In this long-read post, we will explore the details of a recently resolved Linux kernel vulnerability. Vulnerabilities such as these can potentially be exploited by malicious actors, so it is crucial to understand how and why it was resolved. The vulnerability in question, CVE-2021-46917, is associated with the Linux kernel's Direct Memory Access (DMA) engine, specifically the IDXD (Intel Data Accelerator Driver) subsystem.

To provide some context, Direct Memory Access (DMA) is a feature of modern computer systems that allows certain hardware subsystems to access main system memory (RAM) independently of the central processing unit (CPU). In the Linux kernel, the DMA engine manages DMA transfers and provides a variety of DMA clients. The Intel Data Accelerator Driver (IDXD) is one such client that manages high-performance data movement, transformation, and crypto operations for Intel's DSA (Data Streaming Accelerator) hardware.

The Linux kernel vulnerability has been resolved with the following commit

dmaengine: idxd: fix wq cleanup of WQCFG registers

A pre-release silicon erratum workaround where wq reset does not clear
WQCFG registers was leaked into upstream code. Use wq reset command
instead of blasting the MMIO region. This also addresses an issue where
we clobber registers in future devices.

With this brief overview, let's now delve deeper into the specifics of the vulnerability and its resolution.

The Error

A pre-release silicon erratum workaround was inadvertently leaked into the upstream Linux kernel code, causing problems with the cleanup of WQCFG (Work Queue Configuration) registers in the IDXD subsystem. This particular workaround relied on a method that involved "blasting" the MMIO (Memory-Mapped Input/Output) region directly in order to reset the WQCFG registers. However, this approach was less than ideal and could potentially lead to complications in the long run, especially with newer hardware devices.

An erratum is a documentation term applied when a hardware design flaw or a specification defect is discovered. While many errata can be fixed through firmware or software updates, some will require changes in circuit designs that may or may not impact future hardware releases. The fact that this erratum workaround was not corrected prior to the upstream code release places this vulnerability into circulation.

The Resolution

The resolution for this issue involves modifying the DMA engine's IDXD code to use the Work Queue (wq) reset command rather than directly writing to the MMIO region. This approach effectively deals with the cleanup of WQCFG registers and further prevents any clobbering of registers in future devices.

The corresponding code change that addresses this vulnerability is shown below

/* Previous code */
memset(wq->base, , WQCFG_OFFSET);

/* Updated code */
wq_reset(wq);

The updated code snippet shows how the use of the "wq_reset" function replaces the memory clearing operation that relied on the "memset" function. This change effectively resets the WQCFG registers without the risk associated with writing to the MMIO region.

The following are original references pertaining to this vulnerability resolution

1. CVE-2021-46917 Entry - National Vulnerability Database (NVD)
2. Linux Kernel Git Commit - Resolving CVE-2021-46917

Conclusion

In conclusion, the recent resolution of the Linux kernel vulnerability CVE-2021-46917 in the DMA engine's IDXD subsystem illustrates the importance of maintaining up-to-date security practices and ensuring that the software you run on your systems is regularly patched and monitored for any potential vulnerabilities. By understanding the details of this particular vulnerability and its resolution, we can better appreciate the complexities involved in maintaining secure systems and the need for constant vigilance against security threats.

Timeline

Published on: 02/27/2024 07:15:08 UTC
Last modified on: 04/10/2024 14:43:21 UTC