--- id: "CVE-2026-98980" kind: "vulnerability" aliases: ["CVE-2026-98980"] title: "Linux kernel: refwalk path lookup can reveal files hidden under unlinked mounts" severity: "low" cvss: null epss: 0.00209 exploited: false status: "open" category: "runtime" products: [] ecosystems: [] tags: ["linux-kernel", "multi-user-hosts", "runtime", "self-hosted"] not_affected_if: ["no-untrusted-local-users", "single-user-host"] published_at: "2026-10-23T13:17:19+00:00" first_seen_at: "2026-10-23T21:57:33+00:00" updated_at: "2026-10-25T12:38:50+00:00" sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98980", "first_seen_at": "2026-10-23T21:57:33+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-98980", "first_seen_at": "2026-10-25T12:38:50+00:00"}] --- # Linux kernel: refwalk path lookup can reveal files hidden under unlinked mounts A race in the Linux kernel's path lookup (namei) lets a refwalk that is already holding a dentry reference reach an entry whose mount was detached by a concurrent unlink, so it can read the underlying file instead of the mounted one. The sources describe a local reproducer that reads a covered file through an O_PATH-held cloned mount while another thread unlinks the mountpoint. _Summary, check and fix written by a model from the sources; labelled as such._ ## How to check - `uname -r` and compare against the fixed kernel version in the vendor advisory (unknown in the sources) - Check whether unprivileged users can create mount namespaces or call open_tree/move_mount (unknown in the sources) ## Fix - Install the kernel update from your distribution that includes the upstream namei commit adding a DCACHE_CANT_MOUNT check after step_into() (version unknown in the sources) - Reboot into the patched kernel ## Timeline - 2026-10-23T21:57:33+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98980 - 2026-10-25T12:38:50+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-98980 ## From the sources 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 #include #include #include #include #include #include #include #include #include #include #include #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--- --- _Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_