---
id: CVE-2025-39879
title: 'ceph: always call ceph_shift_unused_folios_left()'
summary: |-
  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_ba…
severity: medium
cvss: 5.5
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'
cvssSource: adp
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 0f2b2b7cacf888229cec19126e6791b11fb35f32 <
    523214f0afa1b3d0e696917bc5a4d0737718e3d7
  - >-
    Linux >= ce80b76dd32764cc914975777e058d4fae4f0ea0 <
    289b6615cf553d98509a9b273195d9936da1cfb2
  - >-
    Linux >= ce80b76dd32764cc914975777e058d4fae4f0ea0 <
    cce7c15faaac79b532a07ed6ab8332280ad83762
  - Linux 6.15
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-01-14T19:25:25.258616Z'
published: '2025-09-23'
updated: '2026-10-21'
sourceUpdated: '2026-10-21T12:37:32.477Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2025-39879'
references:
  - url: 'https://git.kernel.org/stable/c/523214f0afa1b3d0e696917bc5a4d0737718e3d7'
  - url: 'https://git.kernel.org/stable/c/289b6615cf553d98509a9b273195d9936da1cfb2'
  - url: 'https://git.kernel.org/stable/c/cce7c15faaac79b532a07ed6ab8332280ad83762'
tags:
  - cve.org
epss: 0.00187
epssPercentile: 0.08239
ingestedAt: '2026-10-21T12:48:53.687Z'
---

## Overview

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.

## Affected

- `Linux >= 0f2b2b7cacf888229cec19126e6791b11fb35f32 < 523214f0afa1b3d0e696917bc5a4d0737718e3d7`
- `Linux >= ce80b76dd32764cc914975777e058d4fae4f0ea0 < 289b6615cf553d98509a9b273195d9936da1cfb2`
- `Linux >= ce80b76dd32764cc914975777e058d4fae4f0ea0 < cce7c15faaac79b532a07ed6ab8332280ad83762`
- `Linux 6.15`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.