{"id":"CVE-2026-98981","summary":"namespace: unhash a dentry before detaching the mounts on it","details":"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 \u003cerrno.h\u003e\n  #include \u003cfcntl.h\u003e\n  #include \u003cpthread.h\u003e\n  #include \u003cstdatomic.h\u003e\n  #include \u003cstdio.h\u003e\n  #include \u003cstdlib.h\u003e\n  #include \u003cstring.h\u003e\n  #include \u003cunistd.h\u003e\n  #include \u003csys/mount.h\u003e\n  #include \u003csys/stat.h\u003e\n  #include \u003csys/syscall.h\u003e\n  #include \u003clinux/openat2.h\u003e\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---","modified":"2026-11-01T02:38:24.635597863Z","published":"2026-10-23T12:19:58.507Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98981.json"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/528b383dc35d1e5caf64ddf5d35f983ae543b0d0"},{"type":"WEB","url":"https://git.kernel.org/stable/c/8300de4aa5d09061bfcf2af5b52c0cdba3873185"},{"type":"WEB","url":"https://git.kernel.org/stable/c/8d71bfc9b3fc15d7103e82ea9dc5057541866e46"},{"type":"WEB","url":"https://git.kernel.org/stable/c/97cc290c57ea04337fd33912d780367add41525c"},{"type":"WEB","url":"https://git.kernel.org/stable/c/b3fa3811ffffa130586c4ff8f7299b7fc69216d5"},{"type":"WEB","url":"https://git.kernel.org/stable/c/bb917ec497f6099c6cb9375596e085a5f6b52ff2"},{"type":"WEB","url":"https://git.kernel.org/stable/c/d29cd99db3db74cf3447be74d25740981789d461"},{"type":"WEB","url":"https://git.kernel.org/stable/c/db90eecbde319527aa9480c039d32a0f5fe80574"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/98xxx/CVE-2026-98981.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-98981"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe"},{"fixed":"b3fa3811ffffa130586c4ff8f7299b7fc69216d5"},{"fixed":"97cc290c57ea04337fd33912d780367add41525c"},{"fixed":"bb917ec497f6099c6cb9375596e085a5f6b52ff2"},{"fixed":"db90eecbde319527aa9480c039d32a0f5fe80574"},{"fixed":"8d71bfc9b3fc15d7103e82ea9dc5057541866e46"},{"fixed":"d29cd99db3db74cf3447be74d25740981789d461"},{"fixed":"8300de4aa5d09061bfcf2af5b52c0cdba3873185"},{"fixed":"528b383dc35d1e5caf64ddf5d35f983ae543b0d0"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-98981.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"3.18.0"},{"fixed":"5.10.273"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.11.0"},{"fixed":"5.15.224"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.16.0"},{"fixed":"6.1.191"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"6.6.160"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.114"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.57"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.2.11"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-98981.json"}}],"schema_version":"1.9.2","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H"}]}