Overview
About vulnerability
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.
Details
- Affected product:
- Debian 10 ELS , Debian 11 ELS , Ubuntu 18.04 ELS , Ubuntu 20.04 ELS
- Affected packages:
- linux @ 5.10.259 (+3 more)