[PATCH 2/2] ceph: fix UAF in check_new_map() on session freed during unlock

[Xiubo Li via B4 Relay]

From: Xiubo Li <xiubo.li@xxxxxxxxx>

check_new_map() iterates mdsc->sessions[] and for each active session
drops mdsc->mutex to perform per-session operations.  The forced-close
path (rank removed from map) correctly takes a reference on s via
ceph_get_mds_session() before releasing mdsc->mutex, but three other
paths do not:

  Path A (address changed):  mutex_unlock → mutex_lock(&s->s_mutex)
  Path B (reconnect):        mutex_unlock → send_mds_reconnect(mdsc, s)
  Path C (active transition): mutex_unlock → mutex_lock(&s->s_mutex)

Without the extra reference, another thread can acquire mdsc->mutex
during the unlock window, call __unregister_session() which drops the
last reference on s, and free it.  The original thread then accesses
freed memory via s->s_mutex.

Fix by adding ceph_get_mds_session(s) before each mutex_unlock and
ceph_put_mds_session(s) after the corresponding mutex_lock, matching
the pattern already used in the forced-close path.

Race timeline (Path A):

  Thread A (check_new_map)             Thread B (another map update
    holds mdsc->mutex                      or session teardown)
  --------------------------           --------------------------
  s = mdsc->sessions[i]
  (refcount == 1, held only by
   sessions[] array)

  mutex_unlock(&mdsc->mutex)
                               --->    acquires mdsc->mutex
                                       __unregister_session(mdsc, s)
                                         sessions[i] = NULL
                                         ceph_put_mds_session(s)
                                           refcount: 1 -> 0
                                           kfree(s)  <--- freed!

  mutex_lock(&s->s_mutex)
  UAF on freed s->s_mutex

Signed-off-by: Xiubo Li <xiubo.li@xxxxxxxxx>
---
 fs/ceph/mds_client.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 18be36c2e9e8..00a401f9f73b 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -5827,9 +5827,11 @@ static void check_new_map(struct ceph_mds_client *mdsc,
 			   ceph_mdsmap_get_addr(newmap, i),
 			   sizeof(struct ceph_entity_addr))) {
 			/* just close it */
+			ceph_get_mds_session(s);
 			mutex_unlock(&mdsc->mutex);
 			mutex_lock(&s->s_mutex);
 			mutex_lock(&mdsc->mutex);
+			ceph_put_mds_session(s);
 			ceph_con_close(&s->s_con);
 			mutex_unlock(&s->s_mutex);
 			s->s_state = CEPH_MDS_SESSION_RESTARTING;
@@ -5844,6 +5846,7 @@ static void check_new_map(struct ceph_mds_client *mdsc,
 		    newstate >= CEPH_MDS_STATE_RECONNECT) {
 			int rc;
 
+			ceph_get_mds_session(s);
 			mutex_unlock(&mdsc->mutex);
 			clear_bit(i, targets);
 			rc = send_mds_reconnect(mdsc, s);
@@ -5852,6 +5855,7 @@ static void check_new_map(struct ceph_mds_client *mdsc,
 					       "mds%d reconnect failed: %d\n",
 					       i, rc);
 			mutex_lock(&mdsc->mutex);
+			ceph_put_mds_session(s);
 		}
 
 		/*
@@ -5864,9 +5868,11 @@ static void check_new_map(struct ceph_mds_client *mdsc,
 				pr_info_client(cl, "mds%d recovery completed\n",
 					       s->s_mds);
 			kick_requests(mdsc, i);
+			ceph_get_mds_session(s);
 			mutex_unlock(&mdsc->mutex);
 			mutex_lock(&s->s_mutex);
 			mutex_lock(&mdsc->mutex);
+			ceph_put_mds_session(s);
 			ceph_kick_flushing_caps(mdsc, s);
 			mutex_unlock(&s->s_mutex);
 			wake_up_session_caps(s, RECONNECT);

-- 
2.53.0