[feature/bugfix] Probe S3 storage for CSP uri, add config flag for extra URIs (#2134)

* [feature/bugfix] Probe S3 storage for CSP uri, add config flag for extra URIs

* env parsing tests, my coy mistress

internal/middleware/contentsecuritypolicy.go

@@ -0,0 +1,144 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package middleware
+
+import (
+	"strings"
+
+	"codeberg.org/gruf/go-debug"
+	"github.com/gin-gonic/gin"
+)
+
+func ContentSecurityPolicy(extraURIs ...string) gin.HandlerFunc {
+	csp := BuildContentSecurityPolicy(extraURIs...)
+
+	return func(c *gin.Context) {
+		// Inform the browser we only load
+		// CSS/JS/media using the given policy.
+		c.Header("Content-Security-Policy", csp)
+	}
+}
+
+func BuildContentSecurityPolicy(extraURIs ...string) string {
+	const (
+		defaultSrc = "default-src"
+		objectSrc  = "object-src"
+		imgSrc     = "img-src"
+		mediaSrc   = "media-src"
+
+		self = "'self'"
+		none = "'none'"
+		blob = "blob:"
+	)
+
+	// CSP values keyed by directive.
+	values := make(map[string][]string, 4)
+
+	/*
+		default-src
+		https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/default-src
+	*/
+
+	if !debug.DEBUG {
+		// Restrictive 'self' policy
+		values[defaultSrc] = []string{self}
+	} else {
+		// If debug is enabled, allow
+		// serving things from localhost
+		// as well (regardless of port).
+		values[defaultSrc] = []string{
+			self,
+			"localhost:*",
+			"ws://localhost:*",
+		}
+	}
+
+	/*
+		object-src
+		https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/object-src
+	*/
+
+	// Disallow object-src as recommended.
+	values[objectSrc] = []string{none}
+
+	/*
+		img-src
+		https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/img-src
+	*/
+
+	// Restrictive 'self' policy,
+	// include extraURIs, and 'blob:'
+	// for previewing uploaded images
+	// (header, avi, emojis) in settings.
+	values[imgSrc] = append(
+		[]string{self, blob},
+		extraURIs...,
+	)
+
+	/*
+		media-src
+		https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/media-src
+	*/
+
+	// Restrictive 'self' policy,
+	// include extraURIs.
+	values[mediaSrc] = append(
+		[]string{self},
+		extraURIs...,
+	)
+
+	/*
+		Assemble policy directives.
+	*/
+
+	// Iterate through an ordered slice rather than
+	// iterating through the map, since we want these
+	// policyDirectives in a determinate order.
+	policyDirectives := make([]string, 4)
+	for i, directive := range []string{
+		defaultSrc,
+		objectSrc,
+		imgSrc,
+		mediaSrc,
+	} {
+		// Each policy directive should look like:
+		// `[directive] [value1] [value2] [etc]`
+
+		// Get assembled values
+		// for this directive.
+		values := values[directive]
+
+		// Prepend values with
+		// the directive name.
+		directiveValues := append(
+			[]string{directive},
+			values...,
+		)
+
+		// Space-separate them.
+		policyDirective := strings.Join(directiveValues, " ")
+
+		// Done.
+		policyDirectives[i] = policyDirective
+	}
+
+	// Content-security-policy looks like this:
+	// `Content-Security-Policy: <policy-directive>; <policy-directive>`
+	// So join each policy directive appropriately.
+	return strings.Join(policyDirectives, "; ")
+}