CVE-2024-53146 - NFSD: Preventing Potential Integer Overflow in the Linux Kernel

The Linux kernel is the core component of the Linux operating system, managing hardware resources, and providing essential functionalities. One of its significant components, NFS (Network File System) daemon (NFSD), recently had to deal with a vulnerability that could lead to integer overflow, a common security risk that arises from improper handling of integers in applications. This vulnerability has been assigned the identifier CVE-2024-53146.

The problem was related to the tag length in NFSD when it reaches a value near the maximum limit of an unsigned 32-bit integer (U32_MAX - 3). This would cause the "length + 4" addition to result in an integer overflow, potentially leading to undefined behavior in the kernel.

To better understand and address this issue, let's delve into the related code snippet.

Original Code Snippet

int decode_cb_compound4res(struct svc_rqst *rqstp, __be32 *p, struct cb_compoundres_state *cstate)
{
    u32 length = be32_to_cpup(p++);
    if (*p++ != rpc_success || !length)
        return ;
    length--; /* trim off the op */
    length *= 4;
    return nfs4svc_decode_compoundres(rqstp, p, cstate);
}

As seen in the code above, the function decode_cb_compound4res() computes the length value, which is then passed to the nfs4svc_decode_compoundres() function. Since the length value is unchecked and could potentially overflow, it poses a security risk.

Fix Code Snippet

To fix this vulnerability, a patch has been proposed to split the integer calculations into separate steps, avoiding any addition on the unsafe length value.

int decode_cb_compound4res(struct svc_rqst *rqstp, __be32 *p, struct cb_compoundres_state *cstate)
{
    u32 length = be32_to_cpup(p++);
    if (*p++ != rpc_success || !length)
        return ;

    u32 safe_length = length - 1U;
    if (safe_length >= U32_MAX / 4U)
        return -EINVAL;
    
    safe_length *= 4U;

    return nfs4svc_decode_compoundres(rqstp, p, cstate);
}

In this updated code, we now compute the safe_length variable by subtracting one from the original length value and then checking if the safe_length is within the acceptable range to avoid integer overflow. If it is outside the safe range, we return an error code (-EINVAL). Finally, we multiply safe_length by 4 and pass it to the nfs4svc_decode_compoundres() function.

With this solution, integer overflow is prevented, and the Linux kernel's NFSD component becomes more secure.

Original References

1. Linux kernel source code
2. CVE-2024-53146 Patch

Exploit Details

An attacker who has access to the NFS daemon's network might send malicious NFS requests with a large tag length value, potentially exploiting this vulnerability to cause an integer overflow. This could lead to undefined behavior, a crash in the kernel, or even arbitrary code execution.

However, with the fix provided in this article, the Linux kernel's NFSD component is now protected from this vulnerability, ensuring it properly handles integer values and reducing the risk of potential exploits.

Timeline

Published on: 12/24/2024 12:15:22 UTC
Last modified on: 01/20/2025 06:19:43 UTC