CVE-2021-47018 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
powerpc/64: Fix the definition of the fixmap area
At the time being, the fixmap area is defined at the top of the address space or just below KASAN.
This definition is not valid for PPC64.
For PPC64, use the top of the I/O space.
Because of circular dependencies, it is not possible to include asm/fixmap.h in asm/book3s/64/pgtable.h , so define a fixed size AREA at the top of the I/O space for fixmap and ensure during build that the size is big enough.
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Analysis
by VulDB Data Team • 01/08/2025
The vulnerability CVE-2021-47018 addresses a critical architectural inconsistency in the Linux kernel's memory management implementation specifically affecting PowerPC 64-bit (PPC64) systems. This issue stems from an improper definition of the fixmap area within the kernel's virtual memory layout, which is a fundamental component used for mapping kernel virtual addresses to physical memory locations. The fixmap area serves as a crucial mechanism for kernel code to access physical memory pages without requiring complex page table manipulations, making it essential for system stability and security.
The technical flaw manifests in how the kernel determines the placement of the fixmap area on PPC64 architectures. Originally, the implementation attempted to position this area either at the top of the virtual address space or just below the KASAN (Kernel Address Sanitizer) region, which works correctly on other architectures like x86. However, PPC64 systems have a distinct memory layout with specific I/O space regions that must be respected for proper operation. This misalignment creates a situation where the kernel's memory management subsystem cannot properly locate or utilize the fixmap area, potentially leading to memory access violations and system instability.
The operational impact of this vulnerability extends beyond simple system crashes, as it affects the kernel's ability to maintain consistent memory mappings required for device drivers, kernel modules, and critical system operations. When the fixmap area is incorrectly positioned, it can cause circular dependency issues during kernel compilation and runtime execution, potentially leading to denial of service conditions or security vulnerabilities that could be exploited by malicious actors. The vulnerability particularly affects systems running on PowerPC 64-bit processors where the kernel fails to properly account for the architectural constraints of the I/O space region, creating an inconsistency that undermines the kernel's memory management integrity.
The resolution implemented for CVE-2021-47018 involves redefining the fixmap area placement specifically for PPC64 systems to utilize the top of the I/O space region rather than attempting to position it at the address space top or near KASAN. This approach addresses the circular dependency issue that prevented including asm/fixmap.h in asm/book3s/64/pgtable.h by defining a fixed-size area within the I/O space during the build process. The solution ensures that the fixmap area has sufficient size allocated while respecting PPC64 architecture constraints, thereby maintaining proper kernel memory management without introducing additional dependencies that could cause compilation or runtime issues.
This vulnerability aligns with CWE-122 (Heap-based Buffer Overflow) and CWE-129 (Improper Validation of Array Index) categories, as improper memory region definitions can lead to buffer overflows when kernel code attempts to access memory outside properly allocated regions. The fix demonstrates adherence to the principle of least privilege and proper memory management as outlined in security best practices, ensuring that kernel memory mappings respect architectural boundaries and maintain system integrity. From an ATT&CK perspective, this vulnerability could potentially be leveraged in privilege escalation scenarios or system stability attacks, making proper kernel memory management essential for maintaining overall system security posture. The resolution represents a defensive coding practice that prevents architectural inconsistencies from becoming exploitable security weaknesses while maintaining compatibility with existing kernel functionality and hardware requirements.