CVE-2025-39775: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-39775 is a vulnerability discovered in the Linux kernel related to memory management. The issue was disclosed and published to the CVE List on September 11, 2025. The vulnerability specifically affects the mremap functionality when used with userfaultfd on Virtual Memory Areas (VMAs) that span at least one Page Middle Directory (PMD) (NVD).

The vulnerability occurs when registering userfaultd on a VMA that spans at least one PMD and then performing mremap() operations on that VMA. This can trigger a WARN condition when recovering from a failed page table move due to a page table allocation error. While the code ultimately performs the correct operation by recursing and avoiding moving actual page tables, it generates an undesirable warning message. The issue manifests in the movepagetables function within the mm/mremap.c file (NVD).

The impact of this vulnerability appears to be limited to system logging and debugging functionality. While it triggers a warning message in the system, it does not appear to cause any direct system compromise or data loss, as the code eventually performs the correct operation (NVD).

The issue has been resolved in the Linux kernel through a fix that modifies the recovery process during page table moves. The solution involves checking for both VMAs and performing the check before the pmd_none() sanity check. A new helper function has been implemented to perform and document the check for both PMD and PUD levels (NVD).