The Linux kernel is one of the most widely used operating systems across the globe, powering millions of devices from small embedded systems to supercomputers. It is constantly being improved and updated to ensure that it remains secure, stable, and efficient. In this article, we will discuss a recently resolved vulnerability in the Linux kernel, specifically in the nvme-pci subsystem. The vulnerability is related to the improper handling of the HMB descriptor table, which could potentially cause unwanted behavior or issues within the system.

Vulnerability Details

The vulnerability, identified as CVE-2024-56756, is related to the nvme-pci subsystem in the Linux kernel. The nvme-pci subsystem is responsible for managing NVMe (Non-volatile Memory express) devices on PCI (Peripheral Component Interconnect) buses. NVMe devices are a type of high-speed solid-state storage commonly used in modern computer systems.

The issue lies in the handling of the HMB (Host Memory Buffer) descriptor table. The HMB descriptor table is sized according to the maximum number of descriptors that could potentially be used for a given device. However, under certain conditions, the __nvme_alloc_host_mem function could break out of its loop earlier than expected due to a memory allocation failure. This results in fewer descriptors being used than originally planned for, leading to an incorrect size being passed to the dma_free_coherent function.

In practical terms, this vulnerability has not been observed to cause significant issues, as the number of descriptors typically remains low and the dma coherent allocator always allocates and frees at least a page. However, it is still important to address this issue in order to maintain the stability and security of the Linux kernel.

The following code snippet is part of the fix implemented to resolve this vulnerability

static void __nvme_free_host_mem(struct nvme_ctrl *ctrl)
{
     struct nvme_hmb *hmb = &ctrl->hmb;
     int i;

     for (i = ; i < hmb->num_descs; i++) {
         dma_free_coherent(ctrl->dev, hmb->descs[i].block_size,
                 hmb->descs[i].block_virt, hmb->descs[i].block_dma);
     }
     kfree(hmb->descs);
}

This change ensures that the correct number of descriptors and their respective sizes are accounted for when freeing the HMB descriptor table.

For further information about this vulnerability and its resolution, you can refer to the original references:

- Linux Kernel Git Commit - Fix HMB descriptor table handling
- Linux Kernel Mailing List - NVMe driver fixes

Exploit Details and Impact

As previously mentioned, this vulnerability has not been observed to cause significant issues in practice. The dma coherent allocator always allocates and frees at least a page, which mitigates the impact of the incorrect size being passed to dma_free_coherent. However, it is essential to address this vulnerability to ensure the overall security, stability, and future-proofing of the Linux kernel and the systems that rely on it.

By applying the fix provided by the Linux kernel developers, users can ensure that their systems are not susceptible to any potential issues or attacks related to this vulnerability. It is highly recommended to keep your Linux kernel installation up to date and apply any relevant patches or updates to protect against known vulnerabilities.

Timeline

Published on: 12/29/2024 12:15:09 UTC
Last modified on: 01/06/2025 20:33:10 UTC