In June 2024, an important security issue was fixed in the Linux kernel’s crypto subsystem, specifically in the Broadcom (bcm) hardware accelerator driver. This vulnerability, now tracked as CVE-2024-56681, could have allowed improper initialization of hashing operations if memory allocation failed, possibly leading to system instability and unexpected behavior.

Let's break down what happened, why it's important, and take a peek at the code behind the fix.

What’s CVE-2024-56681 About?

The vulnerability was in the ahash_hmac_init function used by the Broadcom crypto driver under Linux. The purpose of this function is to initialize hash-based message authentication codes (HMACs), which are widely used in data integrity checks and cryptography. Inside this function, it called another function, ahash_init, to set up the core hashing context.

Here's the catch

- If ahash_init fails (for example, because the system ran out of memory and returned -ENOMEM), ahash_hmac_init would still say everything is OK and return .

This leads the caller to think initialization succeeded, while it actually failed under the hood.

- Consequences: The rest of the cryptographic process might use uninitialized memory, causing security holes, cryptographic failures, or crashes.

Let’s simplify the buggy pattern into a minimal C code snippet

int ahash_init(struct ahash_request *req)
{
    ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
    if (!ctx)
        return -ENOMEM;
    // ...more setup...
    return ;
}

int ahash_hmac_init(struct ahash_request *req)
{
    // Call lower-level init function
    ahash_init(req);  // <-- Error not checked!
    // ...proceed assuming everything is OK...
    return ;  // Always returns success!
}

Here, you see: ahash_hmac_init calls ahash_init, but if ahash_init fails, it’s ignored.

The Patch: Proper Error Handling

The Linux kernel maintainers fixed this by *checking* the return value and properly propagating the error up the call stack. The corrected code now looks more like:

int ahash_hmac_init(struct ahash_request *req)
{
    int ret = ahash_init(req);
    if (ret)
        return ret;  // Propagate error!
    // ...continue setup since we're all good...
    return ;
}

This means any caller (and in the end, any user or kernel subsystem) will be informed right away if something went wrong during initialization.

Can This Be Exploited?

- Privilege Required: You’d need to be able to make use of the hardware crypto driver, which is often only possible as a kernel module or from privileged userspace.
- Impact: If an attack triggers or forces memory allocation failure at the right time (e.g., by exhausting system memory), the crypto driver could be tricked into later using uninitialized memory, potentially leading to unpredictable results—crashes, exposure of sensitive info, or corrupted authentication.
- Practicality: It’s not the easiest bug to exploit, but the risk is that on a stressed or specially crafted system, cryptographic guarantees fail quietly.

### Proof of Concept / Trigger

An attacker could fill up system memory until kmalloc fails, then trick a program or operation to kick off an HMAC operation relying on the Broadcom driver. The vulnerable function would (before the fix) proceed as if everything was fine.

- Commit fixing CVE-2024-56681 in the Linux kernel
- Kernel mailing list discussion
- NVD entry for CVE-2024-56681 _(May be pending update)_

TL;DR

CVE-2024-56681 was a bug where the Linux Broadcom crypto driver didn't check for memory allocation failures during HMAC initialization—fixed now! This highlights why checking for errors every single time is not just a formality, but a foundation of secure code.

Stay safe, and always check those return values!

*This post was written exclusively for you, based on the latest changes in the Linux kernel as of June 2024.*

Timeline

Published on: 12/28/2024 10:15:09 UTC
Last modified on: 05/04/2025 10:02:08 UTC