[PATCH 1/2] ceph: fix UAF in __kick_flushing_caps() on cf entry freed during unlock

From: Xiubo Li <[email protected]>

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.

Signed-off-by: Xiubo Li <[email protected]>
---
 fs/ceph/caps.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
index f8d898ad091e..358ee4f14524 100644
--- a/fs/ceph/caps.c
+++ b/fs/ceph/caps.c
@@ -2626,9 +2626,14 @@ static void __kick_flushing_caps(struct ceph_mds_client *mdsc,
 		}
 	}
 
-	list_for_each_entry(cf, &ci->i_cap_flush_list, i_list) {
-		if (cf->tid < first_tid)
+	cf = list_first_entry(&ci->i_cap_flush_list, struct ceph_cap_flush, i_list);
+	while (&cf->i_list != &ci->i_cap_flush_list) {
+		struct ceph_cap_flush *next;
+
+		if (cf->tid < first_tid) {
+			cf = list_next_entry(cf, i_list);
 			continue;
+		}
 
 		cap = ci->i_auth_cap;
 		if (!(cap && cap->session == session)) {
@@ -2638,6 +2643,7 @@ static void __kick_flushing_caps(struct ceph_mds_client *mdsc,
 		}
 
 		first_tid = cf->tid + 1;
+		next = list_next_entry(cf, i_list);
 
 		if (!cf->is_capsnap) {
 			struct cap_msg_args arg;
@@ -2678,6 +2684,7 @@ static void __kick_flushing_caps(struct ceph_mds_client *mdsc,
 		}
 
 		spin_lock(&ci->i_ceph_lock);
+		cf = next;
 	}
 }
 

-- 
2.53.0