In the vast and complex world of computer systems, especially in the popular open-source operating system Linux, there are always new vulnerabilities coming to the fore that need attention and fixing. Recently, the Linux kernel faced such an issue that has now been resolved. As security researchers work tirelessly to find potential exploits on computer systems, one such vulnerability has been discovered and patched in the Linux kernel relating to the mt76: mt7615 subsystem. This post will provide an in-depth explanation of this security vulnerability, the associated code snippet, the original references, and the exploit details. So, let's dive into understanding CVE-2021-47033: mt76: mt7615 - fixing the Transmit (TX) Socket Buffer (SKB) Direct Memory Access (DMA) unmap issue.

To commence, it is essential to have a brief understanding of the components involved in this vulnerability, namely the mt76 driver and the mt7615 subsystem. The mt76 driver is a family of Linux kernel drivers for MediaTek (MT) devices that utilize the mac80211 framework, supporting the IEEE 802.11 networking protocol. Specifically, the mt7615 is a specific MediaTek chipset for wireless network devices that support fast Wi-Fi speeds and is widely used in routers and other wireless devices.

Now, let's dive into the actual vulnerability. The core issue lies in the way tx_skb_dma was handled in the mt7615 subsystem of the Linux kernel. Here is the specific code snippet that was problematic:

/* Unmap SKBs mainliner
 * The first pointer in txp needs to be unmapped as well */
__skb_queue_head_init(&txp_skb);
skb_queue_splice_init(&wcid->swq, &txp_skb);
skb_queue_walk_safe(&txp_skb, skb, tmp) {
	dev_kfree_skb_any(skb);
}

The issue with this code is that the first pointer in the txp (transmit) was not being unmapped, leading to a potential leak in DMA (Direct Memory Access). DMA is a method that allows input/output (I/O) devices to access the memory directly, without involving the Central Processing Unit (CPU). In this specific case, the txp needed to be unmapped to avoid leaking DMA mapping entries.

Luckily, as the Linux community is vigilant, this issue was identified, and a fix has been proposed and applied to the kernel. The updated code snippet takes into account the unmapping of the first pointer in the txp, thus solving the vulnerability:

/* Unmap SKBs mainliner
 * The first pointer in txp needs to be unmapped as well */
__skb_queue_head_init(&txp_skb);
skb_queue_splice_init(&wcid->swq, &txp_skb);
skb_queue_walk_safe(&txp_skb, skb, tmp) {
	dma_unmap_single(dev, skb_get_p(skb),
			 mt76_tx_len(skb), DMA_TO_DEVICE);
	dev_kfree_skb_any(skb);
}

The new code snippet now includes the function dma_unmap_single, which is responsible for unmapping the skb and mitigate the leak vulnerability.

For a detailed understanding, we recommend going through the original reference of this vulnerability and the fix:

- Linux Kernel Commit that fixes CVE-2021-47033

In conclusion, it is of utmost importance for developers and syst҉em administrators to apply patches and keep their Linux systems up-to-date to ensure the security and integrity of their systems and networks. CVE-2021-47033 is just one example of the many vulnerabilities that can exist in a system. By understanding the details of these exploits, you can better protect your infrastructure from potential threats.

Timeline

Published on: 02/28/2024 09:15:39 UTC
Last modified on: 12/12/2024 15:39:46 UTC