lookup operations to fully close all potential race conditions associated with mount namespace isolation.