CVE-2021-29155: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-29155 was discovered in the Linux kernel through 5.11.x, specifically affecting the kernel/bpf/verifier.c component. The vulnerability was identified by Piotr Krysiuk and Benedict Schlueter, and was disclosed in April 2021. The issue affects the Linux kernel's mechanism to mitigate speculatively out-of-bounds loads, where the BPF verifier performs undesirable out-of-bounds speculation on pointer arithmetic (MITRE CVE, Ubuntu Security).

The vulnerability exists in the kernel's BPF verifier where, for sequences of pointer arithmetic operations, the pointer modification performed by the first operation is not correctly accounted for when restricting subsequent operations. This leads to a gap in the Spectre mitigation implementation. The issue specifically affects how the verifier handles pointer arithmetic in kernel/bpf/verifier.c, allowing for undesirable out-of-bounds speculation (OSS Security).

The vulnerability allows unprivileged BPF programs running on affected systems to bypass Spectre mitigations and execute speculatively out-of-bounds loads from kernel memory. This can be exploited via side-channel attacks to extract sensitive information from kernel memory. A proof-of-concept demonstrated the ability for unprivileged local users to extract contents of a 31 KByte window within the kernel memory (OSS Security).

The vulnerability can be exploited by local unprivileged users who can run BPF programs on the affected system. A working proof-of-concept exploit was developed and shared privately with kernel developers to assist with fix development (OSS Security).

The issue was fixed through a series of patches in the Linux kernel mainline. The fix involved refactoring the BPF verifier logic to properly account for pointer modifications in arithmetic operations. Multiple Linux distributions have released security updates to address this vulnerability, including Ubuntu, Debian, and Fedora (Debian Security, Ubuntu Security).