# CVE-2026-89793: ublk: clear VM_MAYWRITE on read-only ublk char device mmap

> An unprivileged daemon can corrupt kernel-written I/O descriptors by upgrading a read-only mmap to writable, potentially leading to privilege escalation or code execution.

- **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:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-101334, BELL-CVE-2026-89793, DEBIAN-CVE-2026-89793, GHSA-fp98-h54j-vwfj, UBUNTU-CVE-2026-89793

## What is CVE-2026-89793?

CVE-2026-89793 is a high-severity vulnerability in Linux, published on 16 Sep 2026. 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:

ublk: clear VM_MAYWRITE on read-only ublk char device mmap

ublk_ch_mmap() rejects mmap requests with VM_WRITE set, but never
clears VM_MAYWRITE on the resulting read-only mapping. This allows
a userspace daemon to mmap the per-queue command buffer PROT_READ,
then upgrade it to PROT_WRITE via mprotect(), since VM_MAYWRITE was
never cleared.

The command buffer holds struct ublksrv_io_desc entries that are
kernel-written ABI; a writable mapping lets an unprivileged daemon
process corrupt fields such as addr, op_flags, nr_sectors, and
start_sector.

Same bug class as the drm/panthor and drm/vc4 VM_MAYWRITE fixes, and
the 2026-08-13 ptp/vmclock fix (a5edadbae57e).

Verified via mprotect() PoC: before the fix, a PROT_READ mapping can
be upgraded to PROT_READ|PROT_WRITE and a write into the command
buffer corrupts io_desc fields (confirmed under KASAN). After the
fix, mprotect() returns -EACCES.

## Which versions are affected by CVE-2026-89793?


## How do I fix CVE-2026-89793?

Apply the vendor's patch.

## Is CVE-2026-89793 being exploited?

There is no known exploitation of CVE-2026-89793 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89793 and fix it if it is affected.

What it is: CVE-2026-89793 is a high-severity vulnerability in Linux, published on 16 Sep 2026. 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-2026-89793 in the wild and no public exploit code.

Affected product: Linux. 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-2026-89793/ (Markdown: https://insights.sbomify.com/CVE-2026-89793/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89793
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89793
- OSV: https://osv.dev/vulnerability/CVE-2026-89793

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89793/
Summaries are generated and may contain mistakes; the official description above is the record.