CVE-2024-39463 in Linux
Summary
by MITRE • 06/25/2024
In the Linux kernel, the following vulnerability has been resolved:
9p: add missing locking around taking dentry fid list
Fix a use-after-free on dentry's d_fsdata fid list when a thread looks up a fid through dentry while another thread unlinks it:
UAF thread: refcount_t: addition on 0; use-after-free. p9_fid_get linux/./include/net/9p/client.h:262 v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129 v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181 v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314 v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400 vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248
Freed by: p9_fid_destroy (inlined) p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456 p9_fid_put linux/./include/net/9p/client.h:278 v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55 v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518 vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335
The problem is that d_fsdata was not accessed under d_lock, because d_release() normally is only called once the dentry is otherwise no longer accessible but since we also call it explicitly in v9fs_remove that lock is required: move the hlist out of the dentry under lock then unref its fids once they are no longer accessible.
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Analysis
by VulDB Data Team • 04/07/2026
The vulnerability described in CVE-2024-39463 represents a critical use-after-free condition within the Linux kernel's 9p file system implementation that arises from inadequate synchronization around dentry fid list operations. This flaw occurs specifically within the v9fs subsystem which implements the 9p protocol for network file sharing, creating a scenario where concurrent access patterns can lead to memory corruption and potential privilege escalation. The issue stems from the absence of proper locking mechanisms when accessing the dentry's d_fsdata field containing the fid list, which is crucial for maintaining consistency in distributed file system operations.
The technical flaw manifests when one thread performs a fid lookup operation through a dentry while another thread simultaneously unlinks that same dentry, creating a race condition that results in the freed memory being accessed after deallocation. The call stack reveals that the use-after-free occurs during the p9_fid_get operation where a reference count is incremented on what has already been freed memory, with the p9_fid_destroy function being invoked through the p9_client_clunk pathway. The freed memory is managed through the v9fs_dentry_release function which is called during the v9fs_remove operation, demonstrating how the explicit call to d_release creates an opportunity for the race condition to occur.
This vulnerability directly maps to CWE-416, which describes the use of freed memory condition, and aligns with ATT&CK technique T1068, involving privilege escalation through kernel vulnerabilities. The operational impact is severe as this condition can be exploited to achieve arbitrary code execution within kernel space, potentially allowing an attacker to escalate privileges from a regular user to root access. The flaw affects systems running Linux kernels with 9p file system support, particularly those utilizing distributed file sharing environments where concurrent access patterns are common. The race condition occurs because d_release() is called explicitly in v9fs_remove rather than relying on the normal dentry cleanup process, which normally ensures proper synchronization through d_lock.
The fix implemented addresses this by ensuring proper locking around the access to dentry's d_fsdata field, specifically moving the hlist out of the dentry under proper lock protection before proceeding with fid reference counting operations. This approach follows the established pattern of protecting shared data structures through appropriate synchronization primitives, preventing the scenario where one thread accesses freed memory while another thread is in the process of deallocating it. The mitigation strategy requires maintaining the d_lock during the entire duration of fid list operations, ensuring that when a dentry is being unlinked, no other thread can simultaneously perform fid lookups on that same dentry. This solution adheres to kernel security best practices by ensuring proper atomicity in concurrent access to shared kernel data structures, thereby preventing the memory corruption that could lead to privilege escalation or system compromise in environments utilizing the 9p protocol for network file sharing.