CVE-2021-34556 in Linux
Summary
by MITRE • 08/02/2021
In the Linux kernel through 5.13.7, an unprivileged BPF program can obtain sensitive information from kernel memory via a Speculative Store Bypass side-channel attack because the protection mechanism neglects the possibility of uninitialized memory locations on the BPF stack.
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Analysis
by VulDB Data Team • 08/06/2021
The vulnerability identified as CVE-2021-34556 represents a critical security flaw in the Linux kernel affecting versions through 5.13.7. This issue stems from insufficient protection mechanisms against speculative execution side-channel attacks, specifically targeting the BPF (Berkeley Packet Filter) subsystem. The vulnerability exploits a fundamental weakness in how the kernel handles memory access patterns during speculative execution, creating a pathway for unauthorized information disclosure.
The technical flaw manifests through an improper handling of uninitialized memory locations within the BPF stack during speculative execution phases. When an unprivileged BPF program executes, it can trigger speculative store bypass conditions that allow the program to indirectly access kernel memory contents through side-channel information leakage. This occurs because the existing protection mechanisms fail to account for the possibility that memory locations on the BPF stack may remain uninitialized, creating potential attack vectors for information disclosure. The vulnerability specifically targets the interaction between the CPU's speculative execution engine and the kernel's memory management during BPF program execution.
The operational impact of this vulnerability is significant as it allows unprivileged users to potentially extract sensitive kernel memory information through carefully crafted BPF programs. Attackers can leverage this flaw to gain access to confidential data that should remain protected within kernel memory spaces, potentially including cryptographic keys, user credentials, or other sensitive system information. The vulnerability is particularly concerning because it operates without requiring elevated privileges, making it accessible to any user with the ability to load BPF programs. This creates a substantial risk for systems where BPF functionality is enabled, as it effectively bypasses traditional privilege boundaries and provides an indirect information leakage channel.
Mitigation strategies for CVE-2021-34556 include applying the patched kernel version that addresses the specific memory handling issue in the BPF subsystem. System administrators should also consider disabling BPF functionality if it is not essential for system operations, particularly on systems handling sensitive data. Additionally, implementing proper kernel hardening measures such as enabling KPTI (Kernel Page Table Isolation) and ensuring up-to-date microcode updates can help reduce the attack surface. Organizations should conduct thorough security assessments to identify systems running vulnerable kernel versions and implement monitoring for suspicious BPF program activities. The vulnerability aligns with CWE-200 (Information Exposure) and can be categorized under ATT&CK technique T1059.006 (Python) and T1566.001 (Phishing with Malicious Attachment) when considering exploitation vectors, though the primary concern remains the kernel-level information disclosure.