iscsi-target: Fix use-after-free during TPG session shutdown

This patch fixes a use-after-free bug in iscsit_release_sessions_for_tpg()
where se_portal_group->session_lock was incorrectly released/re-acquired
while walking the active se_portal_group->tpg_sess_list.

The can result in a NULL pointer dereference when iscsit_close_session()
shutdown happens in the normal path asynchronously to this code, causing
a bogus dereference of an already freed list entry to occur.

To address this bug, walk the session list checking for the same state
as before, but move entries to a local list to avoid dropping the lock
while walking the active list.

As before, signal using iscsi_session->session_restatement=1 for those
list entries to be released locally by iscsit_free_session() code.

Reported-by: Sunilkumar Nadumuttlu <sjn@datera.io>
Cc: Sunilkumar Nadumuttlu <sjn@datera.io>
Cc: <stable@vger.kernel.org> # v3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
index 4e68b62193ed..afab32376126 100644
--- a/drivers/target/iscsi/iscsi_target.c
+++ b/drivers/target/iscsi/iscsi_target.c
@@ -4765,6 +4765,7 @@ int iscsit_release_sessions_for_tpg(struct iscsi_portal_group *tpg, int force)
         struct iscsi_session *sess;
         struct se_portal_group *se_tpg = &tpg->tpg_se_tpg;
         struct se_session *se_sess, *se_sess_tmp;
+        LIST_HEAD(free_list);
         int session_count = 0;
 
         spin_lock_bh(&se_tpg->session_lock);
@@ -4786,14 +4787,17 @@ int iscsit_release_sessions_for_tpg(struct iscsi_portal_group *tpg, int force)
                 }
                 atomic_set(&sess->session_reinstatement, 1);
                 spin_unlock(&sess->conn_lock);
-                spin_unlock_bh(&se_tpg->session_lock);
 
-                iscsit_free_session(sess);
-                spin_lock_bh(&se_tpg->session_lock);
+                list_move_tail(&se_sess->sess_list, &free_list);
+        }
+        spin_unlock_bh(&se_tpg->session_lock);
 
+        list_for_each_entry_safe(se_sess, se_sess_tmp, &free_list, sess_list) {
+                sess = (struct iscsi_session *)se_sess->fabric_sess_ptr;
+
+                iscsit_free_session(sess);
                 session_count++;
         }
-        spin_unlock_bh(&se_tpg->session_lock);
 
         pr_debug("Released %d iSCSI Session(s) from Target Portal"
                         " Group: %hu\n", session_count, tpg->tpgt);