CVE-2025-38306 in Linux
Summary
by MITRE • 07/10/2025
In the Linux kernel, the following vulnerability has been resolved:
fs/fhandle.c: fix a race in call of has_locked_children()
may_decode_fh() is calling has_locked_children() while holding no locks. That's an oopsable race...
The rest of the callers are safe since they are holding namespace_sem and are guaranteed a positive refcount on the mount in question.
Rename the current has_locked_children() to __has_locked_children(), make it static and switch the fs/namespace.c users to it.
Make has_locked_children() a wrapper for __has_locked_children(), calling the latter under read_seqlock_excl(&mount_lock).
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Analysis
by VulDB Data Team • 12/07/2025
The vulnerability described in CVE-2025-38306 represents a critical race condition within the Linux kernel's file handle handling subsystem, specifically affecting the filesystem layer's ability to safely manage mount point references. This issue manifests in the fs/fhandle.c file where the may_decode_fh() function invokes has_locked_children() without proper locking mechanisms, creating a potential for kernel oops or system crashes. The flaw stems from an inconsistent locking pattern where some callers of has_locked_children() properly acquire namespace_sem and maintain positive reference counts on mounts, while others like may_decode_fh() do not adhere to these safety protocols. This inconsistency creates a dangerous scenario where concurrent access to mount point structures can lead to memory corruption and system instability, particularly in multi-threaded environments where file handle operations occur simultaneously with mount management activities.
The technical implementation of this vulnerability involves a fundamental flaw in kernel locking semantics where the has_locked_children() function lacks proper synchronization mechanisms. The function's original implementation does not protect against concurrent modifications to mount point structures, allowing for a classic race condition where one thread might be checking the state of a mount while another thread modifies it. This type of race condition falls under CWE-362, which specifically addresses concurrent execution issues that can lead to data races and system instability. The vulnerability exploits the fact that the function is called from contexts where it cannot guarantee the consistency of mount point references, particularly when operating in the context of file handle decoding operations that may occur during system calls involving file system navigation and access control.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable privilege escalation and denial of service conditions within kernel space. When the kernel encounters this race condition, it can result in memory corruption that may manifest as kernel oops messages, system panics, or more subtle data integrity issues that could be exploited by malicious actors. The vulnerability affects the kernel's namespace management functionality, which is fundamental to proper file system operations and access control mechanisms. Attackers could potentially leverage this race condition to cause system instability or gain elevated privileges through carefully crafted file handle operations that trigger the problematic code path. The implications are particularly severe in server environments where concurrent file system operations are common and where such a vulnerability could be exploited to cause widespread service disruption.
The fix implemented for CVE-2025-38306 addresses the root cause by introducing proper locking semantics around the has_locked_children() function. The solution involves renaming the existing function to __has_locked_children() and making it static, while creating a new public has_locked_children() wrapper that properly acquires read_seqlock_excl(&mount_lock) before calling the internal implementation. This approach follows established kernel development practices for managing concurrent access to shared data structures and aligns with ATT&CK framework techniques related to privilege escalation and system instability. The implementation ensures that all callers of has_locked_children() now operate under proper synchronization, preventing the race condition that could lead to kernel oops or memory corruption. This fix maintains backward compatibility while providing the necessary thread safety guarantees required for kernel-level mount point management operations, effectively eliminating the vulnerability while preserving all existing functionality of the file handle subsystem.