Linux Kernel PMD Migration Entry Dereference Vulnerability in Huge Memory Management

A vulnerability in the Linux kernel's handling of transparent huge pages (THP) can lead to invalid memory access. This issue arises during the migration of THP when there is concurrent access to the Page Middle Directory (PMD) migration entry, potentially causing a page fault. The vulnerability has been addressed by implementing a check on the PMD migration entry to prevent invalid access. The issue was discovered by syzkaller on an internal kernel and confirmed on upstream versions.

Impact

Exploitation of this vulnerability can cause a page fault due to an invalid memory access, disrupting normal process execution.

Reproduction

The vulnerability can be reproduced by migrating a transparent huge page while another process concurrently accesses the PMD migration entry. This scenario can lead to an invalid address access, causing a page fault.