A critical race condition, tracked as CVE-2024-27022, was discovered and resolved in the Linux kernel, specifically affecting the fork() implementation with HugeTLB (huge page) memory mappings. This issue could potentially allow local attackers to exploit memory corruption by creating conditions for use-after-free, leading to system crashes or privilege escalation.

In this post, we’ll break down the cause, walk through the vulnerable flow, provide exclusive code insights, share references, and explain how the patch addresses the problem.

What Exactly Went Wrong?

The core of the vulnerability lies in how the kernel handled memory mapping (VMAs) for files backed by hugetlbfs during process forking.

- When a process with hugetlbfs-backed VMAs called fork(), memory mappings were being inserted into the system’s interval tree *before* they were fully initialized and safe to use.
- Operations like hugetlbfs_fallocate or hugetlbfs_punch_hole, which modify these areas, could race with the incomplete initialization—leading to dereferencing partially built or non-locked VMAs.

This scenario exposes an unsafe state window that could be abused.

Let’s walk through a simplified (yet accurate) race between two CPUs

// Pseudocode representing the race
CPU 1: fork() process
  dup_mmap()
    i_mmap_lock_write(mapping);
    vma_interval_tree_insert_after(); // <-- VMA now visible!
    i_mmap_unlock_write(mapping);
    hugetlb_dup_vma_private(); // initializes VMA (but now it's already public!)
    tmp->vm_ops->open();
      // vma_lock is allocated here, still outside proper lock

CPU 2: operation on hugetlb VMA (e.g., fallocate, punch_hole)
  i_mmap_lock_write(mapping);
  hugetlb_vmdelete_list()
    vma_interval_tree_foreach()
    hugetlb_vma_trylock_write(); // unsafe, as vma_lock is unset!

Explanation

- When fork() runs, the new VMA is linked to the interval tree before it's fully initialized (lock pointer, ops, etc.).
- Another thread (CPU 2) can see this incomplete VMA and operate on it, leading to unprotected, even erroneous, access.

Create a process mapping large regions of memory via hugetlbfs.

2. Repeatedly fork() while triggering operations like fallocate and punch_hole from other threads/CPUs.
3. With luck or heavy stress, hit the race condition where an incomplete VMA is manipulated—possibly causing a kernel crash (BUG, WARNING) or attacking a use-after-free window.

Potential impacts: Local denial of service (DoS), memory corruption, or (with further chaining) privilege escalation.

The Official Fix

The Linux kernel maintainers patched this by deferring the linking of the file VMA to the interval tree until it’s fully built and ready.

Key change: Instead of instantly exposing the VMA in the tree (and, thus, globally), they now postpone this step until the VMA’s structure, lock, and ops are entirely set.

Patch excerpt (source):

// old - inserts vma into tree *before* full init
vma_interval_tree_insert_after();
hugetlb_dup_vma_private(vma, ...);
tmp->vm_ops->open(tmp);

// new - build VMA *first*, then insert
hugetlb_dup_vma_private(vma, ...);
tmp->vm_ops->open(tmp);
vma_interval_tree_insert_after();

This guarantees: Any code seeing a VMA in the system can assume it’s fully initialized and has a valid lock.

Original References

- Linux kernel commit fixing CVE-2024-27022 (source code)
- Kernel.org stable mailing list announcement
- Thorvald’s bug report

To illustrate, here’s a simple C snippet that could stress this area (for research/testing only!)

#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>

int main() {
    int fd = open("/dev/hugepages/myfile", O_CREAT | O_RDWR, 0777);
    void *addr = mmap(NULL, 2 * 1024 * 1024, PROT_READ | PROT_WRITE,
                      MAP_SHARED, fd, );

    // In parallel, fork processes while also performing punch_hole
    if (fork() == ) {
        // Child: Try to punch holes / fallocate if supported
        // ...
    } else {
        // Parent: Repeatedly fork again
        // ...
    }
    // Clean up
    munmap(addr, 2 * 1024 * 1024);
    close(fd);
    return ;
}

Again, this is a demonstration of the *class* of vulnerability, not an actual exploit!

Impacted Systems & Patch Status

- Affected: Linux kernel versions prior to the fix (late Feb 2024); all systems using hugetlbfs + concurrent VM operations + fork().
- Patched: All major Linux distributions have begun rolling updates. *If you use hugepages/hugetlbfs, apply updates ASAP.*

Summary

CVE-2024-27022 is a subtle but critical example of the dangers in exposing internal kernel objects (like VMAs) before they’re fully ready. It exemplifies how high-concurrency and advanced memory management features like hugepages can lead to exploitable state races.

Recommendation: Always use the latest kernels, especially if you employ hugepages or shared memory in high-concurrency applications!

For full details, check the Linux kernel commit and keep your systems patched.


*Stay safe, and don’t leave your VMAs half-dressed!*

Timeline

Published on: 05/01/2024 06:15:21 UTC
Last modified on: 06/21/2024 14:15:11 UTC