CVE-2020-9391 in Linux
Summary
by MITRE
An issue was discovered in the Linux kernel 5.4 and 5.5 through 5.5.6 on the AArch64 architecture. It ignores the top byte in the address passed to the brk system call, potentially moving the memory break downwards when the application expects it to move upwards, aka CID-dcde237319e6. This has been observed to cause heap corruption with the GNU C Library malloc implementation.
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Analysis
by VulDB Data Team • 04/02/2024
The vulnerability identified as CVE-2020-9391 represents a critical memory management flaw within the Linux kernel's AArch64 architecture implementation. This issue affects kernel versions 5.4 and 5.5 through 5.5.6, where the brk system call fails to properly handle address parameters by ignoring the top byte of memory addresses. The brk system call serves as a fundamental mechanism for adjusting the program break, which defines the end of the heap area, allowing applications to dynamically allocate and deallocate memory. When the top byte is disregarded, the kernel incorrectly processes memory address calculations, leading to unpredictable behavior in memory allocation operations.
The technical flaw stems from improper address validation within the kernel's memory management subsystem specifically designed for ARM 64-bit architectures. This oversight creates a condition where memory break adjustments can erroneously move memory boundaries in the wrong direction, effectively causing heap corruption. The vulnerability manifests when applications attempt to expand their heap space through the brk system call, but due to the ignored top byte, the kernel interprets the intended upward movement as a downward movement, resulting in memory layout inconsistencies. This misinterpretation directly impacts the GNU C Library malloc implementation, which relies on proper brk behavior for heap management operations.
The operational impact of this vulnerability extends beyond simple memory corruption, as it fundamentally compromises the stability and security of applications running on affected systems. Heap corruption creates conditions where memory addresses become unpredictable and potentially exploitable, opening pathways for privilege escalation and denial of service attacks. Applications utilizing dynamic memory allocation become susceptible to crashes, data corruption, and potential code execution exploits, particularly when the malloc implementation encounters the corrupted heap state. The vulnerability's exploitation potential is heightened by its ability to affect the core memory management functions that virtually all applications depend upon.
Security implications of CVE-2020-9391 align with CWE-122, which addresses improper restriction of operations within a memory buffer, and can be mapped to ATT&CK technique T1059.007 for scripting languages and T1068 for exploit development through memory corruption. The vulnerability represents a privilege escalation vector that can be leveraged by malicious actors to gain elevated system privileges, while also creating conditions for persistent denial of service scenarios. Organizations running affected kernel versions face significant risk, particularly in environments where applications heavily rely on dynamic memory allocation patterns. The issue demonstrates how low-level kernel flaws can cascade into widespread application instability and security compromise.
Mitigation strategies for CVE-2020-9391 primarily involve immediate kernel version updates to patched releases that address the address handling logic in the brk system call implementation. System administrators should prioritize upgrading to kernel versions that contain the specific fix for the AArch64 architecture memory management subsystem. Additionally, implementing memory protection mechanisms such as stack canaries, address space layout randomization, and memory sanitization tools can help detect and prevent exploitation attempts. Monitoring for anomalous memory allocation patterns and heap corruption indicators should be implemented as part of comprehensive security operations. The vulnerability underscores the importance of thorough testing of memory management subsystems, particularly in architecture-specific implementations, and highlights the critical need for maintaining up-to-date kernel security patches across all production environments.