1 files changed, 251 insertions, 0 deletions
diff --git a/internal/middleware/headerfilter.go b/internal/middleware/headerfilter.go
new file mode 100644
index 000000000..18c9d1e67
--- /dev/null
+++ b/internal/middleware/headerfilter.go
@@ -0,0 +1,251 @@
+// 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 (
+	"sync"
+
+	"github.com/gin-gonic/gin"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+	"github.com/superseriousbusiness/gotosocial/internal/headerfilter"
+	"github.com/superseriousbusiness/gotosocial/internal/log"
+	"github.com/superseriousbusiness/gotosocial/internal/state"
+)
+
+var (
+	allowMatches = matchstats{m: make(map[string]uint64)}
+	blockMatches = matchstats{m: make(map[string]uint64)}
+)
+
+// matchstats is a simple statistics
+// counter for header filter matches.
+// TODO: replace with otel.
+type matchstats struct {
+	m map[string]uint64
+	l sync.Mutex
+}
+
+func (m *matchstats) Add(hdr, regex string) {
+	m.l.Lock()
+	key := hdr + ":" + regex
+	m.m[key]++
+	m.l.Unlock()
+}
+
+// HeaderFilter returns a gin middleware handler that provides HTTP
+// request blocking (filtering) based on database allow / block filters.
+func HeaderFilter(state *state.State) gin.HandlerFunc {
+	switch mode := config.GetAdvancedHeaderFilterMode(); mode {
+	case config.RequestHeaderFilterModeDisabled:
+		return func(ctx *gin.Context) {}
+
+	case config.RequestHeaderFilterModeAllow:
+		return headerFilterAllowMode(state)
+
+	case config.RequestHeaderFilterModeBlock:
+		return headerFilterBlockMode(state)
+
+	default:
+		panic("unrecognized filter mode: " + mode)
+	}
+}
+
+func headerFilterAllowMode(state *state.State) func(c *gin.Context) {
+	_ = *state //nolint
+	// Allowlist mode: explicit block takes
+	// precedence over explicit allow.
+	//
+	// Headers that have neither block
+	// or allow entries are blocked.
+	return func(c *gin.Context) {
+
+		// Check if header is explicitly blocked.
+		block, err := isHeaderBlocked(state, c)
+		if err != nil {
+			respondInternalServerError(c, err)
+			return
+		}
+
+		if block {
+			respondBlocked(c)
+			return
+		}
+
+		// Check if header is missing explicit allow.
+		notAllow, err := isHeaderNotAllowed(state, c)
+		if err != nil {
+			respondInternalServerError(c, err)
+			return
+		}
+
+		if notAllow {
+			respondBlocked(c)
+			return
+		}
+
+		// Allowed!
+		c.Next()
+	}
+}
+
+func headerFilterBlockMode(state *state.State) func(c *gin.Context) {
+	_ = *state //nolint
+	// Blocklist/default mode: explicit allow
+	// takes precedence over explicit block.
+	//
+	// Headers that have neither block
+	// or allow entries are allowed.
+	return func(c *gin.Context) {
+
+		// Check if header is explicitly allowed.
+		allow, err := isHeaderAllowed(state, c)
+		if err != nil {
+			respondInternalServerError(c, err)
+			return
+		}
+
+		if !allow {
+			// Check if header is explicitly blocked.
+			block, err := isHeaderBlocked(state, c)
+			if err != nil {
+				respondInternalServerError(c, err)
+				return
+			}
+
+			if block {
+				respondBlocked(c)
+				return
+			}
+		}
+
+		// Allowed!
+		c.Next()
+	}
+}
+
+func isHeaderBlocked(state *state.State, c *gin.Context) (bool, error) {
+	var (
+		ctx = c.Request.Context()
+		hdr = c.Request.Header
+	)
+
+	// Perform an explicit is-blocked check on request header.
+	key, expr, err := state.DB.BlockHeaderRegularMatch(ctx, hdr)
+	switch err {
+	case nil:
+		break
+
+	case headerfilter.ErrLargeHeaderValue:
+		log.Warn(ctx, "large header value")
+		key = "*" // block large headers
+
+	default:
+		err := gtserror.Newf("error checking header: %w", err)
+		return false, err
+	}
+
+	if key != "" {
+		if expr != "" {
+			// Increment block matches stat.
+			// TODO: replace expvar with build
+			// taggable metrics types in State{}.
+			blockMatches.Add(key, expr)
+		}
+
+		// A header was matched against!
+		// i.e. this request is blocked.
+		return true, nil
+	}
+
+	return false, nil
+}
+
+func isHeaderAllowed(state *state.State, c *gin.Context) (bool, error) {
+	var (
+		ctx = c.Request.Context()
+		hdr = c.Request.Header
+	)
+
+	// Perform an explicit is-allowed check on request header.
+	key, expr, err := state.DB.AllowHeaderRegularMatch(ctx, hdr)
+	switch err {
+	case nil:
+		break
+
+	case headerfilter.ErrLargeHeaderValue:
+		log.Warn(ctx, "large header value")
+		key = "" // block large headers
+
+	default:
+		err := gtserror.Newf("error checking header: %w", err)
+		return false, err
+	}
+
+	if key != "" {
+		if expr != "" {
+			// Increment allow matches stat.
+			// TODO: replace expvar with build
+			// taggable metrics types in State{}.
+			allowMatches.Add(key, expr)
+		}
+
+		// A header was matched against!
+		// i.e. this request is allowed.
+		return true, nil
+	}
+
+	return false, nil
+}
+
+func isHeaderNotAllowed(state *state.State, c *gin.Context) (bool, error) {
+	var (
+		ctx = c.Request.Context()
+		hdr = c.Request.Header
+	)
+
+	// Perform an explicit is-NOT-allowed check on request header.
+	key, expr, err := state.DB.AllowHeaderInverseMatch(ctx, hdr)
+	switch err {
+	case nil:
+		break
+
+	case headerfilter.ErrLargeHeaderValue:
+		log.Warn(ctx, "large header value")
+		key = "*" // block large headers
+
+	default:
+		err := gtserror.Newf("error checking header: %w", err)
+		return false, err
+	}
+
+	if key != "" {
+		if expr != "" {
+			// Increment allow matches stat.
+			// TODO: replace expvar with build
+			// taggable metrics types in State{}.
+			allowMatches.Add(key, expr)
+		}
+
+		// A header was matched against!
+		// i.e. request is NOT allowed.
+		return true, nil
+	}
+
+	return false, nil
+}