Linux kernel: refwalk path lookup can reveal files hidden under unlinked mounts

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 <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 while there is none */ static atomic_long n_unlink, n_walk, n_mark, n_enoent, n_othe ---truncated---