CVE-2026-89655ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock

Official 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 packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/091137821e1fc88f37e15201abf055c9494ddc61
• https://git.kernel.org/stable/c/01542430081014d80fc9e70e92d6647432ddb7fc
• https://git.kernel.org/stable/c/23eb34a53a53cb1a6dab1eeee830633207ac158d
• https://git.kernel.org/stable/c/19f16f04c2b014a7dd214dc1e42557d8530b16f3
• https://git.kernel.org/stable/c/2701431aa3cc8b23efe6890182e7b04f5e76fab5
• https://git.kernel.org/stable/c/fe46746087b5b9c5bb2d022df6c7819218494ced
• https://git.kernel.org/stable/c/2dba24dcd5050be4b7b119e6f0b01f62203b5d26
• https://git.kernel.org/stable/c/7af4c4f01305b0935adf6d4301b1ec407025485d