A recently discovered vulnerability in the Linux kernel, specifically in the f2fs (Flash-Friendly File System) module, has been addressed and resolved. This vulnerability, assigned CVE-2021-47004, could potentially lead to data corruption and system crashes. This blog post will discuss the details of the vulnerability, provide code snippets to help understand the issue, and outline the steps taken to address it. We'll also include links to the original references for those interested in diving deeper into the subject matter.

Background

The f2fs module was designed to optimize the performance of NAND flash memory storage devices. However, as with any software, vulnerabilities may be discovered that could lead to unintended behavior. In this case, the issue arises from the implementation of the get_victim() function, which is responsible for selecting the victim section and segment during garbage collection (GC) in CP (Checkpoint) disabling mode.

Exploit Details

There are two primary issues when using LFS (Log-Structured File System) or SSR (Static Stream Rate) | AT_SSR mode to select victims:
1. LFS is set to find the source section during GC, and the victim should have no checkpointed data. This is because, after the GC, the section could not be set free for reuse. Previously, the implementation checked valid chpt (checkpoint) blocks in the current segment rather than the section, which was incorrect.
2. SSR | AT_SSR modes are set to find target segments for writes that can be fully filled by checkpointed and newly written blocks. These segments should never be selected, or it can cause panic or data corruption during allocation. A potential case for this issue is described below:

Here's a snippet of the updated code that addresses the issues described above

/* check there is no checkpointed valid blocks to avoid data corruption */
if (__has_curseg_enough_space(sbi)) {
	seq = GET_SEC_FROM_SEG(sbi, curseg->segno);
	for (i = ; i < sbi->segs_per_sec; i++) {
		if (get_valid_blocks(sbi, GET_SEG_FROM_SEC(sbi, seq) + i) > )
			return false;
	}
}

This modification ensures that the get_victim() function behaves as intended by avoiding the selection of segments with checkpointed data, thus mitigating the risk of data corruption and system crashes.

Original References

For more information and a deeper understanding of the vulnerability and its resolution, please refer to the following resources:

1. Linux kernel commit that addresses the vulnerability: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=febbdd29c476508736ac8099937c3c6b1f23640c
2. CVE-2021-47004 details on NIST NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47004

Conclusion

In conclusion, the get_victim() function's vulnerability (CVE-2021-47004) within the Linux kernel's f2fs module has been resolved, thus mitigating the risk of data corruption and system crashes. By becoming aware of these issues, users can ensure they are running secure, updated software. Don't forget to consult the referenced materials for a deeper understanding of the vulnerability and its resolution!

Timeline

Published on: 02/28/2024 09:15:38 UTC
Last modified on: 02/28/2024 14:06:45 UTC