A recent vulnerability, identified as CVE-2024-27007, has been found and resolved in the Linux kernel's userfaultfd (UFFD) subsystem. This bug relates specifically to how huge pages are handled during memory migration. For those managing servers, working with containers, or developing low-level system software, understanding this risk—and what changed—is important.

We'll break down this vulnerability in simple terms, include code snippets for illustration, and provide links to original sources.

What is Userfaultfd?

userfaultfd (UFFD) is a Linux system call that lets user-space processes handle page faults for certain virtual memory areas, like on-demand paging, live migration, or checkpoint/restore operations. It's especially useful for virtual machines and containers.

What Went Wrong: The Vulnerability

The bug comes from *incorrectly handling huge pages* during the UFFDIO_MOVE operation, which moves memory pages in a process's address space. Specifically:
- The function should have waited for page-table entries to clear and made sure the huge page wasn't pinned (used by something else) before changing its mapping/index.

If these steps are skipped or reordered, you can have inconsistencies or memory corruption.

A previous fix (commit d7a08838ab74) addressed this for regular pages but missed huge pages. That means if attackers can repeatedly trigger huge page migrations under certain conditions, they could corrupt memory, crash the system, or maybe escalate privileges.

Exploit Scenario

While this vulnerability is not remotely exploitable without local code execution, a low-privilege user could craft a process that:

Races with system memory migration or swap activity

If the bug occurs, kernel data structures might be corrupted, causing unpredictable behavior or potentially giving the attacker more powerful kernel access.

Code Example (Simplified)

Let's look at how a user-space program could abuse UFFDIO_MOVE with huge pages (most error-checking omitted for brevity):

#define _GNU_SOURCE
#include <linux/userfaultfd.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>

#define PAGE_SIZE (2 * 1024 * 1024) // 2MB huge page

int main() {
    int uffd = syscall(SYS_userfaultfd, O_CLOEXEC | O_NONBLOCK);
    char *addr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
                      MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB, -1, );

    struct uffdio_api ua = { .api = UFFD_API };
    ioctl(uffd, UFFDIO_API, &ua);

    struct uffdio_register ur = {
        .range.start = (unsigned long)addr,
        .range.len = PAGE_SIZE,
        .mode = UFFDIO_REGISTER_MODE_MISSING,
    };
    ioctl(uffd, UFFDIO_REGISTER, &ur);

    // Now, code can trigger UFFDIO_MOVE here under the right races
    // Exploiters would try to race with swapping or memory migration.

    printf("Registered userfaultfd for huge page address: %p\n", addr);
    // ...skip exploitation details for safety...

    return ;
}

Technical Patch Details

- The core issue was that src_folio->{mapping, index} (i.e., the source page's metadata) could be changed before the kernel verified the huge page wasn't pinned and had its page-table cleared, if it was a huge page case.
- The patch moves the code that changes this metadata after all those checks, for huge pages as well as normal ones.

Patch commit

> mm: userfaultfd: fix unexpected change to src_folio when UFFDIO_MOVE fails
>
> Fix huge-page case: ensure correct order of operations in UFFDIO_MOVE logic.

Reference: Upstream Linux commit

Who is Affected

- Linux systems with kernels supporting userfaultfd and huge page migrations (generally kernel 5.x and newer).
- Systems allowing untrusted users to use userfaultfd or unprivileged containers/VMs.

If unprivileged userfaultfd is disabled, exposure is reduced, but always upgrade if possible.

How to Fix

Upgrade your kernel to a version including the fix or apply the patch manually if you build from source.

Workarounds

- Disable userfaultfd for unprivileged users (usually controlled by /proc/sys/vm/unprivileged_userfaultfd).

More Reading

- CVE Report page
- Linux kernel commit e21cb730
- Kernel documentation: userfaultfd

Conclusion

CVE-2024-27007 is another reminder that memory management bugs—even subtle ones—can have real security consequences, especially with advanced features like userfaultfd and huge pages. Check your system for this CVE and update if you use memory migration, containers, or VM live migration.

> Stay secure—track your kernel updates, read advisories, and restrict advanced VM/memory tools unless needed!


*Post written and explained exclusively for clarity and safety—feel free to share with colleagues or link back!*

Timeline

Published on: 05/01/2024 06:15:19 UTC
Last modified on: 05/04/2025 09:02:01 UTC