hooks: Harden when we accept push options that change repo settings

It is possible to change some repo settings (its visibility, and
template status) via `git push` options: `-o repo.private=true`, `-o
repo.template=true`.

Previously, there weren't sufficient permission checks on these, and
anyone who could `git push` to a repository - including via an AGit
workflow! - was able to change either of these settings. To guard
against this, the pre-receive hook will now check if either of these
options are present, and if so, will perform additional permission
checks to ensure that these can only be set by a repository owner or
an administrator. Additionally, changing these settings is disabled for
forks, even for the fork's owner.

There's still a case where the owner of a repository can change the
visibility of it, and it will not propagate to forks (it propagates to
forks when changing the visibility via the API), but that's an
inconsistency, not a security issue.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

routers/private/hook_pre_receive.go

@@ -101,6 +101,57 @@ func (ctx *preReceiveContext) AssertCreatePullRequest() bool {
 	return true
 }
 
+func (ctx *preReceiveContext) canChangeSettings() bool {
+	if !ctx.loadPusherAndPermission() {
+		return false
+	}
+
+	perm, err := access_model.GetUserRepoPermission(ctx, ctx.Repo.Repository, ctx.user)
+	if err != nil {
+		return false
+	}
+	if !perm.IsOwner() && !perm.IsAdmin() {
+		return false
+	}
+
+	if ctx.Repo.Repository.IsFork {
+		return false
+	}
+
+	return true
+}
+
+func (ctx *preReceiveContext) assertChangeSettings() bool {
+	opts := web.GetForm(ctx).(*private.HookOptions)
+
+	if len(opts.GitPushOptions) == 0 {
+		return true
+	}
+
+	_, hasPrivateOpt := opts.GitPushOptions[private.GitPushOptionRepoPrivate]
+	_, hasTemplateOpt := opts.GitPushOptions[private.GitPushOptionRepoTemplate]
+
+	if !hasPrivateOpt && !hasTemplateOpt {
+		// If neither `repo.private` nor `repo.template` is present in
+		// the push options, we're good to go without further permission
+		// checking.
+		return true
+	}
+
+	// Either `repo.private` or `repo.template` is among the push options,
+	// do some permission checks.
+	if !ctx.canChangeSettings() {
+		if ctx.Written() {
+			return false
+		}
+		ctx.JSON(http.StatusForbidden, private.Response{
+			UserMsg: "Permission denied for changing repo settings.",
+		})
+		return false
+	}
+	return true
+}
+
 // HookPreReceive checks whether a individual commit is acceptable
 func HookPreReceive(ctx *gitea_context.PrivateContext) {
 	opts := web.GetForm(ctx).(*private.HookOptions)
@@ -111,6 +162,10 @@ func HookPreReceive(ctx *gitea_context.PrivateContext) {
 		opts:           opts,
 	}
 
+	if !ourCtx.assertChangeSettings() {
+		return
+	}
+
 	// Iterate across the provided old commit IDs
 	for i := range opts.OldCommitIDs {
 		oldCommitID := opts.OldCommitIDs[i]