CVE-2026-64140 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: fix null pointer dereference in proc_show_files()
When a SMB2 client opens a file with a durable v2 handle and then issues SMB2 SESSION_LOGOFF, session_fd_check() clears fp->tcon = NULL on the reconnectable file pointer but leaves the fp registered in global_ft.idr until the durable scavenger fires (up to fp->durable_timeout seconds later).
During that window any read of /proc/fs/ksmbd/files (mode 0400) panics the kernel because proc_show_files() walks global_ft.idr and unconditionally dereferences fp->tcon->id with no NULL guard.
Reproducer requires only a successful SMB2 SESSION_SETUP and a share configured with 'durable handles = yes'. KASAN report on mainline 70390501d194:
general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
RIP: 0010:proc_show_files+0x118/0x740 Call Trace: proc_show_files+0x118/0x740 seq_read_iter+0x4ef/0xe10 proc_reg_read_iter+0x1b7/0x280 ...
Guard the dereference. A durable-disconnected fp legitimately has no tcon; report its tree id as 0 rather than oopsing.
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Analysis
by VulDB Data Team • 07/19/2026
This vulnerability exists within the ksmbd kernel module, which provides SMB2/SMB3 server functionality for Linux systems. The issue stems from improper handling of file pointers during session reconnection scenarios involving durable v2 handles. When an SMB2 client establishes a session with durable handle support and subsequently logs off, the session_fd_check() function properly clears the file pointer's tcon reference to NULL but fails to unregister the file pointer from the global_ft.idr data structure immediately. This creates a temporal window where the file pointer remains registered in the global file table while its tcon reference is already invalid.
The technical flaw manifests in the proc_show_files() function which serves the /proc/fs/ksmbd/files interface for system monitoring and debugging purposes. During kernel read operations on this proc entry, the function iterates through all registered file pointers in global_ft.idr without implementing proper NULL pointer validation before dereferencing fp->tcon->id. This violates standard null pointer dereference protection mechanisms and directly contradicts CWE-476 which addresses null pointer dereference vulnerabilities. The vulnerability represents a classic race condition between resource cleanup and resource access operations.
The operational impact of this vulnerability is severe as it can cause immediate kernel panics and system crashes whenever any process attempts to read the /proc/fs/ksmbd/files file during the window between session logoff and durable scavenger cleanup. This affects systems running ksmbd with durable handle support, creating potential denial-of-service conditions that could be exploited by unauthorized users to disrupt SMB services. The KASAN report confirms the vulnerability occurs at a specific memory address indicating canonical address fault conditions typical of null pointer dereference scenarios, aligning with ATT&CK technique T1499.004 for network disruption through kernel exploitation.
The recommended mitigation involves implementing proper NULL pointer guards in proc_show_files() before accessing fp->tcon->id and ensuring that disconnected file pointers are properly unregistered from global_ft.idr immediately upon session logoff rather than waiting for the durable scavenger timer to expire. This fix should maintain consistency with the principle of least privilege by preventing access to invalid resources while also addressing the fundamental issue of temporal inconsistency in resource management state. The solution must ensure that disconnected file pointers legitimately report tree id as 0 rather than causing kernel oops conditions, thereby maintaining system stability and preventing unauthorized access patterns that could be leveraged for further exploitation.