Merge bitcoin#23631: p2p: Don't use timestamps from inbound peers for…

… Adjusted Time

0c85dc3 p2p: Don't use timestamps from inbound peers (Martin Zumsande)

Pull request description:

  `GetAdjustedTime()` (used e.g. in validation and addrman) returns a time with an offset that is influenced by timestamps that our peers have sent us in their version message.

  Currently, timestamps from all peers are used for this.
  However, I think that it would make sense to ignore the timedata samples from inbound peers, making it much harder for others to influence the Adjusted Time in a targeted way.
  With the extra feeler connections (every 2 minutes on average) and extra block-relay-only connections (every 5 minutes on average) there are also now plenty of opportunities to gather a meaningful number of timedata samples from outbound peers.

  There are some measures in place to prevent abuse: the `-maxtimeadjustment` parameter with a default of 70 minutes, warnings in cases of large deviations, only using the first 200 samples ([explanation](https://github.com/bitcoin/bitcoin/blob/383d350bd5107bfe00e3b90a00cab9a3c1397c72/src/timedata.cpp#L57-L72)), but I think that only using samples from outbound connections in the first place would be an additional safety measure that would make sense.

  See also issue dashpay#4521 for further context and links: There have been several discussions in the past about replacing or abolishing the existing timedata system.

ACKs for top commit:
  jnewbery:
    Concept and code review ACK 0c85dc3
  naumenkogs:
    ACK 0c85dc3
  vasild:
    ACK 0c85dc3

Tree-SHA512: 2d6375305bcae034d68b58b7a07777b40ac430dfed554c88e681a048c527536691e1b7d08c0ef995247d356f8e81aa0a4b983bf2674faf6a416264e5f1af0a96

src/net_processing.cpp

@@ -3651,7 +3651,11 @@ void PeerManagerImpl::ProcessMessage(
 
         int64_t nTimeOffset = nTime - GetTime();
         pfrom.nTimeOffset = nTimeOffset;
-        AddTimeData(pfrom.addr, nTimeOffset);
+        if (!pfrom.IsInboundConn()) {
+            // Don't use timedata samples from inbound peers to make it
+            // harder for others to tamper with our adjusted time.
+            AddTimeData(pfrom.addr, nTimeOffset);
+        }
 
         // Feeler connections exist only to verify if address is online.
         if (pfrom.IsFeelerConn()) {