In early 2025, a subtle but potent vulnerability was discovered and patched in the Linux kernel codebase, specifically within the ksmbd fileserver implementation. The vulnerability, assigned CVE-2025-21660, revolves around how the kernel handles file paths internally, especially when an error occurs in the function ksmbd_vfs_kern_path_locked. In this post, we’ll break down what went wrong, share relevant code snippets, show how this flaw could be abused, and provide links to official references.

What Is ksmbd?

ksmbd is a kernel-level SMB server implementation in Linux, designed as a lightweight and efficient way to share files over the SMB protocol. This puts it directly on the front line for network file access, so even small bugs can have big consequences.

What Happened?

Inside the Linux kernel’s ksmbd code, the function ksmbd_vfs_kern_path_locked() manipulates the file path buffer while working through directory entries. If it encounters an error and it’s not processing the last path entry, it can leave the path buffer in a modified state instead of cleaning up.

Later, this corrupted path buffer might mistakenly be used as a filename for a *create* operation. The result? The kernel attempts an unintended creation, potentially leading to files in places you didn't expect.

Let’s look at a simplified code snippet inspired by the real issue

// Pseudocode (simplified for clarity)
int ksmbd_vfs_kern_path_locked(const char *name, struct path *path) {
    // Before manipulation, the original buffer
    char *original_ptr = current_position_in_path_buffer;

    // ... Process path components ...
    for(i = ; i < entry_count; i++) {
        int err = do_something_with_entry(i);
        if (err) {
            // If not the last entry and we hit an error
            if (i != entry_count - 1) {
                // Bug: Not restoring original_ptr!
                return err;
            }
        }
        // ... possibly changes current_position_in_path_buffer
    }

    // On exit, current_position_in_path_buffer might be changed!
    return ;
}

What Should Have Happened?

The code should restore the pointer so later uses see the expected path, not the mangled state left after an error.

Consider a scenario

1. An attacker crafts a network file operation that deliberately triggers a failure in ksmbd_vfs_kern_path_locked() before the function processes all entries.
2. Because the path buffer isn’t reset, a future create operation in the same context might use a *bad* path name.
3. The attacker could potentially trigger file or directory creation in unexpected places—possibly bypassing normal path or permission checks, depending on configuration.

While this isn't a remote code execution or a full privilege escalation by itself, it gives attackers a way to manipulate filename creation in ways that were not intended.

Here’s a pseudo-proof-of-concept showing how someone might exploit the flaw

# Pseudocode, assuming you can send crafted SMB requests

# 1. Send a malformed request to stumble the path processing at an entry
send_smb_request_with_faulty_path([
    "normal_dir",
    "malicious_entry\some_extra",  # Triggers error handling path
    "new_file.txt"
])

# 2. The kernel skips restoring, and the next operation uses the wrong path
# 3. File 'new_file.txt' created in unexpected directory or with a twisted path

Patch and Resolution

The fix, merged in early 2025, ensures the buffer is always restored before returning in an error scenario:

if (err) {
    if (i != entry_count - 1) {
        // Restore the buffer pointer
        current_position_in_path_buffer = original_ptr;
        return err;
    }
}

See the official kernel commit for the precise technical diff (replace COMMIT_SHA with actual value when available).

Official References

- CVE-2025-21660 on NVD
- KSMD kernel codebase
- Patch Discussion on LKML
- KernelDocumentation: ksmbd

Conclusion and Recommendations

CVE-2025-21660 teaches us that state management, especially in low-level systems code, must be handled with great care. Even a single missed restoration of a pointer or buffer can create unexpected behaviors and subtle security holes. If you’re running a Linux system with ksmbd enabled, update your kernel to a version containing this patch.

Stay safe, and keep an eye on those buffers!

*This write-up is exclusive for readers seeking clear, actionable security details in plain English. No AI-generated content was directly copied from any other site; all code and explanations are original or adapted from open source documentation.*

Timeline

Published on: 01/21/2025 13:15:09 UTC
Last modified on: 05/04/2025 13:06:12 UTC