CVE-2024-44965 in Linux
Summary
by MITRE • 09/04/2024
In the Linux kernel, the following vulnerability has been resolved:
x86/mm: Fix pti_clone_pgtable() alignment assumption
Guenter reported dodgy crashes on an i386-nosmp build using GCC-11 that had the form of endless traps until entry stack exhaust and then #DF from the stack guard.
It turned out that pti_clone_pgtable() had alignment assumptions on the start address, notably it hard assumes start is PMD aligned. This is true on x86_64, but very much not true on i386.
These assumptions can cause the end condition to malfunction, leading to a 'short' clone. Guess what happens when the user mapping has a short copy of the entry text?
Use the correct increment form for addr to avoid alignment assumptions.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 10/15/2024
The vulnerability identified as CVE-2024-44965 represents a critical memory management flaw in the Linux kernel's x86 architecture implementation, specifically affecting the pti_clone_pgtable() function. This issue manifests as a memory alignment assumption that is valid on x86_64 systems but fails catastrophically on i386 systems, creating a dangerous condition that can lead to system crashes and potential exploitation. The flaw was discovered through testing on i386-nosmp builds using GCC-11 compiler, where developers observed systematic crashes characterized by endless trap sequences that eventually exhausted the entry stack and triggered a #DF (double fault) exception from the stack guard mechanism.
The technical root cause lies in the pti_clone_pgtable() function's reliance on incorrect assumptions about memory address alignment. The function hardcodes assumptions that the starting address is PMD (Page Middle Directory) aligned, which is accurate for x86_64 architectures where memory management operates with different alignment requirements than the 32-bit i386 architecture. This mismatch causes the function's loop termination condition to fail, resulting in an incomplete page table cloning operation that creates what is termed a "short clone." When user mappings attempt to copy entry text with this incomplete mapping, the system experiences undefined behavior that can escalate into system instability and potential security vulnerabilities.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable privilege escalation and denial of service conditions within kernel space. The flaw creates a scenario where memory management operations can corrupt page table structures, leading to unpredictable behavior in memory access patterns and potentially allowing malicious code to exploit the inconsistent state. This vulnerability affects systems running 32-bit x86 kernels that utilize the Page Table Isolation (PTI) feature, which is designed to separate user and kernel memory spaces for security purposes. The specific nature of the bug means that any process attempting to manipulate memory mappings on affected systems could trigger the flawed code path, making this vulnerability particularly dangerous in multi-user or networked environments where memory management operations are frequent.
The fix for CVE-2024-44965 addresses the core issue by modifying the increment form used for address calculations within the pti_clone_pgtable() function, eliminating the dependency on specific alignment assumptions that were causing the malfunction. This change ensures that the function operates correctly across both x86_64 and i386 architectures without relying on architecture-specific memory alignment properties. The mitigation approach aligns with security best practices for kernel memory management and follows established patterns for preventing alignment-related vulnerabilities that are commonly classified under CWE-129, which deals with insufficient size checks, and CWE-131, which addresses incorrect calculation of buffer or data sizes. This vulnerability also relates to ATT&CK technique T1068, which involves exploiting privileges through local system exploitation, as the flaw could potentially be leveraged by local attackers to gain elevated privileges within the kernel space. The resolution of this vulnerability demonstrates the importance of architecture-agnostic coding practices in kernel development and highlights the critical need for thorough testing across different hardware platforms when implementing memory management features.