CVE-2026-98980 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
namei: don't reveal overmounted entries in refwalk
In rcuwalk the dentry is validated before it is accepted. step_into() step_into() rechecks d_seq and __follow_mount_rcu() rechecks mount_lock. An entry that gets unlinked in between causes the lookup to retry and miss.
A refwalk doesn't do this. lookup_fast() takes a reference on the hashed dentry and simply accepts it. So an unlink that happens after the reference was taken isn't seen by refwalk. That's fine for a simple file. We just happened to open it before it was unlinked, no problem.
For a mountpoint and specifically a locked mountpoint it very much isn't. The unlink detaches all mounts and then removes the name. Any refwalk that hasn't traversed the mounts yet simply reveals the underlying entry. It's a very narrow window but it can be hit:
reads of a covered file in 60 s, 3 walkers, ~15000 unlinks no widening 27 that step + 200 us 3741
See the appended patch for a more reliable reproducer.
So check the entry after step_into(). unlink(), rmdir() and rename() mark the dentry with dont_mount() before they detach the mounts and remove the dentry.
So a dentry that is marked with DCACHE_CANT_MOUNT and is unhashed by the time its mounts were looked at is a name that was unlinked under the refwalk. DCACHE_CANT_MOUNT is read after the mounts were looked up and it is set before they are detached by detach_mounts(). A refwalk that missed the mounts will see DCACHE_CANT_MOUNT.
Plain d_unlinked() is fine. A rename takes the dentry off its hash chain but ___d_drop() leaves d_hash.pprev set. So only __d_drop() and a rename over the dentry unhash it. The only move that flips IS_ROOT splices in a disconnected alias. That doesn't have DCACHE_CANT_MOUNT set.
Add the to step_into_slowpath(). ".." and LOOKUP_DOWN may legitimately land on an unhashed directory. A refwalk that crossed onto a mount has path.mnt different from nd->path.mnt. A dentry that a filesystem dropped on its own (d_invalidate(), d_drop()) doesn't have the flag set and is treated as before.
// SPDX-License-Identifier: GPL-2.0 /* * unlink_covered: unlink a file that is a mountpoint in a detached copy of * its mount, against walkers 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 while there is none */ static atomic_long n_unlink, n_walk, n_mark, n_enoent, n_othe ---truncated---
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Analysis
by VulDB Data Team • 10/24/2026
The Linux kernel contains a race condition vulnerability within its directory entry lookup mechanism that allows an attacker to access files or directories that have been unlinked but not yet fully removed from the filesystem namespace. This flaw specifically affects reference-based walks (refwalks) used in path resolution, where dentries are validated by taking references rather than relying solely on sequence locks. In a refwalk scenario, if a file is deleted and subsequently becomes a mount point for another volume while a lookup operation is in progress, the kernel may fail to detect that the original entry has been detached from its mounts. This results in the exposure of underlying entries that should have been inaccessible due to the unlinking process, effectively bypassing standard access controls associated with file deletion.
The technical root cause lies in the disparity between how rcuwalk and refwalk handle dentry validation during path traversal. In an rcuwalk context, the kernel rechecks sequence numbers and mount locks before accepting a dentry, ensuring that any unlinked entries are detected and the lookup is retried or aborted appropriately. However, refwalks take a reference to a hashed dentry early in the process and accept it without subsequent verification of its mounting status. When an unlink operation occurs on a file that serves as a mountpoint, the kernel marks the dentry with DCACHE_CANT_MOUNT before detaching all associated mounts and removing the name from the hash chain. If a refwalk has already taken a reference to this dentry but has not yet traversed its potential child mounts, it will miss the detachment event. Consequently, the lookup continues as if the file still exists in its original location, revealing data that was intended to be hidden or removed by the unlink operation.
This vulnerability presents significant operational impacts regarding information disclosure and integrity violations. An attacker who can trigger rapid unlinks of mountpoints while concurrent path resolution operations are active may read sensitive files located beneath those mount points. The attack window is narrow but reproducible under high concurrency, particularly when multiple walkers attempt to open paths simultaneously with frequent unlinking events. This behavior violates the principle that a file should be inaccessible once it has been unlinked from its parent directory, even if temporary mounts were attached to it during the operation. Such exposure could lead to unauthorized access to confidential data or compromise system integrity by allowing operations on files that are logically removed from the active filesystem tree.
Mitigation for this issue involves applying kernel patches that enhance validation checks within the refwalk path resolution logic. Specifically, the fix introduces a check in the step_into_slowpath function to verify whether a dentry has been marked with DCACHE_CANT_MOUNT and is unhashed after mounts have been looked up but before they are fully detached. This ensures that any reference-based walk detects when a file has been unlinked under it during mount detachment, causing the lookup to fail appropriately rather than proceeding to reveal underlying entries. System administrators should update their Linux kernels to versions containing this fix to prevent exploitation of this race condition. Additionally, monitoring for unusual patterns of rapid file deletion and access in high-concurrency environments may help detect attempted exploits before they succeed.
From a classification perspective, this vulnerability aligns with CWE-362, which describes concurrent execution using shared resources with improper synchronization leading to race conditions. The exploitation technique leverages timing differences between filesystem state changes and path resolution operations, mapping closely to ATT&CK techniques related to discovery and potentially data exfiltration if sensitive files are accessed through the exposed paths. Understanding this flaw highlights the complexity of maintaining consistency in kernel-level file system abstractions under concurrent access patterns and underscores the necessity for rigorous validation steps in reference-counted object traversal mechanisms within operating systems.