CVE-2021-31829: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-31829 affects the Linux kernel through version 5.12.1, specifically in the kernel/bpf/verifier.c component. The vulnerability was discovered by Piotr Krysiuk and disclosed on May 4, 2021. The issue involves undesirable speculative loads in the BPF stack area that can lead to disclosure of kernel stack content via side-channel attacks (Openwall, NVD).

The vulnerability stems from insufficient protection of the BPF stack area against speculative loads. The specific concern is that when protecting BPF stack pointer against speculative pointer arithmetic, the BPF stack area itself remains unprotected. Additionally, the BPF stack can contain uninitialized data that might represent sensitive information previously operated on by the kernel. The vulnerability has been assigned a CVSS v3.1 Base Score of 5.5 (Medium) with vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N (NVD).

The vulnerability allows unprivileged local users to extract contents of up to 511 bytes from the BPF stack through side-channel attacks. The extracted contents may include addresses of kernel structures that could be used to defeat Kernel Address Space Layout Randomization (KASLR), potentially facilitating the exploitation of other vulnerabilities (Openwall).

A proof-of-concept exploit was developed that allows unprivileged local users to extract contents from the BPF stack. The PoC was shared privately with kernel developers to assist with fix development (Openwall).

The vulnerability was fixed through patches available in the BPF subsystem public git repository. The fix involves turning register-based arithmetic operations into immediate-based arithmetic operations without the need for masking. The patch is identified as commit 801c6058d14a82179a7ee17a4b532cac6fad067f (GitHub).