CVE-2026-98984 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
dcache: don't put a mountpoint on a dentry that's being removed
rmdir(), unlink() and rename() call dont_mount() on the victim and then detach_mounts() with the victim's inode locked. do_lock_mount() takes the inode lock of the mountpoint and checks cant_mount() so a mount can't show up after detach_mounts().
But attach_recursive_mnt() makes a second mountpoint for the root of the source mount so that the mounts already located at the destination can be put on top of it. No inode is locked for that one and d_set_mounted() only refuses a dentry that is unlinked. Between detach_mounts() and d_delete() the victim is still hashed:
rmrace: b passed dont_mount() and detach_mounts(), sleeping T2: move_mount(S, /tmp/plcant/x, BENEATH) = 0 errno 0 () T1: rmdir(/tmp/plcant/d/b) = 0 errno 0 109 107 0:61 /d/b//deleted /tmp/plcant/x rw,relatime - tmpfs tmpfs rw 108 109 0:63 / /tmp/plcant/x rw,relatime - tmpfs T rw
A bind mount of the directory that's being removed is moved beneath an existing mount while the rmdir() is located between the two calls. The mount ends up on the removed directory and nothing will ever detach it.
Check cant_mount() in d_set_mounted() as well. dont_mount() raises the flag under d_lock before detach_mounts() runs so either the mountpoint is set first and detach_mounts() finds the mount or the flag is seen and the mount is refused.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel contains a race condition vulnerability within the directory cache (dcache) subsystem that allows for improper handling of mount points during file system removal operations. This issue specifically affects scenarios involving rmdir, unlink, and rename calls where a victim dentry is being removed while simultaneously having mounts attached to it or moved beneath existing mounts. The core technical flaw lies in the synchronization logic between detach_mounts() and subsequent mounting operations. When these removal functions are invoked, they call dont_mount() on the target dentry followed by detach_mounts(), which operates with the victim's inode locked. While do_lock_mount() correctly takes the inode lock of a potential mountpoint to check cant_mount() preventing new mounts after detachment, attach_recursive_mnt() creates a second mount point for the root of the source mount without holding an inode lock on that specific dentry. Consequently, d_set_mounted() only refuses mounting if the dentry is already unlinked, but during the window between detach_mounts and d_delete, the victim remains hashed in the directory cache structure.
This timing gap enables a race condition where bind mounts of directories being removed can be moved beneath existing mounts while an rmdir operation is mid-execution. In such cases, the mount becomes attached to the removed directory without any mechanism to subsequently detach it because the standard checks fail due to the lack of proper locking on the newly created mount point dentry. The result is a persistent reference to a deleted or removing inode, which can lead to kernel instability, use-after-free conditions if memory associated with that inode is reclaimed prematurely, or general file system corruption as the VFS layer attempts to manage structures in an inconsistent state. This represents a significant integrity violation within the virtual file system layer where mount namespaces and dentry trees are not kept consistent under concurrent access patterns typical of multi-threaded applications performing rapid directory manipulations alongside mounting operations.
From a classification perspective, this vulnerability aligns with CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization Race Conditions. The failure to properly serialize access to the mount point dentry during critical sections allows for an inconsistent state that persists beyond the intended lifecycle of the object. In terms of attack vectors and defensive mapping, this scenario relates to ATT&CK technique T1059 Command and Scripting Interpreter if exploited via automated scripts, but more critically it falls under system resource management failures often associated with privilege escalation paths or denial of service through kernel panic induced by memory corruption resulting from the dangling mount reference. The lack of atomicity in checking mount eligibility against removal status creates a window exploitable by local users who can trigger concurrent rmdir and move_mount operations on shared tmpfs or other volatile file systems.
Mitigation strategies primarily involve applying vendor-provided kernel patches that address this specific race condition by enhancing the checks within d_set_mounted(). The fix requires checking cant_mount() in addition to existing unlinked status checks, ensuring that if dont_mount has raised its flag under d_lock before detach_mounts runs, any subsequent attempt to set a mount on that dentry is refused. This ensures either the mount point is established first allowing detach_mounts to find and remove it properly, or the removal flag is detected early enough to prevent the invalid attachment entirely. System administrators should ensure their Linux kernels are updated with patches addressing this specific dcachemount race condition. For environments where such operations cannot be fully controlled at the application level, implementing strict access controls on mount capabilities via user namespaces and restricting unprivileged users from performing complex mount namespace manipulations can reduce the attack surface. Regular auditing of file system operation logs for unusual patterns of concurrent removal and mounting may also aid in detecting attempted exploitation before it leads to kernel instability.