CVE-2021-34556: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-34556 is a vulnerability discovered in the Linux kernel through version 5.13.7, where an unprivileged BPF program can obtain sensitive information from kernel memory via a Speculative Store Bypass side-channel attack. The vulnerability exists because the protection mechanism neglects the possibility of uninitialized memory locations on the BPF stack. This vulnerability was discovered by Benedict Schlueter and disclosed on August 1, 2021 (Openwall).

The vulnerability stems from insufficient protection against Speculative Store Bypass in the BPF subsystem. When identifying memory store operations to be protected, uninitialized BPF stack locations are not considered, and the BPF verifier never attempts to protect the first store operation for each BPF stack location. Additionally, the BPF stack is allocated without sanitizing preexisting memory content. This allows later load instructions that depend on the unprotected store to speculatively execute ahead of the store and use unsanitized memory (Openwall, Kernel Patch).

When exploited, this vulnerability allows an unprivileged attacker to disclose the content of arbitrary kernel memory through a side-channel attack. This can lead to information disclosure of sensitive kernel data, potentially compromising system security (NVD, Openwall).

The vulnerability has been fixed in the Linux kernel through patches that reimplement the mitigation to follow techniques recommended by CPU vendors. The fix is available in the mainline kernel git repository. For systems that cannot be immediately patched, the vulnerability can be mitigated by setting sysctl kernel.unprivileged_bpf_disabled=1, which disables BPF use by unprivileged users (Debian LTS, Openwall).