CVE-2023-52480: Fixing a Race Condition Vulnerability in Linux Kernel's ksmbd

A vulnerability labeled CVE-2023-52480 has been discovered and resolved in the Linux kernel's ksmbd component. It involves a race condition between session lookup and expire functionalities. This post outlines the details of the vulnerability, the code snippet showcasing the problem and its solution, and links to original references. Additionally, it discusses the exploit's details and possible ramifications if not addressed.

Vulnerability Details

The vulnerability stems from a race condition between two threads A and B, where Thread A performs a ksmbd_session_lookup, and Thread B performs a smb2_sess_setup. Thread A attempts to update 'last_active' for the session object; however, a execution scheduling conflict may cause Thread B to destroy this session, leading to an Unauthorized Access to Freed memory (UAF) vulnerability, as shown below:

Thread A (ksmbd_session_lookup)      |      Thread B (smb2_sess_setup)
                                      |
   sess = xa_load                     |
                                      |
                                      |        xa_erase(&conn->sessions, sess->id);
                                      |
                                      |        ksmbd_session_destroy(sess) --> kfree(sess)
                                      |
   // UAF!                            |
   sess->last_active = jiffies        |
                                      +

Patch Solution

To address this race condition, the patch introduces a new Read-Write Semaphores (rwsem) mechanism that establishes a synchronizing mutual exclusion between ksmbd_session_lookup and ksmbd_expire_session functions. This approach ensures that the session object will not be destroyed while being accessed or updated by another thread, eliminating the UAF vulnerability.

Original References

- Linux Kernel Mailing List (LKML) patch announcement
- GitHub commit

Exploit Details

An attacker could potentially exploit the UAF vulnerability to cause a denial of service (DoS) by crashing the ksmbd service or, worse, execute malicious code with kernel-level privileges. This could lead to unauthorized system access or information disclosure, rendering affected systems at high risk.

Conclusion

The CVE-2023-52480 vulnerability has been addressed by implementing proper synchronization mechanisms among concurrent threads to prevent the race condition. It is advised for users to update their Linux kernel to include the necessary patches to mitigate the associated risks and ensure maximum system security.

Timeline

Published on: 02/29/2024 06:15:46 UTC
Last modified on: 05/29/2024 05:12:35 UTC