[Bug 1887405] [NEW] Race condition while processing security_groups_member_updated events (ipset)

Fri Mar 12 14:44:55 UTC 2021

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# Summary

Race condition while processing security_groups_member_updated events
(ipset)

# Overview

We have a customer that uses heat templates to deploy large environments
(e.g. 21 instances) with a significant number of security groups (e.g.
60) that use bi-directional remote group references for both ingress and
egress filtering.  These heat stacks are deployed using a CI pipeline
and intermittently suffer from application layer failures due to broken
network connectivity.  We found that this was caused by the ipsets used
to implement remote_group memberships missing IPs from their member
lists.  Troubleshooting suggests this is caused by a race condition,
which I've attempted to describe in detail below.

Version: `54e1a6b1bc378c0745afc03987d0fea241b826ae` (HEAD of
stable/rocky as of Jan 26, 2020), though I suspect this issue persists
through master.

I'm working on getting some multi-node environments deployed (I don't
think it's possible to reproduce this with a single hypervisor) and hope
to provide reproduction steps on Rocky and master soon.  I wanted to get
this report submitted as-is with the hopes that an experienced Neutron
dev might be able to spot possible solutions or provide diagnostic
insight that I am not yet able to produce.

I suspect this report may be easier to read with some markdown, so
please feel free to read it in a gist:
https://gist.github.com/cfarquhar/20fddf2000a83216021bd15b512f772b

Also, this diagram is probably critical to following along: https
://user-images.githubusercontent.com/1253665/87317744-0a75b180-c4ed-
11ea-9bad-085019c0f954.png

# Race condition symptoms

Given the following security groups/rules:

```
| secgroup name | secgroup id                          | direction | remote group                         | dest port |
|---------------|--------------------------------------|-----------|--------------------------------------|-----------|
| server        | fcd6cf12-2ac9-4704-9208-7c6cb83d1a71 | ingress   | b52c8c54-b97a-477d-8b68-f4075e7595d9 | 9092      |
| client        | b52c8c54-b97a-477d-8b68-f4075e7595d9 | egress    | fcd6cf12-2ac9-4704-9208-7c6cb83d1a71 | 9092      |
```

And the following instances:

```
| instance name | hypervisor | ip          | secgroup assignment |
|---------------|------------|-------------|---------------------|
| server01      | compute01  | 192.168.0.1 | server              |
| server02      | compute02  | 192.168.0.2 | server              |
| server03      | compute03  | 192.168.0.3 | server              |
| client01      | compute04  | 192.168.0.4 | client              |
```

We would expect to find the following ipset representing the `server`
security group members on `compute04`:

```
# ipset list NIPv4fcd6cf12-2ac9-4704-9208-
Name: NIPv4fcd6cf12-2ac9-4704-9208-
Type: hash:net
Revision: 6
Header: family inet hashsize 1024 maxelem 65536
Size in memory: 536
References: 4
Number of entries: 3
Members:
192.168.0.1
192.168.0.2
192.168.0.3
```

What we actually get when the race condition is triggered is an
incomplete list of members in the ipset.  The member list could contain
anywhere between zero and two of the expected IPs.

# Triggering the race condition

The problem occurs when `security_group_member_updated` events arrive between `port_update` steps 12 and 22 (see diagram and process details below).  
  - `port_update` step 12 retrieves the remote security groups' member lists, which are not necessarily complete yet.
  - `port_update` step 22 adds the port to `IptablesFirewallDriver.ports()`.

This results in `security_group_member_updated` step 3 looking for the
port to apply the updated member list to (in
`IptablesFirewallDriver.ports()`) BEFORE it has been added by
`port_update`'s step 22. This causes the membership update event to
effectively be discarded.  We are then left with whatever the remote
security group's member list was when the `port_update` process
retrieved it at step 12.  This state persists until something triggers
the port being re-added to the `updated_ports` list (e.g. agent restart,
another remote group membership change, local security group
addition/removal, etc).

# Race condition details

The race condition occurs in the linuxbridge agent between the two following operations:
  1) Processing a `port_update` event when an instance is first created
  2) Processing `security_group_member_updated` events for the instance's remote security groups.

Either of these operations can result in creating or mutating an ipset
from `IpsetManager.set_members()`.  The relevant control flow sequence
for each operation is listed below.  I've left out any branches that did
not seem to be relevant to the race condition.

## Processing a `port_update` event:
  1) We receive an RPC port_update event via `LinuxBridgeRpcCallbacks.port_update()`, which adds the tap device to the `LinuxBridgeRpcCallbacks.updated_devices` list
  2) Sleep until the next `CommonAgentLoop.daemon_loop()` iteration
  3) `CommonAgentLoop.daemon_loop()` calls `CommonAgentLoop.scan_devices()`
  4) `CommonAgentLoop.scan_devices()` calls `LinuxBridgeRpcCallbacks.get_and_clear_updated_devices()` to retrieve and clear the `updated_devices` list from step 1.
  5) `CommonAgentLoop.scan_devices()` performs some calculations and returns control to `CommonAgentLoop.daemon_loop()` along with a list of added or updated devices.
  6) `CommonAgentLoop.daemon_loop()` calls `CommonAgentLoop.process_network_devices()` when there are added or updated devices from step 5.
  7) `CommonAgentLoop.process_network_devices()` calls `SecurityGroupAgentRpc.setup_port_filters()` with the added or updated devices from step 5.
  8) `SecurityGroupAgentRpc.setup_port_filters()` checks for devices that were added to the `SecurityGroupAgentRpc.devices_to_refilter` list (which happens in step 4 of the `security_group_member_updated` process) and appends them to the list of updated devices.  This is where devices from the `security_group_member_updated` process would have been processed if they weren't lost due to the race condition.
  9)  `SecurityGroupAgentRpc.setup_port_filters()` calls `SecurityGroupAgentRpc.prepare_devices_filter()` with the updated port IDs
  10) `SecurityGroupAgentRpc.prepare_devices_filter()` calls `SecurityGroupAgentRpc._apply_port_filter()` with the updated port IDs
  11) `SecurityGroupAgentRpc._apply_port_filter()` calls `SecurityGroupServerRpcApi.security_group_info_for_devices()` with the tap device id
  12) `SecurityGroupServerRpcApi.security_group_info_for_devices()` retrieves detailed information about the port via RPC.  The important detail here is a `sg_member_ids` dict which contains a list of member IPs for each remote security groups applicable to the port.
  13) `SecurityGroupServerRpcApi.security_group_info_for_devices()` returns control to `SecurityGroupAgentRpc._apply_port_filter()` along with the port and security group details.
  14) `SecurityGroupAgentRpc._apply_port_filter()` calls `SecurityGroupAgentRpc._update_security_group_info()` with the remote security groups and their member IP list.
  15) `SecurityGroupAgentRpc._update_security_group_info()` iterates over the remote security groups and calls `IptablesFirewallDriver.update_security_group_members()` for each one
  16) `IptablesFirewallDriver.update_security_group_members()` calls `IptablesFirewallDriver._update_ipset_members()`
  17) `IptablesFirewallDriver._update_ipset_members()` calls `IpsetManager.set_members()`
  18) `IpsetManager.set_members()` calls a number of methods to create or mutate the ipset using linux's `ipset` commands.
  19) The stack unwinds back up to `SecurityGroupAgentRpc._apply_port_filter()` (last seen in step 11)
  20) `SecurityGroupAgentRpc._apply_port_filter()` calls `IptablesFirewallDriver.prepare_port_filter()` with the port
  21) `IptablesFirewallDriver.prepare_port_filter()` calls `IptablesFirewallDriver._set_ports()` with the port details retrieved in step 12
  22) `IptablesFirewallDriver._set_ports()` adds the port to `IptablesFirewallDriver.filtered_ports`, which indicates the port is known to exist on the hypervisor.  It will now be returned by IptablesFirewallDriver.ports().

## Processing a `security_group_member_updated` event:
  1) We receive an RPC security_group_member_updated event via `SecurityGroupAgentRpc.security_groups_member_updated()`
  2) `SecurityGroupAgentRpc.security_groups_member_updated()` calls `SecurityGroupAgentRpc._security_group_updated()` with the list of security groups whose membership was updated.
  3) `SecurityGroupAgentRpc._security_group_updated()` consults `IptablesFirewallDriver.ports()` to find ports affected by the membership updates.  In the race condition, `port_update` has not yet reached step 22 to add the port we need to find, so we stop here and the following step does not occur.
  4) `SecurityGroupAgentRpc._security_group_updated()` adds each affected port from the previous step to the `SecurityGroupAgentRpc.devices_to_refilter` list.  This list will be processed next time we reach `port_event` step 8, but in the race condition we never get here.

** Affects: neutron
     Importance: Medium
     Assignee: Charles Farquhar (cfarquhar)
         Status: Fix Released

** Affects: neutron (Ubuntu)
     Importance: Undecided
         Status: New

** Tags: sg-fw
-- 
Race condition while processing security_groups_member_updated events (ipset)
https://bugs.launchpad.net/bugs/1887405
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