CVE-2024-36943 in Linux
Summary
by MITRE • 05/30/2024
In the Linux kernel, the following vulnerability has been resolved:
fs/proc/task_mmu: fix loss of young/dirty bits during pagemap scan
make_uffd_wp_pte() was previously doing:
pte = ptep_get(ptep); ptep_modify_prot_start(ptep); pte = pte_mkuffd_wp(pte); ptep_modify_prot_commit(ptep, pte);
But if another thread accessed or dirtied the pte between the first 2 calls, this could lead to loss of that information. Since ptep_modify_prot_start() gets and clears atomically, the following is the correct pattern and prevents any possible race. Any access after the first call would see an invalid pte and cause a fault:
pte = ptep_modify_prot_start(ptep); pte = pte_mkuffd_wp(pte); ptep_modify_prot_commit(ptep, pte);
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Analysis
by VulDB Data Team • 10/01/2025
This vulnerability exists in the linux kernel's memory management subsystem, specifically within the proc filesystem's task memory management interface. The issue affects the pagemap scan functionality which is used to expose memory mapping information to user space processes through the /proc filesystem. The vulnerability stems from a race condition in the make_uffd_wp_pte() function that handles userfaultfd write protection operations on page table entries. According to the common weakness enumeration framework, this represents a race condition vulnerability categorized under CWE-362, which deals with concurrent execution using locks and other synchronization mechanisms.
The technical flaw occurs when the kernel attempts to modify page table entries to enable userfaultfd write protection. The original implementation called ptep_get() to retrieve the page table entry, then invoked ptep_modify_prot_start() which atomically retrieves and clears the entry, followed by pte_mkuffd_wp() to mark the entry for userfaultfd write protection, and finally ptep_modify_prot_commit() to commit the modified entry. However, if another thread accessed or dirtied the page table entry between the ptep_get() call and ptep_modify_prot_start() call, that access information would be lost. This happens because ptep_modify_prot_start() clears the entry atomically, so any subsequent access to the entry would see an invalid state and trigger a page fault rather than preserving the original access information.
The operational impact of this vulnerability is significant for systems that rely on precise memory tracking and monitoring through the pagemap interface. When a process performs memory scans or monitoring operations, the loss of young/dirty bits can result in incorrect memory access tracking, potentially leading to data corruption or information leakage. Attackers could exploit this by crafting specific timing conditions to cause memory access information to be lost during concurrent memory management operations. The vulnerability affects systems using userfaultfd functionality and could impact memory management monitoring tools, performance profilers, and security auditing systems that depend on accurate pagemap information. This aligns with attack techniques described in the attack tree framework where adversaries may leverage race conditions to manipulate system state or information flow. The fix ensures that the page table entry modification sequence properly handles concurrent access by using the correct atomic pattern that prevents race conditions and preserves access information during the modification process.
The recommended mitigations include updating to a patched kernel version that implements the corrected page table entry modification sequence. System administrators should prioritize applying this patch across all production systems, particularly those running memory-intensive applications or security monitoring tools. Monitoring should be implemented to detect unusual memory access patterns that might indicate exploitation attempts. Additionally, organizations should review their memory management monitoring configurations to ensure they account for proper handling of page table entry modifications. The fix aligns with secure coding practices that emphasize proper synchronization mechanisms and atomic operations in concurrent environments, addressing the fundamental race condition that could allow information loss during critical memory management operations.