p2p, rpc: don't allow past absolute timestamp in `setban`

src/rpc/net.cpp

@@ -741,6 +741,10 @@ static RPCHelpMan setban()
 
         const bool absolute{request.params[3].isNull() ? false : request.params[3].get_bool()};
 
+        if (absolute && banTime < GetTime()) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Error: Absolute timestamp is in the past");
+        }
+
         if (isSubnet) {
             node.banman->Ban(subNet, banTime, absolute);
             if (node.connman) {

test/functional/p2p_disconnect_ban.py

@@ -46,6 +46,9 @@ class DisconnectBanTest(BitcoinTestFramework):
         assert_raises_rpc_error(-30, "Error: Invalid IP/Subnet", self.nodes[1].setban, "127.0.0.1/42", "add")
         assert_equal(len(self.nodes[1].listbanned()), 1)  # still only one banned ip because 127.0.0.1 is within the range of 127.0.0.0/24
 
+        self.log.info("setban: fail to ban with past absolute timestamp")
+        assert_raises_rpc_error(-8, "Error: Absolute timestamp is in the past", self.nodes[1].setban, "127.27.0.1", "add", 123, True)
+
         self.log.info("setban remove: fail to unban a non-banned subnet")
         assert_raises_rpc_error(-30, "Error: Unban failed", self.nodes[1].setban, "127.0.0.1", "remove")
         assert_equal(len(self.nodes[1].listbanned()), 1)