ksmbd: fix user-after-free from session log off

There is racy issue between smb2 session log off and smb2 session setup. It will cause user-after-free from session log off. This add session_lock when setting SMB2_SESSION_EXPIRED and referece count to session struct not to free session while it is being used. Reported-by: [email protected] # ZDI-CAN-25282 Signed-off-by: Namjae Jeon <[email protected]>
cifsd-team · Oct 13, 2024 · 6f6416f · 6f6416f
1 parent c1d26c6
commit 6f6416f
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Showing 4 changed files with 34 additions and 6 deletions.
diff --git a/mgmt/user_session.c b/mgmt/user_session.c
@@ -177,9 +177,10 @@ static void ksmbd_expire_session(struct ksmbd_conn *conn)
 
 	down_write(&conn->session_lock);
 	xa_for_each(&conn->sessions, id, sess) {
-		if (sess->state != SMB2_SESSION_VALID ||
-		    time_after(jiffies,
-			       sess->last_active + SMB2_SESSION_TIMEOUT)) {
+		if (atomic_read(&sess->refcnt) == 0 &&
+		    (sess->state != SMB2_SESSION_VALID ||
+		     time_after(jiffies,
+			       sess->last_active + SMB2_SESSION_TIMEOUT))) {
 			xa_erase(&conn->sessions, sess->id);
 #ifdef CONFIG_SMB_INSECURE_SERVER
 			if (hash_hashed(&sess->hlist))
@@ -284,8 +285,6 @@ struct ksmbd_session *ksmbd_session_lookup_slowpath(unsigned long long id)
 
 	down_read(&sessions_table_lock);
 	sess = __session_lookup(id);
-	if (sess)
-		sess->last_active = jiffies;
 	up_read(&sessions_table_lock);
 
 	return sess;
@@ -304,6 +303,22 @@ struct ksmbd_session *ksmbd_session_lookup_all(struct ksmbd_conn *conn,
 	return sess;
 }
 
+void ksmbd_user_session_get(struct ksmbd_session *sess)
+{
+	atomic_inc(&sess->refcnt);
+}
+
+void ksmbd_user_session_put(struct ksmbd_session *sess)
+{
+	if (!sess)
+		return;
+
+	if (atomic_read(&sess->refcnt) <= 0)
+		WARN_ON(1);
+	else
+		atomic_dec(&sess->refcnt);
+}
+
 struct preauth_session *ksmbd_preauth_session_alloc(struct ksmbd_conn *conn,
 						    u64 sess_id)
 {
@@ -417,6 +432,7 @@ static struct ksmbd_session *__session_create(int protocol)
 	xa_init(&sess->rpc_handle_list);
 	sess->sequence_number = 1;
 	rwlock_init(&sess->tree_conns_lock);
+	atomic_set(&sess->refcnt, 1);
 
 	switch (protocol) {
 #ifdef CONFIG_SMB_INSECURE_SERVER

diff --git a/mgmt/user_session.h b/mgmt/user_session.h
@@ -64,6 +64,8 @@ struct ksmbd_session {
 	struct ksmbd_file_table		file_table;
 	unsigned long			last_active;
 	rwlock_t			tree_conns_lock;
+
+	atomic_t			refcnt;
 };
 
 static inline int test_session_flag(struct ksmbd_session *sess, int bit)
@@ -97,6 +99,8 @@ void ksmbd_sessions_deregister(struct ksmbd_conn *conn);
 struct ksmbd_session *__session_lookup(unsigned long long id);
 struct ksmbd_session *ksmbd_session_lookup_all(struct ksmbd_conn *conn,
 					       unsigned long long id);
+void ksmbd_user_session_get(struct ksmbd_session *sess);
+void ksmbd_user_session_put(struct ksmbd_session *sess);
 void destroy_previous_session(struct ksmbd_conn *conn,
 			      struct ksmbd_user *user, u64 id);
 struct preauth_session *ksmbd_preauth_session_alloc(struct ksmbd_conn *conn,

diff --git a/server.c b/server.c
@@ -238,6 +238,8 @@ static void __handle_ksmbd_work(struct ksmbd_work *work,
 	} while (is_chained == true);
 
 send:
+	if (work->sess)
+		ksmbd_user_session_put(work->sess);
 	if (work->tcon)
 		ksmbd_tree_connect_put(work->tcon);
 	smb3_preauth_hash_rsp(work);

diff --git a/smb2pdu.c b/smb2pdu.c
@@ -611,8 +611,10 @@ int smb2_check_user_session(struct ksmbd_work *work)
 
 	/* Check for validity of user session */
 	work->sess = ksmbd_session_lookup_all(conn, sess_id);
-	if (work->sess)
+	if (work->sess) {
+		ksmbd_user_session_get(work->sess);
 		return 1;
+	}
 	ksmbd_debug(SMB, "Invalid user session, Uid %llu\n", sess_id);
 	return -ENOENT;
 }
@@ -1758,6 +1760,7 @@ int smb2_sess_setup(struct ksmbd_work *work)
 		}
 
 		conn->binding = true;
+		ksmbd_user_session_get(sess);
 	} else if ((conn->dialect < SMB30_PROT_ID ||
 		    server_conf.flags & KSMBD_GLOBAL_FLAG_SMB3_MULTICHANNEL) &&
 		   (req->Flags & SMB2_SESSION_REQ_FLAG_BINDING)) {
@@ -1784,6 +1787,7 @@ int smb2_sess_setup(struct ksmbd_work *work)
 		}
 
 		conn->binding = false;
+		ksmbd_user_session_get(sess);
 	}
 	work->sess = sess;
 
@@ -2245,7 +2249,9 @@ int smb2_session_logoff(struct ksmbd_work *work)
 	}
 
 	ksmbd_destroy_file_table(&sess->file_table);
+	down_write(&conn->session_lock);
 	sess->state = SMB2_SESSION_EXPIRED;
+	up_write(&conn->session_lock);
 
 	ksmbd_free_user(sess->user);
 	sess->user = NULL;