CVE-2026-89655: ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock
In the Linux kernel, the following vulnerability has been resolved:
ceph: fix UAF in kickflushingcaps() on cf entry freed during unlock
listforeachentry() iterates ci->icapflushlist but drops icephlock to send cap messages. During the unlock window, handlecapflushack() can acquire icephlock, detach cf entries with tid <= flushtid from the list, release icephlock, and free them via cephfreecapflush() outside any lock. When the original thread reacquires icephlock and the for-loop macro advances via cf = listnextentry(cf, ilist), it dereferences cf->ilist.next on freed memory.
The race timeline:
kickflushingcaps() handlecapflushack() ----------------------- ----------------------- holds icephlock <--- iterates to cf (tid=10) prepares FLUSH message drops icephlock <--- sendcap() ── FLUSH(tid=10) MDS sends FLUSHACK(tid=10) ---> acquires icephlock cf->tid(10) <= flushtid(10), detaches cf from icapflushlist drops icephlock cephfreecapflush(cf) <- frees it! acquires icephlock <--- for-loop advances: cf = listnextentry(cf, ilist) -- UAF on freed cf->ilist.next
The cf was just sent by kickflushingcaps itself via sendcap(). The MDS may respond with FLUSHACK quickly enough that handlecapflushack() frees cf before kickflushingcaps can finish the iteration.
Fix by converting to a manual while loop: save the next pointer under icephlock before dropping it, then use the saved pointer after reacquiring, so the potentially-freed cf is never accessed again.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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In __kick_flushing_caps(), replace the list_for_each_entry() iteration with a manual while loop that saves the next pointer under i_ceph_lock before dropping the lock, then uses the saved pointer after reacquiring the lock so a potentially freed cf entry is never accessed.
Event History
Frequently Asked Questions
Are specific affected or fixed Linux kernel versions identified?
No. The available information does not name affected or fixed kernel versions.
Where can I find the referenced fixes?
Three stable kernel commits are referenced: 2701431aa3cc8b23efe6890182e7b04f5e76fab5, fe46746087b5b9c5bb2d022df6c7819218494ced, and 2dba24dcd5050be4b7b119e6f0b01f62203b5d26.