The Common Vulnerabilities and Exposures (CVE) system has recently recorded a new vulnerability, CVE-2023-1095, that affects the Linux Kernel's nf_tables_updtable function. This vulnerability results in a NULL pointer dereference, which can cause the kernel to crash, leading to a denial of service. In this post, we'll delve into how this vulnerability came to be, how it occurs, and how it can be potentially exploited.

Background on nf_tables

Before we dive into the vulnerability, let's first understand what nf_tables is. Netfilter is a framework inside the Linux kernel responsible for packet filtering, network address translation, and other packet mangling tasks. nf_tables is a new subsystem in the Netfilter framework designed to offer better performance, expressiveness, and ease of use compared to the older iptables, ip6tables, and other tools.

The Vulnerability: CVE-2023-1095

As mentioned earlier, this vulnerability resides in the nf_tables_updtable function in the Linux kernel and is caused by a specific error condition that leads to a NULL pointer dereference.

Let's look at the code snippet that causes the issue

int nf_tables_updtable(struct nft_ctx *ctx, struct nft_table *table)
{
    struct nft_trans *trans;
    int err;

    trans = nft_trans_alloc(ctx, NFT_TRANS_TABLE, sizeof(*trans));
    if (IS_ERR(trans))
        return PTR_ERR(trans);

    nft_trans_table_update(trans, table);

    err = nf_tables_table_enable(ctx, trans);
    if (err < ) {
        nft_trans_destroy(trans);
        return err;
    }

    list_add_tail(&trans->list, &ctx->net->nft.commit_list);
    return err;
}

In the above code, nf_tables_updtable creates a new transaction object using nft_trans_alloc. Then, it calls the nf_tables_table_enable function. If that function returns an error, it proceeds to call nft_trans_destroy, which subsequently causes a NULL pointer dereference.

Let's have a quick look at the nft_trans_destroy function

void nft_trans_destroy(struct nft_trans *trans)
{
    list_del(&trans->elist); // NULL pointer dereference occurs here
    kfree(trans); // frees the transaction object
}

The problem occurs because the transaction object was never placed on a list before calling nft_trans_destroy. Therefore, the transaction's list head is uninitialized, leading to a NULL pointer dereference in the list_del function.

Potential Exploit and Mitigation

Although CVE-2023-1095 presents a kernel crash possibility, exploiting it for malicious intent may prove challenging as it requires specific conditions. Attackers may attempt to trigger the nf_tables_table_enable error, causing a denial-of-service (DoS) attack. However, the likelihood of this exploit depends on the system's overall configuration and an attacker's ability to craft specifically tailored network packets.

In terms of mitigation, the vulnerability might be addressed by ensuring that a transaction object is added to a proper list before calling the nft_trans_destroy function. This should prevent the NULL pointer dereference from happening. It's important to follow the latest security advisories related to the Linux Kernel and apply updates and patches as they become available.

Original References

1. Linux Kernel Mailing List (LKML) Bug Report
2. CVE-2023-1095 on the CVE List

Conclusion

CVE-2023-1095 serves as a reminder that even widely used, robust systems like the Linux kernel can be susceptible to vulnerabilities. While this particular issue may not be trivial to exploit, it's crucial to stay informed of the latest vulnerabilities and ensure that your systems are up to date. By maintaining a strong security posture, we can prevent potential exploitation of such vulnerabilities and ensure the stability and safety of our digital systems.

Timeline

Published on: 02/28/2023 23:15:00 UTC
Last modified on: 03/06/2023 14:41:00 UTC