CVE-2021-35477 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 a certain preempting store operation does not necessarily occur before a store operation that has an attacker-controlled value.
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Analysis
by VulDB Data Team • 08/06/2021
The vulnerability identified as CVE-2021-35477 represents a critical side-channel attack vector within the Linux kernel that exploits speculative execution mechanisms to leak kernel memory contents. This flaw affects kernel versions through 5.13.7 and stems from improper handling of preempting store operations during BPF program execution. The issue specifically manifests when an unprivileged BPF (Berkeley Packet Filter) program can leverage speculative store bypass techniques to access sensitive data that should remain protected within kernel memory spaces. The vulnerability operates through the processor's speculative execution engine where certain memory operations are executed before their dependencies are fully resolved, creating potential information leakage pathways.
The technical implementation of this vulnerability involves a specific race condition between memory store operations that occurs during BPF program execution contexts. When a BPF program runs with unprivileged permissions, it can manipulate the timing and ordering of memory operations such that a speculative store bypass attack becomes feasible. The core flaw lies in the assumption that certain preempting store operations will always occur before attacker-controlled store operations, which is not guaranteed under all execution conditions. This misordering allows an attacker to potentially observe the contents of kernel memory through side-channel information leakage, where the timing differences in memory access patterns reveal sensitive data.
From an operational impact perspective, this vulnerability poses significant risks to system security and confidentiality. An unprivileged user can potentially extract kernel memory contents including cryptographic keys, passwords, and other sensitive information that should be protected from user-space access. The attack requires only basic user privileges and does not necessitate root access or elevated permissions, making it particularly dangerous in multi-user environments. The vulnerability affects systems running Linux kernel versions up to 5.13.7 and can be exploited across various hardware platforms that support speculative execution, including those with intel and amd processors. The impact extends beyond simple information disclosure as the leaked data could potentially enable further attacks or compromise system integrity.
Security mitigations for CVE-2021-35477 primarily focus on kernel-level patches and hardware mitigations. The primary fix involves updating to Linux kernel versions 5.13.8 or later, where the kernel developers implemented proper ordering constraints for memory operations during BPF execution. Additionally, system administrators should consider disabling BPF functionality if it is not required for specific use cases, though this may impact network monitoring and security tools that depend on BPF capabilities. Hardware mitigations include enabling microcode updates that address speculative execution vulnerabilities and potentially disabling speculative execution features where possible. The vulnerability aligns with CWE-200 (Information Exposure) and maps to ATT&CK technique T1005 (Data from Local System) and T1552 (Unsecured Credentials), representing a sophisticated attack vector that leverages modern processor features against kernel memory protection mechanisms.
The broader implications of this vulnerability highlight the challenges in securing modern operating systems against side-channel attacks that exploit hardware-level features. It demonstrates how seemingly benign kernel operations can create attack surfaces when combined with advanced processor capabilities. The vulnerability also underscores the importance of proper memory ordering guarantees in kernel code and the need for comprehensive testing of speculative execution behaviors. Organizations should implement regular kernel updates and maintain awareness of emerging vulnerabilities that target the interaction between software and hardware security features. The complexity of this attack vector requires security teams to consider not just traditional access control mechanisms but also the subtle interactions between kernel memory management and processor-level speculative execution features.