CVE-2026-98977 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
namespace: handle mount locking for automounts correctly
If mounts are propagated across user namespaces, attach_recursive_mnt() locks every copy of the source mount to protect overmounts from vanishing and revealing the underlying files or directories.
The user namespace is taken from the caller's mount namespaces since this is where the mounts end up. Except, that's not always true. Automounts may legitimately get popped in by tasks located in a different mount and user namespace during path lookup.
Then check doesn't make sense at that point. The copy in the namespace of the parent - which may be the host's - is now locked and the host cannot change the flags of its own mounts anymore.
Here's the reproducer:
- host has debugfs mounted nosuid,nodev,noexec and shared - tracefs automount below it is not yet active - hand a child process a directory descriptor on that mount - child process enters auser and mount namespace - child process' namespace now holds a locked copy of the host's debugfs mount that receives propagation from it - child process stats "tracing/." through the descriptor - lookup runs on the host's mount so the automount lands below the host's mount - propagation puts a copy below the child's copy - both try to clear the flags on the mount they got, with a bind remount:
host, on its own automount: MS_REMOUNT|MS_BIND = EPERM child, on the copy in its namespace: MS_REMOUNT|MS_BIND = 0
So the lock landed on the host's mount instead of the child's copy. Congrats. So we need to compare with the owner of the namespace the mount actually gets mounted on. For all regular cases that is the caller's mount namespace and so nothing changes.
Detached trees in anonymous mount namespaces by be handed over via SCM_RIGHTS or inherited in other ways on purpose so the attaching task's mount namespace is authoritative, not the creator of the detached tree.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel vulnerability identified as a flaw in handling mount locking for automounts within user namespaces represents a critical logic error in how recursive mount propagation and lock acquisition are coordinated across distinct namespace boundaries. This issue specifically affects the attach_recursive_mnt function, which is responsible for managing the attachment of mounts during operations such as bind remounting or automatic mounting triggered by path lookups. The core technical flaw arises from an incorrect assumption regarding where locks should be applied when propagating mount structures between user namespaces. Specifically, the code originally took the user namespace from the caller's current context to determine which copy of a source mount needed locking. This approach was designed to protect overmounts from vanishing and revealing underlying files or directories that might otherwise become accessible due to race conditions during propagation. However, this logic fails when automounts are involved because these mounts can be legitimately introduced by tasks operating in different mount and user namespaces than the one initiating the operation.
When a task located in a child namespace performs an action like statting a directory on an automount point that resides under a shared host mount, the kernel's path lookup mechanism may trigger the activation of that automount. This process causes a copy of the parent mount to be inserted into the child's namespace structure while simultaneously maintaining its presence and lock status in the original host namespace. Because the locking logic incorrectly targeted the caller's namespace context rather than the actual destination namespace where the mount is being attached, it resulted in the wrong instance of the mount tree being locked. In practice, this meant that a copy of the host's debugfs or tracefs mount remained locked under the parent namespace even though the operation was intended to affect only the child's view. This mismatch creates a situation where the host system retains an exclusive lock on its own mounts, preventing it from performing legitimate administrative actions such as changing flags via bind remounts, while the child process believes it has successfully manipulated its local copy without conflict.
The operational impact of this vulnerability is primarily characterized by denial-of-service conditions and potential privilege escalation vectors through resource exhaustion or state inconsistency. For system administrators running hosts with shared filesystems like debugfs or tracefs, a malicious user in an unprivileged container can trigger the automount logic to lock down host-level mounts indefinitely. This effectively blocks other processes on the host from remounting those filesystems, which can disrupt monitoring tools, debugging utilities, and security auditing systems that rely on dynamic flag changes. Furthermore, because the locking mechanism is intended to prevent race conditions where underlying files might be exposed prematurely or inconsistently, a failure in this logic could theoretically allow an attacker to manipulate mount states in ways that bypass expected isolation boundaries between user namespaces. Although direct code execution is not implied by this specific lock inversion bug, the ability to freeze host-level filesystem operations constitutes a significant availability risk and undermines the integrity of namespace-based containerization security models.
To mitigate this vulnerability, kernel developers have corrected the logic within attach_recursive_mnt to ensure that locks are applied based on the owner of the namespace where the mount actually gets attached, rather than relying solely on the caller's current context. This adjustment ensures that for all regular cases involving standard propagation scenarios, behavior remains unchanged as the caller’s namespace typically aligns with the destination. However, in complex scenarios involving detached trees handed over via SCM_RIGHTS or inherited across namespaces, the authoritative mount namespace is correctly identified as that of the attaching task rather than the creator of the tree. This fix restores proper synchronization between host and container views of shared mounts. System administrators should apply kernel updates that include this patch immediately to prevent potential denial-of-service attacks against critical filesystem infrastructure. Additionally, organizations relying on strict isolation in multi-tenant environments must verify their kernel versions are patched to ensure that namespace boundaries remain robust against lock inversion exploits.
From a classification perspective, this vulnerability aligns with CWE-667, which describes improper locking mechanisms that can lead to deadlocks or race conditions affecting system stability and security. It also relates to CWE-200, as the exposure of internal kernel state through inconsistent mount flags could potentially aid further reconnaissance by attackers attempting to map host resources from within a container. In terms of adversarial tactics, this flaw maps to MITRE ATT&CK technique T1564.008, specifically hiding execution artifacts or manipulating system configuration via filesystem manipulation techniques that exploit misconfigured access controls and locking primitives in operating systems. The resolution emphasizes the importance of precise namespace context tracking during recursive operations involving shared mounts, ensuring that security boundaries enforced by user namespaces are not inadvertently weakened by incorrect lock ownership assumptions.