Public bug reported: # 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: Undecided Status: New ** Attachment added: "ipset race condition diagram 20200713a.png" https://bugs.launchpad.net/bugs/1887405/+attachment/5392348/+files/ipset%20race%20condition%20diagram%2020200713a.png -- You received this bug notification because you are a member of Yahoo! Engineering Team, which is subscribed to neutron. https://bugs.launchpad.net/bugs/1887405 Title: Race condition while processing security_groups_member_updated events (ipset) Status in neutron: New Bug description: # 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. 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