CVE-2021-46951 - A Fix for Integer Underflow in the Linux Kernel TPM EFI Module

The Linux Kernel is the core software component of all Linux distributions. It is responsible for managing hardware resources and facilitating communication between hardware and software components. In this "long read" post, we will discuss the recently resolved vulnerability related to an integer underflow within the trusted platform module (TPM) support in the Linux kernel, referred to as CVE-2021-46951. We will dive into the root cause of the issue, walk through the code changes that fixed the vulnerability, and provide links to related resources.

Issue Details

The vulnerability CVE-2021-46951, described as "tpm: efi: Use local variable for calculating final log size," happens when the tpm_read_log_efi function is called multiple times, causing the global variable efi_tpm_final_log_size to eventually become a negative number due to the subtraction of final_events_preboot_size. Using a local variable instead of a global one prevents this integer underflow.

Root Cause

The root cause of this issue is the usage of a global variable (efi_tpm_final_log_size) for calculating final log size. When the tpm_read_log_efi function is called multiple times, the global variable becomes a negative number, leading to an integer underflow and potentially causing unexpected behavior or crashes in the kernel.

Fix:

To fix this vulnerability, a local variable is used instead of the global variable for calculating the final log size. The change in the code ensures that repeated calls to tpm_read_log_efi don't affect the value of the global variable, thus preventing the integer underflow.

References

1. The Linux kernel Git repository, where you can find the commit that introduced the fix for CVE-2021-46951: Linux kernel commit

2. The CVE database entry for CVE-2021-46951, providing a brief summary of the issue and related details: CVE-2021-46951

Conclusion

CVE-2021-46951 highlights the importance of proper variable scoping and careful handling of integer operations in software programming, especially in critical components like the Linux kernel. Fortunately, a timely fix has been introduced to address this vulnerability, so it is essential to ensure that your kernel is updated to a version that includes the patch for CVE-2021-46951.

Stay informed about kernel vulnerabilities and fixes by subscribing to the Linux Kernel Mailing List and regularly checking the CVE database.

Timeline

Published on: 02/27/2024 19:04:06 UTC
Last modified on: 04/10/2024 20:15:55 UTC