# CVE-2026-93048: mtd: part: reject MTDPART_OFS_RETAIN in mtd_add_partition()

> A local attacker can trigger an integer underflow in the Linux kernel MTD subsystem via BLKPG ioctl, causing a warning and potential denial of service.

- **Severity:** Not scored
- **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:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102063, BELL-CVE-2026-93048, DEBIAN-CVE-2026-93048, GHSA-2p6r-8jww-x7j3, UBUNTU-CVE-2026-93048

## What is CVE-2026-93048?

CVE-2026-93048 is a vulnerability in Linux, published on 17 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:

mtd: part: reject MTDPART_OFS_RETAIN in mtd_add_partition()

mtd_add_partition() does not reject the special offset value
MTDPART_OFS_RETAIN (-3), which leads to a WARN_ON in
add_mtd_device() when called through the BLKPG ioctl on NAND
devices. The RETAIN value depends on cur_offset being the end of
the previous partition, but in the dynamic partition path
cur_offset equals the offset argument itself, causing undefined
behavior.

Commit 5daa7b21496a ("mtd: prepare partition add and del functions
for ioctl requests") introduced mtd_add_partition() and correctly
rejected MTDPART_OFS_APPEND (-1) and MTDPART_OFS_NXTBLK (-2),
since those special offsets rely on cur_offset tracking the
previous partition's end. However, commit 1a31368bf92e ("mtd: add a flags
for partitions which should just leave smth. after them")
later added MTDPART_OFS_RETAIN (-3) for the static
partition table path without updating mtd_add_partition() to
also reject this value.

With offset=-3 passed via BLKPG, the RETAIN size calculation in
allocate_partition() underflows (parent_size - 0xFFFFFFFFFFFFFFFD
= parent_size + 3). If the underflow result does not appear to
leave enough space, allocate_partition() jumps to out_register via
goto, skipping erasesize initialization. This results in
erasesize=0, which triggers:

WARN_ON((!mtd->erasesize || !master->_erase) &&
		!(mtd->flags & MTD_NO_ERASE))

in add_mtd_device(). If the underflow result appears to leave
enough space, a bogus partition size is calculated, but the
"out of reach" sanity check catches the invalid offset and
creates a disabled empty partition (offset=0, size=0) instead
of returning an error.

Fix this by adding MTDPART_OFS_RETAIN to the rejection list in
mtd_add_partition(), consistent with the existing handling of
APPEND and NXTBLK.

## Which versions are affected by CVE-2026-93048?


## How do I fix CVE-2026-93048?

Apply the vendor's patch.

## Is CVE-2026-93048 being exploited?

There is no known exploitation of CVE-2026-93048 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-93048 and fix it if it is affected.

What it is: CVE-2026-93048 is a vulnerability in Linux, published on 17 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-2026-93048 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-2026-93048/ (Markdown: https://insights.sbomify.com/CVE-2026-93048/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-93048
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-93048
- OSV: https://osv.dev/vulnerability/CVE-2026-93048

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-93048/
Summaries are generated and may contain mistakes; the official description above is the record.