# CVE-2026-98974: xfs: fix exchange-range-to-eof file size exchange


- **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
- **Fix available:** Yes
- **Published:** 23 Oct 2026
- **Updated:** 24 Oct 2026
- **Also known as:** BELL-CVE-2026-98974, DEBIAN-CVE-2026-98974, GHSA-2g9r-fc27-29gx, UBUNTU-CVE-2026-98974

## What is CVE-2026-98974?

CVE-2026-98974 is a high-severity vulnerability in Linux, published on 23 Oct 2026. 

### Official description

In the Linux kernel, the following vulnerability has been resolved:

xfs: fix exchange-range-to-eof file size exchange

Norbert Szetei posted a patch containing an incomplete description of a
bug in XFS_IOC_EXCHANGE_RANGE's TO_EOF flag.  The bug tricks
exchange-range into modifying a file so that it has shared blocks
starting beyond EOF, which is never allowed because files never have
written data beyond EOF, and you can only share written data blocks.
This is key to all the incorrect behavior that follows.

Eventually I got from Norbert a description of what's going wrong:

> Fair. Here it is with physical block numbers, 4k blocks. peer is the file
> that gets rewritten and file1 is the one that ends up with the flag
> cleared.
>
> Start, nothing shared:
>
>	peer size 4096   -> [100]
>	file1 size 12288 -> [200] [201] [202]
>	donor1 size 4096 -> [300]
>	donor2 size 4096 -> [400]
>
> 1. FICLONERANGE peer's block into file1's middle slot:
>
>	peer size 4096   -> [100]
>	file1 size 12288 -> [200] [100] [202] reflink flag set
>                             ^^^^^ both files own block 100
>
> 2. XFS_IOC_EXCHANGE_RANGE file1 against donor1, file1_offset 0,
> file2_offset 8192, length 0, TO_EOF. One block is exchanged, and the sizes
> are exchanged with it:
>
>	file1 size 4096   -> [200] [100] [300] reflink flag set
>                              ^^^^^^^^^^^ still owned, now past the size
>	donor1 size 12288 -> [202]
>
> file1 says it is one block long while it still owns three. Nothing was
> unmapped. file2_offset is block 2, so this exchange cannot clear a flag.
>
> 3. XFS_IOC_EXCHANGE_RANGE file1 against donor2, both offsets 0, length 4096.
> By i_disk_size this is two one-block files exchanging everything, so
> xmi_can_exchange_reflink_flags() agrees and the flag is cleared:
>
>	file1 size 4096   -> [400] [100] [300] reflink flag CLEARED
>	donor2 size 4096  -> [200]
>
> Block 100 is still shared with peer.
>
> 4. pwrite(file1, 4096, 4096), which is file1's second slot, block 100.
> xfs_is_cow_inode(file1) is false, so no CoW:
>
>	peer size 4096 -> [100] same block, new contents
>
> Block 100 is the only one that matters here. PoC below, it reads a file
> called peer in the current directory. The filesystem needs reflink and
> exchange-range, which mkfs.xfs 7.x turns on by default.

So yes, TO_EOF is screwing up the file size even if it's actually
exchanging the data block correctly.  The following is the intended
behavior of XFS_EXCHANGE_RANGE_TO_EOF, as described by its manpage:

"Ignore the length parameter.  All bytes in file1_fd from file1_offset
 to EOF are moved to file2_fd, and file2's size is set to
 (file2_offset+(file1_length-file1_offset)).  Meanwhile, all bytes in
 file2 from file2_offset to EOF are moved to file1 and file1's size is
 set to (file1_offset+(file2_length-file2_offset))."

In other words, if we only exchange a portion of the file data, then we
should only exchange the portion of the file size that relates to the
exchanged data.  Unfortunately, the code swaps the ondisk file size and
the incore file sizes, which is incorrect if the caller passes in a
nonzero offset.

Having confused the file sizes, the PoC uses a second exchange-range
call on what looks like a non-reflinked single-block file.  Because the
file size is set incorrectly, exchange-range thinks it's exchanging the
full contents of two files and clears the reflink flag on the broken
file.  That enables an extending write of the broken file to rewrite the
shared block that's just past EOF.  This has become known colloquially
as refluxfs.

Once the file sizes are set correctly, the second exchange-range no
longer thinks that it's doing a full-contents swap, so it won't clear
the reflink flag on either of its file arguments.

Norbert followed up to an earlier version of this patch with a second
complaint about us not handling the i_disk_size state correctly:

> The offsets are bounded against the incore i_size but subtracted from
>
---truncated---

## Which versions are affected by CVE-2026-98974?


## How do I fix CVE-2026-98974?

Upgrade to a fixed version.

## Is CVE-2026-98974 being exploited?

There is no known exploitation of CVE-2026-98974 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98974 and fix it if it is affected.

What it is: CVE-2026-98974 is a high-severity vulnerability in Linux, published on 23 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-98974 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-2026-98974/ (Markdown: https://insights.sbomify.com/CVE-2026-98974/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98974
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98974
- OSV: https://osv.dev/vulnerability/CVE-2026-98974

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98974/
Summaries are generated and may contain mistakes; the official description above is the record.