CVE-2025-68211 ksm: use range-walk function to jump over holes in scan_get_next_rmap_item

What is CVE-2025-68211?

CVE-2025-68211 is a medium-severity denial of service vulnerability in Linux kernel, published on 16 Dec 2025. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

In the Linux kernel, the following vulnerability has been resolved:

ksm: use range-walk function to jump over holes in scan_get_next_rmap_item

Currently, scan_get_next_rmap_item() walks every page address in a VMA to locate mergeable pages. This becomes highly inefficient when scanning large virtual memory areas that contain mostly unmapped regions, causing ksmd to use large amount of cpu without deduplicating much pages.

This patch replaces the per-address lookup with a range walk using walk_page_range(). The range walker allows KSM to skip over entire unmapped holes in a VMA, avoiding unnecessary lookups. This problem was previously discussed in [1].

Consider the following test program which creates a 32 TiB mapping in the virtual address space but only populates a single page:

#include <unistd.h> #include <stdio.h> #include <sys/mman.h>

/* 32 TiB */ const size_t size = 32ul * 1024 * 1024 * 1024 * 1024;

int main() { char *area = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_NORESERVE | MAP_PRIVATE | MAP_ANON, -1, 0);

if (area == MAP_FAILED) { perror("mmap() failed\n"); return -1; }

/* Populate a single page such that we get an anon_vma. */ *area = 0;

/* Enable KSM. */ madvise(area, size, MADV_MERGEABLE); pause(); return 0; }

$ ./ksm-sparse & $ echo 1 > /sys/kernel/mm/ksm/run

Without this patch ksmd uses 100% of the cpu for a long time (more then 1 hour in my test machine) scanning all the 32 TiB virtual address space that contain only one mapped page. This makes ksmd essentially deadlocked not able to deduplicate anything of value. With this patch ksmd walks only the one mapped page and skips the rest of the 32 TiB virtual address space, making the scan fast using little cpu.

Which versions are affected by CVE-2025-68211?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2025-68211?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2025-68211 being exploited?

There is no known exploitation of CVE-2025-68211 in the wild and no public exploit code.

Other vulnerabilities in Linux kernel

• CVE-2026-53266 Linux Kernel Out-of-Bounds Write Exploited High 8.8
• CVE-2013-6282 Linux Kernel Improper Input Validation Exploited High 8.8
• CVE-2022-0185 Linux Kernel Heap-Based Buffer Overflow Exploited High 8.4
• CVE-2013-2094 Linux Kernel Privilege Escalation Exploited High 8.4
• CVE-2026-53362 Linux Kernel Unspecified Exploited High 7.8

All Linux kernel vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as AZL-72419, AZL-78398, BELL-CVE-2025-68211, DEBIAN-CVE-2025-68211, GHSA-qhqr-mc3h-hmjp, ROOT-OS-DEBIAN-11-CVE-2025-68211 and 5 more.