# CVE-2026-89655

## Summary

- **CVE ID:** CVE-2026-89655
- **Severity:** CRITICAL
- **CVSS Score:** 9.8 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H)
- **CWE:** N/A
- **Published:** Sep 11, 2026
- **Last Modified:** Sep 14, 2026

## Description

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

ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock

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.

## Affected Products

- Linux — Linux (1da177e4c3f41524e886b7f1b8a0c1fc7321cac2)
- Linux — Linux (0)
- Linux — Linux (6.12.109)
- Linux — Linux (6.18.50)
- Linux — Linux (7.2.4)
- Linux — Linux (7.3-rc1)
- Linux — Linux (e4500b5e35c213e0f97be7cb69328c0877203a79)
- Linux — Linux (4.8)
- Linux — Linux (5.10.270)
- Linux — Linux (5.15.221)
- Linux — Linux (6.1.188)
- Linux — Linux (6.6.157)

## References

- [CNA](https://git.kernel.org/stable/c/2701431aa3cc8b23efe6890182e7b04f5e76fab5)
- [CNA](https://git.kernel.org/stable/c/fe46746087b5b9c5bb2d022df6c7819218494ced)
- [CNA](https://git.kernel.org/stable/c/2dba24dcd5050be4b7b119e6f0b01f62203b5d26)
- [CNA](https://git.kernel.org/stable/c/7af4c4f01305b0935adf6d4301b1ec407025485d)
- [CNA](https://git.kernel.org/stable/c/091137821e1fc88f37e15201abf055c9494ddc61)
- [CNA](https://git.kernel.org/stable/c/01542430081014d80fc9e70e92d6647432ddb7fc)
- [CNA](https://git.kernel.org/stable/c/23eb34a53a53cb1a6dab1eeee830633207ac158d)
- [CNA](https://git.kernel.org/stable/c/19f16f04c2b014a7dd214dc1e42557d8530b16f3)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.70%
- **EPSS Percentile:** 51.6

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-18._