[HDFS-15588] Arbitrarily low values for `dfs.block.access.token.lifetime` aren't safe and can cause a healthy datanode to be excluded

• Type: Bug
• Status: Patch Available
• Priority: Major
• Resolution: Unresolved
• Component/s: hdfs, hdfs-client, security
• Assignee: Unassigned
• Reporter: sr2020
• Created: 18/Sep/20 19:53
• Updated: 19/Sep/20 04:48

Description

Problem:
Setting `dfs.block.access.token.lifetime` to arbitrarily low values (like 1) means the lifetime of a block token is very short, as a result some healthy datanodes could be wrongly excluded by the client due to the `InvalidBlockTokenException`.

More specifically, in `nextBlockOutputStream`, the client tries to get the `accessToken` from the namenode and use it to talk to datanode. And the lifetime of `accessToken` could set to very small (like 1 min) by setting `dfs.block.access.token.lifetime`. In some extreme conditions (like a VM migration, temporary network issue, or a stop-the-world GC), the `accessToken` could become expired when the client tries to use it to talk to the datanode. If expired, `createBlockOutputStream` will return false (and mask the `InvalidBlockTokenException`), so the client will think the datanode is unhealthy, mark the it as "excluded" and will never read/write on it.

Related code in `nextBlockOutputStream`:

// Connect to first DataNode in the list.
success = createBlockOutputStream(nodes, nextStorageTypes, nextStorageIDs,
    0L, false);

if (!success) {
  LOG.warn("Abandoning " + block);
  dfsClient.namenode.abandonBlock(block.getCurrentBlock(),
      stat.getFileId(), src, dfsClient.clientName);
  block.setCurrentBlock(null);
  final DatanodeInfo badNode = nodes[errorState.getBadNodeIndex()];
  LOG.warn("Excluding datanode " + badNode);
  excludedNodes.put(badNode, badNode);
}

 

Proposed solution:
A simple retry on the same datanode after catching `InvalidBlockTokenException` can solve this problem (assuming the extreme conditions won't happen often). Since currently the `dfs.block.access.token.lifetime` can even accept values like 0, we can also choose to prevent the users from setting `dfs.block.access.token.lifetime` to a small value (e.g., we can enforce a minimum value of 5mins for this parameter).

We submit a patch for retrying after catching `InvalidBlockTokenException` in `nextBlockOutputStream`. We can also provide a patch for enforcing a larger minimum value for `dfs.block.access.token.lifetime` if it is a better way to handle this.