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CVE-2026-89655
ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/ceph. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-89655 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixedDLA-4841-1 · 2026-10-17 · text | 6.1.190-1 | Update the kernel and reboot. DLA-4841-1 shipped the fix on 2026-10-17, in 6.1.190-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6528-1 · 2026-09-29 · text | 6.12.111-1 | Update the kernel and reboot. DSA-6528-1 shipped the fix on 2026-09-29, in 6.12.111-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4817-1 · 2026-10-04 · text | 6.12.111-1~deb12u1 | Update the kernel and reboot. DLA-4817-1 shipped the fix on 2026-10-04, in 6.12.111-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
list_for_each_entry() iterates ci->i_cap_flush_list but drops i_ceph_lock to send cap messages. During the unlock window, handle_cap_flush_ack() can acquire i_ceph_lock, detach cf entries with tid <= flush_tid from the list, release i_ceph_lock, and free them via ceph_free_cap_flush() outside any lock. When the original thread reacquires i_ceph_lock and the for-loop macro advances via cf = list_next_entry(cf, i_list), it dereferences cf->i_list.next on freed memory. The race timeline: __kick_flushing_caps() handle_cap_flush_ack() ----------------------- ----------------------- holds i_ceph_lock <--- iterates to cf (tid=10) prepares FLUSH message drops i_ceph_lock <--- __send_cap() ── FLUSH(tid=10) MDS sends FLUSH_ACK(tid=10) ---> acquires i_ceph_lock cf->tid(10) <= flush_tid(10), detaches cf from i_cap_flush_list drops i_ceph_lock ceph_free_cap_flush(cf) <- frees it! acquires i_ceph_lock <--- for-loop advances: cf = list_next_entry(cf, i_list) -- UAF on freed cf->i_list.next The cf was just sent by __kick_flushing_caps itself via __send_cap(). The MDS may respond with FLUSH_ACK quickly enough that handle_cap_flush_ack() frees cf before __kick_flushing_caps can finish the iteration. Fix by converting to a manual while loop: save the next pointer under i_ceph_lock before dropping it, then use the saved pointer after reacquiring, so the potentially-freed cf is never accessed again.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
fs/ceph/caps.c
- What this is
- CEPH DISTRIBUTED FILE SYSTEM CLIENT (CEPH)MAINTAINERS section matched on fs/ceph/ · ceph-devel@vger.kernel.org · Supported
- Kernel module
- ceph
Check it on the machine
grep -w CONFIG_CEPH_FS /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
lsmod | grep -w cephPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sid, bookworm · linux-6.12m
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switch
- CONFIG_CEPH_FS
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-09-11 (37 days ago)source
- CVSS
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.71%, higher than 52.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/ceph
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 4.8 | 5.10.270 | 091137821e1f |
| 4.8 | 5.15.221 | 015424300810 |
| 4.8 | 6.1.188 | 23eb34a53a53 |
| 4.8 | 6.6.157 | 19f16f04c2b0 |
| 4.8 | 6.12.109 | 2701431aa3cc |
| 4.8 | 6.18.50 | fe46746087b5 |
| 4.8 | 7.2.4 | 2dba24dcd505 |
| 4.8 | 7.3 | 7af4c4f01305 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4841-1 | 2026-10-17 | bookworm 6.1.190-1 | tracker |
| DLA-4817-1 | 2026-10-04 | bookworm 6.12.111-1~deb12u1 | tracker |
| DSA-6528-1 | 2026-09-29 | trixie 6.12.111-1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:NAttack vector, network
- handle_cap_flush_ack() runs in the ceph-msgr kworker on CEPH_MSG_CLIENT_CAPS FLUSH_ACK from the MDS over TCP; a malicious or compromised MDS frees the ceph_cap_flush while __kick_flushing_caps() still holds that pointer after dropping i_ceph_lock to send FLUSH.
- AC:LAttack complexity, low
- The MDS controls both sides of the race: it can induce kicking without s_mutex (reconnect sets CEPH_I_KICK_FLUSH, then delayed ceph_check_caps or revoke-queued writeback) and immediately send FLUSH_ACK for the known flush tid in the unlock window around __send_cap().
- PR:NPrivileges required, none
- The attacker is the remote MDS peer on an already-connected kernel CephFS client and needs no Linux UID, capability, or init-namespace root on the victim, matching other MDS-driven client CVEs such as CVE-2026-89651.
- UI:NUser interaction, none
- Once CephFS is mounted (standard in Rook/Kubernetes/OpenStack), cap-flush kicking runs from mdsc delayed work and writeback, and FLUSH_ACK is processed automatically in kworker context with no further victim mount or interactive step at exploit time.
- S:UScope, unchanged
- The use-after-free is of a ceph_cap_flush object in the host kernel heap and stays inside the same kernel security authority; it does not cross a VM, IOMMU, or sandbox boundary.
- C:HConfidentiality impact, high
- After ceph_free_cap_flush(), __kick_flushing_caps() does list_next_entry() on the freed cf; reclaiming the dedicated ceph_cap_flush slab with attacker-controlled objects yields a fake next pointer and arbitrary kernel read, scored High per UAF guidance.
- I:HIntegrity impact, high
- The iterator then treats the sprayed object as a live ceph_cap_flush (tid, caps, is_capsnap, i_list) and continues sending cap state from it, giving a heap-reuse write and control-flow hijack primitive scored High for use-after-free.
- A:HAvailability impact, high
- Dereferencing freed cf->i_list.next oopses or panics the kicker thread even without full exploitation, and a malicious MDS can retrigger the race on every cap flush.