# CVE-2024-46787: userfaultfd: fix checks for huge PMDs

> A race condition in the Linux kernel's userfaultfd subsystem can cause a kernel oops or potentially allow unauthorized memory access.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Sep 2024
- **Updated:** 5 Aug 2026
- **Also known as:** AZL-53265, BELL-CVE-2024-46787, DEBIAN-CVE-2024-46787, GHSA-r8fr-h8q4-2f98, ROOT-OS-DEBIAN-11-CVE-2024-46787, ROOT-OS-DEBIAN-12-CVE-2024-46787, ROOT-OS-UBUNTU-2204-CVE-2024-46787, UBUNTU-CVE-2024-46787

## What is CVE-2024-46787?

CVE-2024-46787 is a high-severity vulnerability in Linux kernel, published on 18 Sep 2024. 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:

userfaultfd: fix checks for huge PMDs

Patch series "userfaultfd: fix races around pmd_trans_huge() check", v2.

The pmd_trans_huge() code in mfill_atomic() is wrong in three different
ways depending on kernel version:

1. The pmd_trans_huge() check is racy and can lead to a BUG_ON() (if you hit
   the right two race windows) - I've tested this in a kernel build with
   some extra mdelay() calls. See the commit message for a description
   of the race scenario.
   On older kernels (before 6.5), I think the same bug can even
   theoretically lead to accessing transhuge page contents as a page table
   if you hit the right 5 narrow race windows (I haven't tested this case).
2. As pointed out by Qi Zheng, pmd_trans_huge() is not sufficient for
   detecting PMDs that don't point to page tables.
   On older kernels (before 6.5), you'd just have to win a single fairly
   wide race to hit this.
   I've tested this on 6.1 stable by racing migration (with a mdelay()
   patched into try_to_migrate()) against UFFDIO_ZEROPAGE - on my x86
   VM, that causes a kernel oops in ptlock_ptr().
3. On newer kernels (>=6.5), for shmem mappings, khugepaged is allowed
   to yank page tables out from under us (though I haven't tested that),
   so I think the BUG_ON() checks in mfill_atomic() are just wrong.

I decided to write two separate fixes for these (one fix for bugs 1+2, one
fix for bug 3), so that the first fix can be backported to kernels
affected by bugs 1+2.

This patch (of 2):

This fixes two issues.

I discovered that the following race can occur:

  mfill_atomic                other thread
  ============                ============
                              <zap PMD>
  pmdp_get_lockless() [reads none pmd]
  <bail if trans_huge>
  <if none:>
                              <pagefault creates transhuge zeropage>
    __pte_alloc [no-op]
                              <zap PMD>
  <bail if pmd_trans_huge(*dst_pmd)>
  BUG_ON(pmd_none(*dst_pmd))

I have experimentally verified this in a kernel with extra mdelay() calls;
the BUG_ON(pmd_none(*dst_pmd)) triggers.

On kernels newer than commit 0d940a9b270b ("mm/pgtable: allow
pte_offset_map[_lock]() to fail"), this can't lead to anything worse than
a BUG_ON(), since the page table access helpers are actually designed to
deal with page tables concurrently disappearing; but on older kernels
(<=6.4), I think we could probably theoretically race past the two
BUG_ON() checks and end up treating a hugepage as a page table.

The second issue is that, as Qi Zheng pointed out, there are other types
of huge PMDs that pmd_trans_huge() can't catch: devmap PMDs and swap PMDs
(in particular, migration PMDs).

On <=6.4, this is worse than the first issue: If mfill_atomic() runs on a
PMD that contains a migration entry (which just requires winning a single,
fairly wide race), it will pass the PMD to pte_offset_map_lock(), which
assumes that the PMD points to a page table.

Breakage follows: First, the kernel tries to take the PTE lock (which will
crash or maybe worse if there is no "struct page" for the address bits in
the migration entry PMD - I think at least on X86 there usually is no
corresponding "struct page" thanks to the PTE inversion mitigation, amd64
looks different).

If that didn't crash, the kernel would next try to write a PTE into what
it wrongly thinks is a page table.

As part of fixing these issues, get rid of the check for pmd_trans_huge()
before __pte_alloc() - that's redundant, we're going to have to check for
that after the __pte_alloc() anyway.

Backport note: pmdp_get_lockless() is pmd_read_atomic() in older kernels.

## Which versions are affected by CVE-2024-46787?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2024-46787?

Apply the vendor's patch.

## Is CVE-2024-46787 being exploited?

There is no known exploitation of CVE-2024-46787 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2024-46787 and fix it if it is affected.

What it is: CVE-2024-46787 is a high-severity vulnerability in Linux kernel, published on 18 Sep 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2024-46787 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2024-46787/ (Markdown: https://insights.sbomify.com/CVE-2024-46787/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2024-46787
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-46787
- OSV: https://osv.dev/vulnerability/CVE-2024-46787

---
From sbomify Insights: https://insights.sbomify.com/CVE-2024-46787/
Summaries are generated and may contain mistakes; the official description above is the record.