# CVE-2025-39879: ceph: always call ceph_shift_unused_folios_left()

> A logic error in the Linux kernel Ceph client causes NULL pointer dereferences, leading to kernel crashes and 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:** 23 Sep 2025
- **Updated:** 21 Oct 2026
- **Also known as:** BELL-CVE-2025-39879, DEBIAN-CVE-2025-39879, GHSA-hq23-ggcp-mx5x, UBUNTU-CVE-2025-39879

## What is CVE-2025-39879?

CVE-2025-39879 is a medium-severity vulnerability in Linux kernel, published on 23 Sep 2025. 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:

ceph: always call ceph_shift_unused_folios_left()

The function ceph_process_folio_batch() sets folio_batch entries to
NULL, which is an illegal state.  Before folio_batch_release() crashes
due to this API violation, the function ceph_shift_unused_folios_left()
is supposed to remove those NULLs from the array.

However, since commit ce80b76dd327 ("ceph: introduce
ceph_process_folio_batch() method"), this shifting doesn't happen
anymore because the "for" loop got moved to ceph_process_folio_batch(),
and now the `i` variable that remains in ceph_writepages_start()
doesn't get incremented anymore, making the shifting effectively
unreachable much of the time.

Later, commit 1551ec61dc55 ("ceph: introduce ceph_submit_write()
method") added more preconditions for doing the shift, replacing the
`i` check (with something that is still just as broken):

- if ceph_process_folio_batch() fails, shifting never happens

- if ceph_move_dirty_page_in_page_array() was never called (because
  ceph_process_folio_batch() has returned early for some of various
  reasons), shifting never happens

- if `processed_in_fbatch` is zero (because ceph_process_folio_batch()
  has returned early for some of the reasons mentioned above or
  because ceph_move_dirty_page_in_page_array() has failed), shifting
  never happens

Since those two commits, any problem in ceph_process_folio_batch()
could crash the kernel, e.g. this way:

 BUG: kernel NULL pointer dereference, address: 0000000000000034
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 PGD 0 P4D 0
 Oops: Oops: 0002 [#1] SMP NOPTI
 CPU: 172 UID: 0 PID: 2342707 Comm: kworker/u778:8 Not tainted 6.15.10-cm4all1-es #714 NONE
 Hardware name: Dell Inc. PowerEdge R7615/0G9DHV, BIOS 1.6.10 12/08/2023
 Workqueue: writeback wb_workfn (flush-ceph-1)
 RIP: 0010:folios_put_refs+0x85/0x140
 Code: 83 c5 01 39 e8 7e 76 48 63 c5 49 8b 5c c4 08 b8 01 00 00 00 4d 85 ed 74 05 41 8b 44 ad 00 48 8b 15 b0 >
 RSP: 0018:ffffb880af8db778 EFLAGS: 00010207
 RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000003
 RDX: ffffe377cc3b0000 RSI: 0000000000000000 RDI: ffffb880af8db8c0
 RBP: 0000000000000000 R08: 000000000000007d R09: 000000000102b86f
 R10: 0000000000000001 R11: 00000000000000ac R12: ffffb880af8db8c0
 R13: 0000000000000000 R14: 0000000000000000 R15: ffff9bd262c97000
 FS:  0000000000000000(0000) GS:ffff9c8efc303000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000034 CR3: 0000000160958004 CR4: 0000000000770ef0
 PKRU: 55555554
 Call Trace:
  <TASK>
  ceph_writepages_start+0xeb9/0x1410

The crash can be reproduced easily by changing the
ceph_check_page_before_write() return value to `-E2BIG`.

(Interestingly, the crash happens only if `huge_zero_folio` has
already been allocated; without `huge_zero_folio`,
is_huge_zero_folio(NULL) returns true and folios_put_refs() skips NULL
entries instead of dereferencing them.  That makes reproducing the bug
somewhat unreliable.  See
https://lore.kernel.org/20250826231626.218675-1-max.kellermann@ionos.com
for a discussion of this detail.)

My suggestion is to move the ceph_shift_unused_folios_left() to right
after ceph_process_folio_batch() to ensure it always gets called to
fix up the illegal folio_batch state.

Weakness: CWE-476 (NULL Pointer Dereference)

## Which versions are affected by CVE-2025-39879?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-39879?

Apply the vendor's patch.

## Is CVE-2025-39879 being exploited?

There is no known exploitation of CVE-2025-39879 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-2025-39879 and fix it if it is affected.

What it is: CVE-2025-39879 is a medium-severity vulnerability in Linux kernel, published on 23 Sep 2025. 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-2025-39879 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-2025-39879/ (Markdown: https://insights.sbomify.com/CVE-2025-39879/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39879
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39879
- OSV: https://osv.dev/vulnerability/CVE-2025-39879

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39879/
Summaries are generated and may contain mistakes; the official description above is the record.