namespace: unhash a dentry before detaching the mounts on it

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-98981
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-23 13:17:19 UTC
Updated2026-10-23 13:17:19 UTC
DescriptionIn 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---

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe db90eecbde319527aa9480c039d32a0f5fe80574 git Not specified
CNA Linux Linux affected 8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe 8d71bfc9b3fc15d7103e82ea9dc5057541866e46 git Not specified
CNA Linux Linux affected 8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe d29cd99db3db74cf3447be74d25740981789d461 git Not specified
CNA Linux Linux affected 8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe 8300de4aa5d09061bfcf2af5b52c0cdba3873185 git Not specified
CNA Linux Linux affected 8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe 528b383dc35d1e5caf64ddf5d35f983ae543b0d0 git Not specified
CNA Linux Linux affected 3.18 Not specified
CNA Linux Linux unaffected 3.18 semver Not specified
CNA Linux Linux unaffected 6.6.160 6.6.* semver Not specified
CNA Linux Linux unaffected 6.12.114 6.12.* semver Not specified
CNA Linux Linux unaffected 6.18.57 6.18.* semver Not specified
CNA Linux Linux unaffected 7.2.11 7.2.* semver Not specified
CNA Linux Linux unaffected 7.3 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/d29cd99db3db74cf3447be74d25740981789d461 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/528b383dc35d1e5caf64ddf5d35f983ae543b0d0 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/8300de4aa5d09061bfcf2af5b52c0cdba3873185 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/8d71bfc9b3fc15d7103e82ea9dc5057541866e46 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/db90eecbde319527aa9480c039d32a0f5fe80574 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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