CVE-2025-37988: Vulnerability in Linux Linux

Severity: highType: vulnerabilityCVE-2025-37988

In the Linux kernel, the following vulnerability has been resolved: fix a couple of races in MNT_TREE_BENEATH handling by do_move_mount() Normally do_lock_mount(path, _) is locking a mountpoint pinned by *path and at the time when matching unlock_mount() unlocks that location it is still pinned by the same thing. Unfortunately, for 'beneath' case it's no longer that simple - the object being locked is not the one *path points to. It's the mountpoint of path->mnt. The thing is, without sufficient locking ->mnt_parent may change under us and none of the locks are held at that point. The rules are * mount_lock stabilizes m->mnt_parent for any mount m. * namespace_sem stabilizes m->mnt_parent, provided that m is mounted. * if either of the above holds and refcount of m is positive, we are guaranteed the same for refcount of m->mnt_parent. namespace_sem nests inside inode_lock(), so do_lock_mount() has to take inode_lock() before grabbing namespace_sem. It does recheck that path->mnt is still mounted in the same place after getting namespace_sem, and it does take care to pin the dentry. It is needed, since otherwise we might end up with racing mount --move (or umount) happening while we were getting locks; in that case dentry would no longer be a mountpoint and could've been evicted on memory pressure along with its inode - not something you want when grabbing lock on that inode. However, pinning a dentry is not enough - the matching mount is also pinned only by the fact that path->mnt is mounted on top it and at that point we are not holding any locks whatsoever, so the same kind of races could end up with all references to that mount gone just as we are about to enter inode_lock(). If that happens, we are left with filesystem being shut down while we are holding a dentry reference on it; results are not pretty. What we need to do is grab both dentry and mount at the same time; that makes inode_lock() safe *and* avoids the problem with fs getting shut down under us. After taking namespace_sem we verify that path->mnt is still mounted (which stabilizes its ->mnt_parent) and check that it's still mounted at the same place. From that point on to the matching namespace_unlock() we are guaranteed that mount/dentry pair we'd grabbed are also pinned by being the mountpoint of path->mnt, so we can quietly drop both the dentry reference (as the current code does) and mnt one - it's OK to do under namespace_sem, since we are not dropping the final refs. That solves the problem on do_lock_mount() side; unlock_mount() also has one, since dentry is guaranteed to stay pinned only until the namespace_unlock(). That's easy to fix - just have inode_unlock() done earlier, while it's still pinned by mp->m_dentry.

AI Analysis

Technical Summary

CVE-2025-37988 is a race condition vulnerability in the Linux kernel's mount handling subsystem, specifically within the do_move_mount() function and its associated locking mechanisms. The vulnerability arises from improper synchronization when handling mountpoints beneath a given path, leading to potential inconsistencies in mount references and dentry (directory entry) pinning. The core issue is that the locking sequence does not adequately stabilize the mountpoint's parent references, allowing for race conditions where mounts can be moved or unmounted concurrently while locks are being acquired. This can result in the filesystem being shut down or unmounted while references to its dentries are still held, potentially causing use-after-free conditions, memory corruption, or kernel crashes. The fix involves ensuring that both the dentry and the mount are pinned simultaneously under proper locking (namespace_sem and inode_lock), preventing the mount from being removed or altered during lock acquisition. Additionally, unlock_mount() was corrected to release inode locks earlier to maintain consistency and avoid dangling references. This vulnerability affects Linux kernel versions identified by the given commit hashes and was publicly disclosed on May 20, 2025. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

Potential Impact

For European organizations, this vulnerability poses a significant risk to systems running affected Linux kernel versions, especially those relying on complex mount namespaces and dynamic mount operations such as containerized environments, cloud infrastructure, and virtualized platforms. Exploitation could lead to kernel crashes, denial of service (DoS), or potentially privilege escalation if attackers can manipulate mount operations to corrupt kernel memory. This can disrupt critical services, data availability, and system integrity. Given the widespread use of Linux in European data centers, government agencies, and enterprises, the impact could be broad, affecting cloud providers, telecom operators, and industrial control systems. The vulnerability's exploitation complexity is moderate due to the need to race mount operations, but once exploited, it can cause severe system instability or compromise. The absence of known exploits currently reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits post-disclosure.

Mitigation Recommendations

European organizations should prioritize updating their Linux kernels to versions containing the patch for CVE-2025-37988 as soon as vendor updates become available. In the interim, system administrators should audit and restrict mount namespace operations to trusted users only, minimizing the risk of race conditions triggered by unprivileged users. Container orchestration platforms should be configured to limit mount manipulations and isolate workloads to reduce attack surface. Monitoring kernel logs for unusual mount or unmount operations and implementing kernel integrity monitoring can help detect exploitation attempts. For environments where immediate patching is not feasible, consider applying kernel lockdown features or mandatory access controls (e.g., SELinux, AppArmor) to restrict mount-related syscalls. Additionally, thorough testing of kernel updates in staging environments is recommended to ensure stability before production deployment.