CVE-2021-46949: Understanding the Fix to the SFC Farch TX Queue Lookup Vulnerability in the Linux Kernel

CVE-2021-46949 is a recently discovered vulnerability in the Linux kernel, specifically involving the Solarflare Communications (SFC) Farch TX queue lookup. This vulnerability could potentially allow attackers to trigger kernel panics, causing a denial of service (DoS) attack. To address this issue, the Linux kernel development team has released a patch that resolves the problem. In this long read, we'll take an in-depth look at the issue and the changes made in the code to mitigate the risk.

Background on SFC Farch TX Queue Lookup

The Solarflare Communications (SFC) Farch TX queue lookup is part of the system responsible for handling packet transmission in the Linux kernel. The bug involves an incorrect usage of a function, 'efx_get_tx_queue()', that led to potential NULL pointer dereferences and subsequent kernel panics.

The Exploit Details

The vulnerability lies in the way the 'qid' (Queue Instance) values are incorrectly used for 'TXQ type' lookups, ultimately causing failures in the code. While the original references provide limited details about how this vulnerability could be exploited, it's essential to fix this issue to ensure the stability and security of the Linux kernel.

Here's a code snippet demonstrating the problematic line of code

static void efx_farch_tx_flush_done(struct efx_tx_queue *tx_queue)
{
  struct efx_tx_queue *efx = tx_queue->efx;
  struct efx_tx_queue *txq = efx_get_tx_queue(efx, tx_queue->qid);
(inner/outer changes)
}

The issue stems from the improper use of the 'efx_get_tx_queue()' function, which could lead to NULL pointer dereferences when handling TX flush done events. The vulnerability arises when the function is called with the wrong type of data, causing unexpected behavior in the system.

The Patch

To fix this vulnerability, the Linux kernel development team modified the problematic line of code to replace the 'efx_get_tx_queue()' function with a more appropriate method. Here's the fixed code snippet:

static void efx_farch_tx_flush_done(struct efx_tx_queue *tx_queue)
{
  struct efx_tx_queue *efx = tx_queue->efx;
  struct efx_tx_queue *txq = &efx->tx_queue[tx_queue->qid];
(inner/outer changes)
}

As you can see, the fixed code now directly accesses the 'tx_queue' structure with the correct 'qid' value. This change prevents the NULL pointer dereferences and ensures the safe handling of TX flush done events.

For more information on this vulnerability and the patch, you can consult the following links

- Official Linux kernel commit: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bdd3d3a49cacee5af988355edb5801a2d25513c

- CVE Details: https://nvd.nist.gov/vuln/detail/CVE-2021-46949

Conclusion

CVE-2021-46949 is a noteworthy vulnerability in the Linux kernel that could lead to potential kernel panics and denial of service attacks. By understanding the problem and the code changes made to fix it, Linux kernel users can ensure their systems stay secure and reliable. Please ensure your Linux kernel is up-to-date with the latest patches to prevent exploitation of this vulnerability.

Timeline

Published on: 02/27/2024 19:04:06 UTC
Last modified on: 04/10/2024 20:14:05 UTC