In June 2024, a crucial bug in the Linux kernel’s huge page (hugetlb) memory subsystem was identified and patched. This long read breaks down what happened, how the bug worked, potential consequences, and the patch changes, using simple language and exclusive, step-by-step walkthroughs for researchers and system administrators.

What is CVE-2024-39477?

CVE-2024-39477 describes a flaw in how the Linux kernel's hugetlb (huge page) memory manager handled certain memory errors, specifically ENOMEM (out of memory) conditions inside the mm/hugetlb.c code. The issue could cause inconsistencies in kernel memory accounting and, in rare cases, trigger kernel panics or potential memory leaks.

If the kernel runs out of memory during reservation, it would incorrectly proceed as if it succeeded

- This could cause kernel functions to operate on invalid/unallocated memory areas

Here is the original bug report:

sysbot splat on __unmap_hugepage_range()

The Original Vulnerability: Step-by-Step

Let’s walk through what went wrong, in plain English.

Error Handling:

- If BIG memory allocation fails (e.g., running low on memory and kmalloc returns NULL), vma_needs_reservation() returns -ENOMEM.

Old code would continue and call vma_add_reservation() even though nothing was allocated.

- This created an *accounting mismatch*—other functions (like region_del() and region_abort()) would think there are reservations to manage, and behave incorrectly.

The Problem in Pseudocode

int ret = vma_needs_reservation(vma, start, end);
if (ret >= ) {
    vma_add_reservation(vma, start, end);
}

But if ret == -ENOMEM, it shouldn't do anything else—however, old code would continue anyway!

Fix Overview

The patch simply checks for -ENOMEM from vma_needs_reservation() and skips further steps if memory allocation failed. This avoids later kernel confusion or accesses to missing memory.

Before: Vulnerable Code

int ret = vma_needs_reservation(vma, start, end);
if (ret)
    vma_add_reservation(vma, start, end);
...

// No special handling for -ENOMEM

After: Fixed Code

int ret = vma_needs_reservation(vma, start, end);
if (ret < ) {
    if (ret == -ENOMEM)
        hugetlb_restore_reserve = false; // Pretend we consumed the reserve
    return ret;
}
if (ret)
    vma_add_reservation(vma, start, end);

On -ENOMEM, we do not call vma_add_reservation(), and we clear the reservation flag.

Reference:
Exact Patch Commit on Kernel.org
(*Replace PATCH_COMMIT_HASH with the actual one once available*)

What Could Go Wrong?

- Use-After-Free / Null Dereference: Later code expects reservation structures to exist but they do not.
- Kernel Panic / Crash: System instability or outright kernel panics (observed in “splat” reports).
- Denial of Service: Remote code, an unprivileged local user, or misconfigured applications can *possibly* crash the machine by exhausting memory under certain conditions.

Exploit Sketch (DoS)

// Sample pseudo-code to stress kernel reservations and cause ENOMEM
#include <sys/mman.h>
#include <unistd.h>

int main() {
    size_t hugepage_size = 2 * 1024 * 1024; // 2MB Huge page
    for (;;) {
        void *ptr = mmap(NULL, hugepage_size, PROT_READ|PROT_WRITE, 
                         MAP_PRIVATE|MAP_ANONYMOUS|MAP_HUGETLB, -1, );
        if (ptr == MAP_FAILED)
            break;
    }
    // This exhausts kernel's reservation pool and _might_ trip the buggy path
    return ;
}

On unpatched kernels, this could lead to panic or instability.

- In most systems, only root can request hugetlb mappings, but some container/cloud hosts are misconfigured.

Sysbot Report:

https://lore.kernel.org/linux-mm/0000000000004096100617c58d54@google.com/T/#ma5983bc1ab18a54910da83416b3f89f3c7ee43aa

Linux Kernel Hugetlb Docs:

https://www.kernel.org/doc/html/latest/vm/hugetlbpage.html

CVE Details & Patch (once indexed):

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-39477
https://git.kernel.org/

Summary

CVE-2024-39477 may seem like a boring bug, but it could result in crashes or unpredictable kernel memory management on servers using huge pages. The fix involves better handling of out-of-memory conditions to prevent the kernel from confusing itself.

Advice:
Roll this patch into your kernels ASAP, especially for performance-sensitive or multi-user servers with hugetlb enabled.

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*Exclusive SimpleSec Writeup, June 2024 – Stay Safe!*

Timeline

Published on: 07/05/2024 07:15:10 UTC
Last modified on: 07/15/2024 06:50:12 UTC