# CVE-2021-47531: drm/msm: Fix mmap to include VM_IO and VM_DONTDUMP

> Missing VM flags in DRM mmap implementation cause kernel crashes during logout on specific Chromebooks, leading to denial of service.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 24 May 2024
- **Updated:** 11 May 2026
- **Also known as:** DEBIAN-CVE-2021-47531, UBUNTU-CVE-2021-47531

## What is CVE-2021-47531?

CVE-2021-47531 is a medium-severity denial of service vulnerability in Linux kernel, published on 24 May 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:

drm/msm: Fix mmap to include VM_IO and VM_DONTDUMP

In commit 510410bfc034 ("drm/msm: Implement mmap as GEM object
function") we switched to a new/cleaner method of doing things. That's
good, but we missed a little bit.

Before that commit, we used to _first_ run through the
drm_gem_mmap_obj() case where `obj->funcs->mmap()` was NULL. That meant
that we ran:

  vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
  vma->vm_page_prot = pgprot_writecombine(vm_get_page_prot(vma->vm_flags));
  vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);

...and _then_ we modified those mappings with our own. Now that
`obj->funcs->mmap()` is no longer NULL we don't run the default
code. It looks like the fact that the vm_flags got VM_IO / VM_DONTDUMP
was important because we're now getting crashes on Chromebooks that
use ARC++ while logging out. Specifically a crash that looks like this
(this is on a 5.10 kernel w/ relevant backports but also seen on a
5.15 kernel):

  Unable to handle kernel paging request at virtual address ffffffc008000000
  Mem abort info:
    ESR = 0x96000006
    EC = 0x25: DABT (current EL), IL = 32 bits
    SET = 0, FnV = 0
    EA = 0, S1PTW = 0
  Data abort info:
    ISV = 0, ISS = 0x00000006
    CM = 0, WnR = 0
  swapper pgtable: 4k pages, 39-bit VAs, pgdp=000000008293d000
  [ffffffc008000000] pgd=00000001002b3003, p4d=00000001002b3003,
                     pud=00000001002b3003, pmd=0000000000000000
  Internal error: Oops: 96000006 [#1] PREEMPT SMP
  [...]
  CPU: 7 PID: 15734 Comm: crash_dump64 Tainted: G W 5.10.67 #1 [...]
  Hardware name: Qualcomm Technologies, Inc. sc7280 IDP SKU2 platform (DT)
  pstate: 80400009 (Nzcv daif +PAN -UAO -TCO BTYPE=--)
  pc : __arch_copy_to_user+0xc0/0x30c
  lr : copyout+0xac/0x14c
  [...]
  Call trace:
   __arch_copy_to_user+0xc0/0x30c
   copy_page_to_iter+0x1a0/0x294
   process_vm_rw_core+0x240/0x408
   process_vm_rw+0x110/0x16c
   __arm64_sys_process_vm_readv+0x30/0x3c
   el0_svc_common+0xf8/0x250
   do_el0_svc+0x30/0x80
   el0_svc+0x10/0x1c
   el0_sync_handler+0x78/0x108
   el0_sync+0x184/0x1c0
  Code: f8408423 f80008c3 910020c6 36100082 (b8404423)

Let's add the two flags back in.

While we're at it, the fact that we aren't running the default means
that we _don't_ need to clear out VM_PFNMAP, so remove that and save
an instruction.

NOTE: it was confirmed that VM_IO was the important flag to fix the
problem I was seeing, but adding back VM_DONTDUMP seems like a sane
thing to do so I'm doing that too.

## Which versions are affected by CVE-2021-47531?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2021-47531?

Apply the vendor's patch.

## Is CVE-2021-47531 being exploited?

There is no known exploitation of CVE-2021-47531 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-2021-47531 and fix it if it is affected.

What it is: CVE-2021-47531 is a medium-severity denial of service vulnerability in Linux kernel, published on 24 May 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-2021-47531 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-2021-47531/ (Markdown: https://insights.sbomify.com/CVE-2021-47531/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-47531
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-47531
- OSV: https://osv.dev/vulnerability/CVE-2021-47531

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-47531/
Summaries are generated and may contain mistakes; the official description above is the record.