{"id":"CVE-2026-98981","state":"PUBLISHED","title":"namespace: unhash a dentry before detaching the mounts on it","description":"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---","publishedAt":"2026-10-23T12:19:58.507Z","updatedAt":"2026-10-30T12:40:41.226Z","severity":{"score":8.8,"label":"High","version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H","source":"Linux"},"assigner":"Linux","products":["Linux Linux"],"affected":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"b3fa3811ffffa130586c4ff8f7299b7fc69216d5","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"97cc290c57ea04337fd33912d780367add41525c","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"bb917ec497f6099c6cb9375596e085a5f6b52ff2","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"db90eecbde319527aa9480c039d32a0f5fe80574","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"8d71bfc9b3fc15d7103e82ea9dc5057541866e46","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"d29cd99db3db74cf3447be74d25740981789d461","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"8300de4aa5d09061bfcf2af5b52c0cdba3873185","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"528b383dc35d1e5caf64ddf5d35f983ae543b0d0","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["fs/namespace.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"3.18","status":"affected"},{"version":"0","status":"unaffected","lessThan":"3.18","versionType":"semver"},{"version":"5.10.273","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.224","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.191","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.160","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.114","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.57","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.11","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["fs/namespace.c"]}],"references":["https://git.kernel.org/stable/c/b3fa3811ffffa130586c4ff8f7299b7fc69216d5","https://git.kernel.org/stable/c/97cc290c57ea04337fd33912d780367add41525c","https://git.kernel.org/stable/c/bb917ec497f6099c6cb9375596e085a5f6b52ff2","https://git.kernel.org/stable/c/db90eecbde319527aa9480c039d32a0f5fe80574","https://git.kernel.org/stable/c/8d71bfc9b3fc15d7103e82ea9dc5057541866e46","https://git.kernel.org/stable/c/d29cd99db3db74cf3447be74d25740981789d461","https://git.kernel.org/stable/c/8300de4aa5d09061bfcf2af5b52c0cdba3873185","https://git.kernel.org/stable/c/528b383dc35d1e5caf64ddf5d35f983ae543b0d0"],"weaknesses":[],"ingestedAt":"2026-10-30T13:00:06.648Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/98xxx/CVE-2026-98981.json","commit":"cb14bf1b43b0c9f05f7f40eae2246263fd430780","baselineCommit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-98981","state":"PUBLISHED","datePublished":"2026-10-23T12:19:58.507Z","dateUpdated":"2026-10-30T12:40:41.226Z","assignerShortName":"Linux","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateReserved":"2026-09-25T10:25:14.384Z"},"containers":{"cna":{"title":"namespace: unhash a dentry before detaching the mounts on it","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":[{"vendor":"Linux","product":"Linux","defaultStatus":"unaffected","versions":[{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"b3fa3811ffffa130586c4ff8f7299b7fc69216d5","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"97cc290c57ea04337fd33912d780367add41525c","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"bb917ec497f6099c6cb9375596e085a5f6b52ff2","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"db90eecbde319527aa9480c039d32a0f5fe80574","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"8d71bfc9b3fc15d7103e82ea9dc5057541866e46","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"d29cd99db3db74cf3447be74d25740981789d461","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"8300de4aa5d09061bfcf2af5b52c0cdba3873185","versionType":"git"},{"version":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe","status":"affected","lessThan":"528b383dc35d1e5caf64ddf5d35f983ae543b0d0","versionType":"git"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["fs/namespace.c"]},{"vendor":"Linux","product":"Linux","defaultStatus":"affected","versions":[{"version":"3.18","status":"affected"},{"version":"0","status":"unaffected","lessThan":"3.18","versionType":"semver"},{"version":"5.10.273","status":"unaffected","lessThanOrEqual":"5.10.*","versionType":"semver"},{"version":"5.15.224","status":"unaffected","lessThanOrEqual":"5.15.*","versionType":"semver"},{"version":"6.1.191","status":"unaffected","lessThanOrEqual":"6.1.*","versionType":"semver"},{"version":"6.6.160","status":"unaffected","lessThanOrEqual":"6.6.*","versionType":"semver"},{"version":"6.12.114","status":"unaffected","lessThanOrEqual":"6.12.*","versionType":"semver"},{"version":"6.18.57","status":"unaffected","lessThanOrEqual":"6.18.*","versionType":"semver"},{"version":"7.2.11","status":"unaffected","lessThanOrEqual":"7.2.*","versionType":"semver"},{"version":"7.3","status":"unaffected","lessThanOrEqual":"*","versionType":"original_commit_for_fix"}],"repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["fs/namespace.c"]}],"references":[{"url":"https://git.kernel.org/stable/c/b3fa3811ffffa130586c4ff8f7299b7fc69216d5"},{"url":"https://git.kernel.org/stable/c/97cc290c57ea04337fd33912d780367add41525c"},{"url":"https://git.kernel.org/stable/c/bb917ec497f6099c6cb9375596e085a5f6b52ff2"},{"url":"https://git.kernel.org/stable/c/db90eecbde319527aa9480c039d32a0f5fe80574"},{"url":"https://git.kernel.org/stable/c/8d71bfc9b3fc15d7103e82ea9dc5057541866e46"},{"url":"https://git.kernel.org/stable/c/d29cd99db3db74cf3447be74d25740981789d461"},{"url":"https://git.kernel.org/stable/c/8300de4aa5d09061bfcf2af5b52c0cdba3873185"},{"url":"https://git.kernel.org/stable/c/528b383dc35d1e5caf64ddf5d35f983ae543b0d0"}],"metrics":[{"cvssV3_1":{"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"},"scenarios":[{"lang":"en","value":"AV:L - The trigger uses local unlinkat(), rmdir(), or renameat2() into vfs_unlink(), vfs_rmdir(), or vfs_rename(), which call the patched __detach_mounts(), while a concurrent openat2() path walk races the detach; no network data reaches this path.\nAC:L - The attacker controls both sides by unlinking the underlying entry while concurrent lookup_fast() walkers open it through the covered mount; the reproducer obtained 11,938 disclosures in 16,140 unlinks.\nPR:L - may_delete_dentry() requires ordinary write and execute permission on the parent, while may_mount() uses CAP_SYS_ADMIN in the mount namespace's user namespace; an unprivileged user can obtain that capability with a new user/mount namespace and mount tmpfs because it has FS_USERNS_MOUNT.\nUI:N - The attacker creates the mount arrangement and directly drives both the removal syscall and concurrent openat2() calls; no separate victim action is required.\nS:C - __detach_mounts() removes mounts attached to the shared dentry across namespaces, allowing a walk in the covered A' namespace to reach the underlying A object. This crosses the mount-namespace isolation boundary the covering mount enforced.\nC:H - The stale positive dentry accepted by lookup_fast() and step_into() can be opened and read after its covering mount disappears; the reproducer reads the complete SECRET_F contents rather than a bounded metadata fragment.\nI:H - The incorrectly uncovered dentry proceeds to do_open(), where may_open() permits O_WRONLY under normal inode permissions and O_TRUNC reaches handle_truncate(); thus a mount-hidden writable object, or the target of an uncovered symlink, can be modified or erased.\nA:H - A stale covered symlink can reach pick_link() and resolve a persistent target, after which do_open() can invoke handle_truncate() for O_TRUNC. Erasing a mount-hidden critical file can cause complete loss of the dependent resource or service."}]}],"providerMetadata":{"shortName":"Linux","orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","dateUpdated":"2026-10-30T12:40:41.226Z"},"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.18","versionEndExcluding":"5.10.273"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.18","versionEndExcluding":"5.15.224"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.18","versionEndExcluding":"6.1.191"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.18","versionEndExcluding":"6.6.160"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.18","versionEndExcluding":"6.12.114"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.18","versionEndExcluding":"6.18.57"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.18","versionEndExcluding":"7.2.11"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"3.18","versionEndExcluding":"7.3"}]}]}],"x_generator":{"engine":"bippy-1.2.0"}}}}}