# CVE-2024-38636: f2fs: multidev: fix to recognize valid zero block address

> A flaw in the Linux kernel's F2FS filesystem causes a kernel warning and potential crash when reading from multi-device setups with zero block addresses.

- **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:** 21 Jun 2024
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2024-38636, DEBIAN-CVE-2024-38636, GHSA-pwgj-c4vj-63r9, UBUNTU-CVE-2024-38636

## What is CVE-2024-38636?

CVE-2024-38636 is a medium-severity vulnerability in Linux kernel, published on 21 Jun 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:

f2fs: multidev: fix to recognize valid zero block address

As reported by Yi Zhang in mailing list [1], kernel warning was catched
during zbd/010 test as below:

./check zbd/010
zbd/010 (test gap zone support with F2FS)                    [failed]
    runtime    ...  3.752s
    something found in dmesg:
    [ 4378.146781] run blktests zbd/010 at 2024-02-18 11:31:13
    [ 4378.192349] null_blk: module loaded
    [ 4378.209860] null_blk: disk nullb0 created
    [ 4378.413285] scsi_debug:sdebug_driver_probe: scsi_debug: trim
poll_queues to 0. poll_q/nr_hw = (0/1)
    [ 4378.422334] scsi host15: scsi_debug: version 0191 [20210520]
                     dev_size_mb=1024, opts=0x0, submit_queues=1, statistics=0
    [ 4378.434922] scsi 15:0:0:0: Direct-Access-ZBC Linux
scsi_debug       0191 PQ: 0 ANSI: 7
    [ 4378.443343] scsi 15:0:0:0: Power-on or device reset occurred
    [ 4378.449371] sd 15:0:0:0: Attached scsi generic sg5 type 20
    [ 4378.449418] sd 15:0:0:0: [sdf] Host-managed zoned block device
    ...
    (See '/mnt/tests/gitlab.com/api/v4/projects/19168116/repository/archive.zip/storage/blktests/blk/blktests/results/nodev/zbd/010.dmesg'

WARNING: CPU: 22 PID: 44011 at fs/iomap/iter.c:51
CPU: 22 PID: 44011 Comm: fio Not tainted 6.8.0-rc3+ #1
RIP: 0010:iomap_iter+0x32b/0x350
Call Trace:
 <TASK>
 __iomap_dio_rw+0x1df/0x830
 f2fs_file_read_iter+0x156/0x3d0 [f2fs]
 aio_read+0x138/0x210
 io_submit_one+0x188/0x8c0
 __x64_sys_io_submit+0x8c/0x1a0
 do_syscall_64+0x86/0x170
 entry_SYSCALL_64_after_hwframe+0x6e/0x76

Shinichiro Kawasaki helps to analyse this issue and proposes a potential
fixing patch in [2].

Quoted from reply of Shinichiro Kawasaki:

"I confirmed that the trigger commit is dbf8e63f48af as Yi reported. I took a
look in the commit, but it looks fine to me. So I thought the cause is not
in the commit diff.

I found the WARN is printed when the f2fs is set up with multiple devices,
and read requests are mapped to the very first block of the second device in the
direct read path. In this case, f2fs_map_blocks() and f2fs_map_blocks_cached()
modify map->m_pblk as the physical block address from each block device. It
becomes zero when it is mapped to the first block of the device. However,
f2fs_iomap_begin() assumes that map->m_pblk is the physical block address of the
whole f2fs, across the all block devices. It compares map->m_pblk against
NULL_ADDR == 0, then go into the unexpected branch and sets the invalid
iomap->length. The WARN catches the invalid iomap->length.

This WARN is printed even for non-zoned block devices, by following steps.

 - Create two (non-zoned) null_blk devices memory backed with 128MB size each:
   nullb0 and nullb1.
 # mkfs.f2fs /dev/nullb0 -c /dev/nullb1
 # mount -t f2fs /dev/nullb0 "${mount_dir}"
 # dd if=/dev/zero of="${mount_dir}/test.dat" bs=1M count=192
 # dd if="${mount_dir}/test.dat" of=/dev/null bs=1M count=192 iflag=direct

..."

So, the root cause of this issue is: when multi-devices feature is on,
f2fs_map_blocks() may return zero blkaddr in non-primary device, which is
a verified valid block address, however, f2fs_iomap_begin() treats it as
an invalid block address, and then it triggers the warning in iomap
framework code.

Finally, as discussed, we decide to use a more simple and direct way that
checking (map.m_flags & F2FS_MAP_MAPPED) condition instead of
(map.m_pblk != NULL_ADDR) to fix this issue.

Thanks a lot for the effort of Yi Zhang and Shinichiro Kawasaki on this
issue.

[1] https://lore.kernel.org/linux-f2fs-devel/CAHj4cs-kfojYC9i0G73PRkYzcxCTex=-vugRFeP40g_URGvnfQ@mail.gmail.com/
[2] https://lore.kernel.org/linux-f2fs-devel/gngdj77k4picagsfdtiaa7gpgnup6fsgwzsltx6milmhegmjff@iax2n4wvrqye/

## Which versions are affected by CVE-2024-38636?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2024-38636?

Apply the vendor's patch.

## Is CVE-2024-38636 being exploited?

There is no known exploitation of CVE-2024-38636 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-2024-38636 and fix it if it is affected.

What it is: CVE-2024-38636 is a medium-severity vulnerability in Linux kernel, published on 21 Jun 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-2024-38636 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-2024-38636/ (Markdown: https://insights.sbomify.com/CVE-2024-38636/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2024-38636
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-38636
- OSV: https://osv.dev/vulnerability/CVE-2024-38636

---
From sbomify Insights: https://insights.sbomify.com/CVE-2024-38636/
Summaries are generated and may contain mistakes; the official description above is the record.