Add runner-level config to enforce PR security

This addresses actions#494 at the runner level to give "just enough protection"
to allow using self-hosted runners with public repos.

By default, the current behaviour is unchanged -- all jobs passed to the
runner are executed.

If the `.runner` config file has this block added to it:

```
  "pullRequestSecurity": {}
```

Then by only PRs from "CONTRIBUTORS" (as defined by the field in
https://docs.github.com/en/free-pro-team@latest/graphql/reference/objects#pullrequest
-- nothing for us to have to work out ourselves.)

It is also possible to explicitly list users that are allowed to run
jobs on this worker:

```
  "pullRequestSecurity": {
    "allowedAuthors": ["ashb"]
  }
```

Or to _only_ allow the given users, but not all contributors:

```
  "pullRequestSecurity": {
    "allowContributors": false,
    "allowedAuthors": ["ashb"]
  }
```

Owners of the repo are always allowed to run jobs.

src/Runner.Common/ConfigurationStore.cs

@@ -1,6 +1,7 @@
 using GitHub.Runner.Common.Util;
 using GitHub.Runner.Sdk;
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Runtime.Serialization;
@@ -45,6 +46,9 @@ public sealed class RunnerSettings
         [DataMember(EmitDefaultValue = false)]
         public string MonitorSocketAddress { get; set; }
 
+        [DataMember(Name = "PullRequestSecurity", EmitDefaultValue = false)]
+        public PullRequestSecuritySettings PullRequestSecuritySettings { get; set; }
+
         [IgnoreDataMember]
         public bool IsHostedServer
         {
@@ -98,6 +102,18 @@ private void OnSerializing(StreamingContext context)
         }
     }
 
+    [DataContract]
+    public sealed class PullRequestSecuritySettings
+    {
+        // pullRequestSecurity is optional in the config -- if the key is
+        // defined, assume that we only want collaborators to run PRs.
+        [DataMember(EmitDefaultValue = false)]
+        public HashSet<string> AllowedAuthors = new HashSet<string>();
+
+        [DataMember(EmitDefaultValue = false)]
+        public bool AllowContributors = true;
+    }
+
     [ServiceLocator(Default = typeof(ConfigurationStore))]
     public interface IConfigurationStore : IRunnerService
     {

src/Runner.Worker/GitHubContext.cs

@@ -56,5 +56,16 @@ public GitHubContext ShallowCopy()
 
             return copy;
         }
+
+        public bool IsPullRequest()
+        {
+            PipelineContextData data;
+            if (TryGetValue("event_name", out data))
+            {
+              var eventName = data as StringContextData;
+              return eventName == "pull_request";
+            }
+            return false;
+        }
     }
 }

src/Runner.Worker/JobRunner.cs

@@ -12,6 +12,7 @@
 using System.Net.Http;
 using GitHub.Runner.Common;
 using GitHub.Runner.Sdk;
+using GitHub.DistributedTask.Pipelines.ContextData;
 
 namespace GitHub.Runner.Worker
 {
@@ -63,6 +64,14 @@ public async Task<TaskResult> RunAsync(Pipelines.AgentJobRequestMessage message,
                 jobContext.InitializeJob(message, jobRequestCancellationToken);
                 Trace.Info("Starting the job execution context.");
                 jobContext.Start();
+                var githubContext = jobContext.ExpressionValues["github"] as GitHubContext;
+
+                if (!JobPassesSecurityRestrictions(jobContext))
+                {
+                    jobContext.Error("Running job on this worker disallowed by security policy");
+                    return await CompleteJobAsync(jobServer, jobContext, message, TaskResult.Failed);
+                }
+
                 jobContext.Debug($"Starting: {message.JobDisplayName}");
 
                 runnerShutdownRegistration = HostContext.RunnerShutdownToken.Register(() =>
@@ -189,6 +198,81 @@ public async Task<TaskResult> RunAsync(Pipelines.AgentJobRequestMessage message,
             }
         }
 
+        private bool JobPassesSecurityRestrictions(IExecutionContext jobContext)
+        {
+            var gitHubContext = jobContext.ExpressionValues["github"] as GitHubContext;
+
+            try {
+              if (gitHubContext.IsPullRequest())
+              {
+                  return OkayToRunPullRequest(gitHubContext);
+              }
+
+              return true;
+            }
+            catch (Exception ex)
+            {
+                Trace.Error("Caught exception in JobPassesSecurityRestrictions");
+                Trace.Error("As a safety precaution we are not allowing this job to run");
+                Trace.Error(ex);
+                return false;
+            }
+        }
+
+        private bool OkayToRunPullRequest(GitHubContext gitHubContext)
+        {
+            var configStore = HostContext.GetService<IConfigurationStore>();
+            var settings = configStore.GetSettings();
+            var prSecuritySettings = settings.PullRequestSecuritySettings;
+
+            if (prSecuritySettings is null) {
+                Trace.Info("No pullRequestSecurity defined in settings, allowing this build");
+                return true;
+            }
+
+            var githubEvent = gitHubContext["event"] as DictionaryContextData;
+            var prData = githubEvent["pull_request"] as DictionaryContextData;
+
+            var authorAssociation = prData.TryGetValue("author_association", out var value)
+              ? value as StringContextData : null;
+
+
+            // TODO: Allow COLLABORATOR, MEMBER too -- possibly by a config setting
+            if (authorAssociation == "OWNER")
+            {
+                Trace.Info("PR is from the repo owner, always allowed");
+                return true;
+            }
+            else if (prSecuritySettings.AllowContributors && authorAssociation == "COLLABORATOR") {
+                Trace.Info("PR is from the repo collaborator, allowing");
+                return true;
+            }
+
+            var prHead = prData["head"] as DictionaryContextData;
+            var prUser = prHead["user"] as DictionaryContextData;
+            var prUserLogin = prUser["login"] as StringContextData;
+
+            Trace.Info($"GitHub PR author is {prUserLogin as StringContextData}");
+
+            if (prUserLogin == null)
+            {
+                Trace.Info("Unable to get PR author, not allowing PR to run");
+                return false;
+            }
+
+            if (prSecuritySettings.AllowedAuthors.Contains(prUserLogin))
+            {
+                Trace.Info("Author in PR allowed list");
+                return true;
+            }
+            else
+            {
+                Trace.Info($"Not running job as author ({prUserLogin}) is not in {{{string.Join(", ", prSecuritySettings.AllowedAuthors)}}}");
+
+                return false;
+            }
+        }
+
         private async Task<TaskResult> CompleteJobAsync(IJobServer jobServer, IExecutionContext jobContext, Pipelines.AgentJobRequestMessage message, TaskResult? taskResult = null)
         {
             jobContext.Debug($"Finishing: {message.JobDisplayName}");