logs: only allow organization members to access them

The bot's logs are accessible to everyone at the /logs endpoint. This is
dangerous because the logs may contain sensitive info.

Fix this by only allowing se-edu organization members to view the logs.

Note that our access control doesn't actually check if the current user
is a member of se-edu, but rather whether the user has access to the
se-edu-bot installation in the se-edu organization. This should be
equivalent.

README.adoc

@@ -274,6 +274,8 @@ which provides some useful base functionality such as splitting the `webhookMidd
 To facilitate debugging problems in production,
 se-edu-bot exposes its logs via the `/logs` endpoint.
 
+You need to be a member of the `se-edu` organization in order to access them.
+
 == Coding standard
 
 We follow the oss-generic coding standard.

lib/Auth.ts

@@ -3,6 +3,7 @@ import koaCompose = require('koa-compose');
 import koaRoute = require('koa-route');
 import simpleOauth2 = require('simple-oauth2');
 import createError = require('http-errors');
+import * as github from '../lib/github';
 
 /**
  * Options to pass to {@link Auth}.
@@ -59,6 +60,44 @@ export class Auth {
         ]);
     }
 
+    createAccessControlByInstallationId(installationId: number): Koa.Middleware {
+        return async (ctx: Koa.Context, next?: () => Promise<void>): Promise<void> => {
+            const ghUserApi = this.getGhUserApi(ctx);
+            if (!ghUserApi) {
+                ctx.redirect(this.getLoginRedirect(ctx));
+                return;
+            }
+
+            // Check that the user has access to our installation
+            const installationIds: number[] = [];
+            await github.forEachPage(ghUserApi, {
+                url: 'user/installations',
+            }, body => {
+                const installations: any[] = body.installations;
+                installations.forEach(installation => installationIds.push(installation.id));
+            });
+
+            if (!installationIds.includes(installationId)) {
+                throw createError(403, 'User is not authorized to access this page');
+            }
+
+            if (next) {
+                await next();
+            }
+        };
+    }
+
+    private getGhUserApi(ctx: Koa.Context): github.RequestApi | undefined {
+        const accessToken = ctx.cookies.get(this.accessTokenCookieName);
+        if (!accessToken) {
+            return;
+        }
+        return github.createAccessTokenApi({
+            accessToken,
+            userAgent: this.userAgent,
+        });
+    }
+
     private async loginMiddleware(ctx: Koa.Context): Promise<void> {
         const authorizationUri = this.oauth2.authorizationCode.authorizeURL({
             redirect_uri: this.getOauthRedirectUri(ctx),
@@ -112,6 +151,11 @@ export class Auth {
         }
         return redirect;
     }
+
+    private getLoginRedirect(ctx: Koa.Context): string {
+        const redirect = `${ctx.path}${ctx.search}`;
+        return `${this.loginRoute}?redirect=${encodeURIComponent(redirect)}`;
+    }
 }
 
 export default Auth;

server.ts

@@ -100,6 +100,7 @@ export function createApp(appConfig: AppConfig): Koa {
         fileName: logFileName,
     });
     app.use(koaRoute.get('/logs', koaCompose<Koa.Context>([
+        auth.createAccessControlByInstallationId(appConfig.githubInstallationId),
         async ctx => {
             ctx.type = 'text';
             ctx.body = logger.createReadableStream();