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Linux KernelOperating system · Linux

CVE-2025-39879

MEDIUM · 5.5 CVSS v3.1 Published 2025-09-23
Fix available
A fix is available. Upgrade to 6.16.8 or later.
See remediation →
57/100
Remediation priority · Elevated
Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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/[email protected] 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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis
How this class of weakness works · CWE-476

The code follows a pointer that is null, crashing the process. An attacker who can reliably trigger it turns the crash into a denial of service. The fix is checking for null before use and handling the failure path gracefully.

General guidance for the null pointer dereference class — the official description and references above are authoritative for this specific CVE. Want a bespoke review and a reviewed fix? Ask our team →

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:>= 6.15, < 6.16.8= 6.17

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 6.16.8 or later
Fixed in 6.16.8
Vendor patch git.kernel.org →
Recommended fix High confidence

Linux Kernel 6.16.8 or later, or 6.18+

  1. Upgrade the Linux kernel to version 6.16.8 or later, or to version 6.18 or later
  2. After kernel upgrade, verify the fix is present by checking that the ceph_shift_unused_folios_left() function is called after ceph_process_folio_batch() in the kernel source code
  3. Reboot the system with the new kernel to apply the fix
  4. Monitor system logs after booting to ensure no NULL pointer dereference errors occur in ceph writeback operations
Caveat Kernel upgrades may require matching system utilities and drivers; ensure compatibility with hardware and deployed software before production deployment

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $2,850. Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

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