fix(sec): only degrade permission check for git push

- A permission check is done when incoming SSH connections are handled (this is
run before git hooks). If this check is for write access and AGit flow
is supported, then this check is degraded to a read check. The
motivation behind this is that for AGit flow the user does not need
write permissions but only read permissions.
- The `if` condition cannot check if this is for AGit flow, as the Git
protocol has not run yet and thus has to delay this permission check.
This `if` condition failed to consider that this also might be run for
LFS which does not care about AGit flow and would not do a delayed
permission check, so ensure that this degradition only happens when the
`git-receive-pack` command is being run (which roughly equals to `git
push`).
- Clarify code comment.
- Added integration test.

(cherry picked from commit 60c1af244a)

Conflicts:
	tests/integration/git_test.go
   - t.Context() does not exist
   - tests do not loop over Git object formats

routers/private/serv.go

@@ -296,8 +296,14 @@ func ServCommand(ctx *context.PrivateContext) {
 				return
 			}
 		} else {
-			// Because of the special ref "refs/for" we will need to delay write permission check
-			if git.SupportProcReceive && unitType == unit.TypeCode {
+			// We don't know yet which references the Git client wants to write to,
+			// but for AGit flow we have to degrade this check to a read permission.
+			// So if we support proc-receive (needed for AGit flow) and the unit type
+			// is code and we know the Git client wants to write to us, then degrade
+			// the permission check to read. The pre-receive hook will do another
+			// permission check which ensure for non AGit flow references the write
+			// permission is checked.
+			if git.SupportProcReceive && unitType == unit.TypeCode && ctx.FormString("verb") == "git-receive-pack" {
 				mode = perm.AccessModeRead
 			}
 

tests/integration/git_test.go

@@ -5,15 +5,18 @@ package integration
 
 import (
 	"bytes"
+	"context"
 	"encoding/hex"
 	"fmt"
 	"math/rand"
 	"net/http"
 	"net/url"
 	"os"
+	"os/exec"
 	"path"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"testing"
 	"time"
 
@@ -34,6 +37,7 @@ import (
 	files_service "code.gitea.io/gitea/services/repository/files"
 	"code.gitea.io/gitea/tests"
 
+	"github.com/kballard/go-shellquote"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -109,7 +113,10 @@ func testGit(t *testing.T, u *url.URL) {
 
 		// Setup key the user ssh key
 		withKeyFile(t, keyname, func(keyFile string) {
-			t.Run("CreateUserKey", doAPICreateUserKey(sshContext, "test-key", keyFile))
+			var publicKeyID int64
+			t.Run("CreateUserKey", doAPICreateUserKey(sshContext, "test-key", keyFile, func(t *testing.T, pk api.PublicKey) {
+				publicKeyID = pk.ID
+			}))
 
 			// Setup remote link
 			// TODO: get url from api
@@ -136,6 +143,7 @@ func testGit(t *testing.T, u *url.URL) {
 			})
 
 			t.Run("PushCreate", doPushCreate(sshContext, sshURL))
+			t.Run("LFS no access", doLFSNoAccess(sshContext, publicKeyID))
 		})
 	})
 }
@@ -1118,3 +1126,26 @@ func TestDataAsync_Issue29101(t *testing.T) {
 		defer r2.Close()
 	})
 }
+
+func doLFSNoAccess(ctx APITestContext, publicKeyID int64) func(*testing.T) {
+	return func(t *testing.T) {
+		// This is set in withKeyFile
+		sshCommand := os.Getenv("GIT_SSH_COMMAND")
+
+		// Sanity check, because we are going to execute whatever is in here.
+		require.True(t, strings.HasPrefix(sshCommand, "ssh "))
+
+		// We really have to split on the arguments and pass them individually.
+		sshOptions, err := shellquote.Split(strings.TrimPrefix(sshCommand, "ssh "))
+		require.NoError(t, err)
+
+		sshOptions = append(sshOptions, "-p "+strconv.Itoa(setting.SSH.ListenPort), "git@"+setting.SSH.ListenHost)
+
+		cmd := exec.CommandContext(context.Background(), "ssh", append(sshOptions, "git-lfs-authenticate", "user40/repo60.git", "upload")...)
+		stderr := bytes.Buffer{}
+		cmd.Stderr = &stderr
+
+		require.ErrorContains(t, cmd.Run(), "exit status 1")
+		assert.Contains(t, stderr.String(), fmt.Sprintf("Forgejo: User: 2:user2 with Key: %d:test-key is not authorized to write to user40/repo60.", publicKeyID))
+	}
+}