CVE-2026-98981
Full vulnerability record from the local CVENow database.
Description
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---
View original CVE JSON
{
"id": "CVE-2026-98981",
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"published": "2026-10-23T13:17:19.890",
"lastModified": "2026-10-30T13:21:33.987",
"vulnStatus": "Received",
"cveTags": [],
"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnamespace: unhash a dentry before detaching the mounts on it\n\nunlink(), rmdir() and rename() remove the entry from the filesystem,\ncall detach_mounts() on the entry with the inode locked and then call\nd_delete() once the inode is unlocked.\n\nThe thing is that between detach_mounts() and d_delete() the dentry is\nstill hashed and positive. But after detach_mounts() nothing covers the\ndentry anymore. A lookup that finds this dentry in the dcache can\nuncover what the mounts hid.\n\nThat's a problem when unlinking files or directories that are\nmountpoints in other mount namespaces. Everybody who had the underlying\nentry covered can race the detach_mount() call until d_delete() has run.\n\nThe race window isn't all that small. It encompasses namespace_unlock()\nwith a full synchronize_rcu_expedited() grace period and the\ninode_unlock() of the entry.\n\nIn my experiments three walkers that kept trying read an overmounted\nfile in 11938 of the 16140 unlinks that removed its mountpoint from a\nbind mount of the filesystem within a minute.\n\nFun fact, d_invalidate() has the same ordering problem but gets it\nright. It unhashes the dentry first and detaches the mounts afterwards.\nLet's do the same in __detach_mounts():\n\n- A lookup that hasn't found the dentry yet misses it in the dcache and\n waits for the directory lock that the caller holds until the name is\n gone for good.\n\n- A lockless lookup that found it already fails the mount_lock check\n while the dentry still counts as a mountpoint and the d_seq check once\n it doesn't, and retries.\n\nThis fixes the lockless path. We still need to fix the reference count\nlookup in a follow-up patch.\n\nSo d_drop() the dentry. The dentry stays positive and held but can't be\nfound anymore. Then proceed with the detach and unlink.\n\nReproducer:\n\nThe reads were counted with the program below, three walkers for 60 s\nin a VM with 4 CPUs. The race window is stretched with the debug patch\npasted here. fs.detach_race_walk_us sleeps in lookup_fast() once it\nfound a mountpoint dentry and before its mounts are crossed.\nfs.detach_race_unlink_us sleeps in vfs_unlink() between detach_mounts()\nand d_delete().\n\n // SPDX-License-Identifier: GPL-2.0\n /*\n * unlink_covered: unlink a file that is a mountpoint in a detached copy of\n * its mount, against lookups that open it through that copy.\n *\n * A is a tmpfs with the file f1 (content SECRET_F) and the plain file\n * MARK_A. A' is an open_tree(OPEN_TREE_CLONE) copy of A with MARK_A bound\n * on A'/f1, held through an O_PATH fd on its root once the tree fd is\n * closed. Walkers open f1 through that fd while the driver unlinks A/f1,\n * where nothing is mounted on it. A walker may read MARK_A or get ENOENT.\n * A read of SECRET_F is a hit: the name was found after its mount was\n * gone. The lockless walks use openat2(RESOLVE_CACHED).\n *\n * usage: unlink_covered [-t seconds] [-w walkers]\n */\n #ifndef _GNU_SOURCE\n #define _GNU_SOURCE\n #endif\n #include <errno.h>\n #include <fcntl.h>\n #include <pthread.h>\n #include <stdatomic.h>\n #include <stdio.h>\n #include <stdlib.h>\n #include <string.h>\n #include <unistd.h>\n #include <sys/mount.h>\n #include <sys/stat.h>\n #include <sys/syscall.h>\n #include <linux/openat2.h>\n\n #ifndef __NR_open_tree\n #define __NR_open_tree 428\n #endif\n #ifndef __NR_move_mount\n #define __NR_move_mount 429\n #endif\n #ifndef __NR_openat2\n #define __NR_openat2 437\n #endif\n #ifndef OPEN_TREE_CLONE\n #define OPEN_TREE_CLONE 1\n #endif\n #ifndef OPEN_TREE_CLOEXEC\n #define OPEN_TREE_CLOEXEC O_CLOEXEC\n #endif\n #ifndef MOVE_MOUNT_F_EMPTY_PATH\n #define MOVE_MOUNT_F_EMPTY_PATH 0x00000004\n #endif\n\n #define WORK \"/tmp/uc\"\n #define ADIR WORK \"/A\"\n\n static int duration = 60, nwalkers = 3;\n static atomic_int stop, writer_waiting;\n static pthread_rwlock_t cur_lock = PTHREAD_RWLOCK_INITIALIZER;\n static int cur_fd = -1;\t\t/* the root of A', -1 wh\n---truncated---"
}
],
"affected": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"affectedData": [
{
"vendor": "Linux",
"product": "Linux",
"defaultStatus": "unaffected",
"programFiles": [
"fs/namespace.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"versions": [
{
"version": "8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe",
"lessThan": "b3fa3811ffffa130586c4ff8f7299b7fc69216d5",
"versionType": "git",
"status": "affected"
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"status": "affected"
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"status": "affected"
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}
]
},
{
"vendor": "Linux",
"product": "Linux",
"defaultStatus": "affected",
"programFiles": [
"fs/namespace.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"versions": [
{
"version": "3.18",
"status": "affected"
},
{
"version": "0",
"lessThan": "3.18",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "5.10.273",
"lessThanOrEqual": "5.10.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "5.15.224",
"lessThanOrEqual": "5.15.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "6.1.191",
"lessThanOrEqual": "6.1.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "6.6.160",
"lessThanOrEqual": "6.6.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "6.12.114",
"lessThanOrEqual": "6.12.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "6.18.57",
"lessThanOrEqual": "6.18.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "7.2.11",
"lessThanOrEqual": "7.2.*",
"versionType": "semver",
"status": "unaffected"
},
{
"version": "7.3",
"lessThanOrEqual": "*",
"versionType": "original_commit_for_fix",
"status": "unaffected"
}
]
}
]
}
],
"metrics": {
"cvssMetricV31": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"type": "Secondary",
"cvssData": {
"version": "3.1",
"vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H",
"baseScore": 8.8,
"baseSeverity": "HIGH",
"attackVector": "LOCAL",
"attackComplexity": "LOW",
"privilegesRequired": "LOW",
"userInteraction": "NONE",
"scope": "CHANGED",
"confidentialityImpact": "HIGH",
"integrityImpact": "HIGH",
"availabilityImpact": "HIGH"
},
"exploitabilityScore": 2.0,
"impactScore": 6.0
}
]
},
"references": [
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"url": "https://git.kernel.org/stable/c/528b383dc35d1e5caf64ddf5d35f983ae543b0d0",
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