CVE-2023-4385 - A deep-dive into the NULL pointer dereference flaw

The Linux Kernel strives to be a robust and efficient operating system, making it a popular choice for various devices ranging from servers to embedded systems. However, like any large-scale software, the kernel is not entirely without its flaws. One such vulnerability, CVE-2023-4385, is a NULL pointer dereference flaw located in the journaling file system (JFS) in the Linux Kernel. This post will provide a detailed analysis of the vulnerability, including code snippets, links to relevant references, and details on how an attacker may exploit the issue.

Overview of JFS and dbFree

The journaling file system, or JFS, is a file system used by Linux for organizing and managing data on storage devices. One key component of JFS is the dbFree function in fs/jfs/jfs_dmap.c, which is responsible for managing the allocation and release of data blocks for JFS.

Reference: https://elixir.bootlin.com/linux/latest/source/fs/jfs/jfs_dmap.c

The Vulnerability

CVE-2023-4385 stems from a NULL pointer dereference flaw in the dbFree function. This means that the function attempts to access memory using a NULL pointer, which is not a valid memory address. The main issue here lies in the failure to perform a sanity check to ensure that the pointer is not NULL before dereferencing it. The following code snippet highlights the problematic code:

void dbFree(dmtree_t *tp, int blkno, int nblocks)
{
    int size;

    size = tp->dmt_stree[DMTROOT];
    ...
}

In this code, the tp pointer is not checked before it's dereferenced. If tp is NULL, attempting to access tp->dmt_stree[DMTROOT] will result in a NULL pointer dereference.

Exploit Details

An attacker with local access to the system could exploit this flaw to crash the system. The process would involve crafting a specific JFS operation that would result in the dbFree function being called with a NULL pointer, causing the system to crash due to the NULL pointer dereference. Because the attacker would not need elevated privileges to cause this crash, the potential impact is significant.

Mitigation

To mitigate this issue, a sanity check must be added to the dbFree function to ensure that the tp pointer is not NULL before dereferencing it. The following code demonstrates a possible patch:

void dbFree(dmtree_t *tp, int blkno, int nblocks)
{
    int size;

    if (!tp)
        return; // <-- This is the added sanity check

    size = tp->dmt_stree[DMTROOT];
    ...
}

By checking that the tp pointer is not NULL before dereferencing it, the risk of a NULL pointer dereference is eliminated, effectively mitigating the vulnerability.

References

1. Linux kernel source code: https://elixir.bootlin.com/linux/latest/source
2. JFS: https://en.wikipedia.org/wiki/JFS_(file_system)
3. Linux kernel mailing list discussion: https://lore.kernel.org/lkml/CA+55aFz+_zqMfHiGHuAvxw01XK8h1tG4LkBQaPcaPGkdmHaCxw@mail.gmail.com/

Conclusion

The CVE-2023-4385 vulnerability highlights the importance of thorough code review and continual testing to ensure the safety and stability of critical software systems like the Linux kernel. By understanding the root cause of the vulnerability and implementing the appropriate mitigation strategies, developers and system administrators can better protect their systems from potential exploits.

Timeline

Published on: 08/16/2023 17:15:00 UTC
Last modified on: 08/22/2023 22:45:00 UTC