From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 551E1C433EF for ; Mon, 4 Oct 2021 19:11:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2E8AB61452 for ; Mon, 4 Oct 2021 19:11:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233226AbhJDTMs (ORCPT ); Mon, 4 Oct 2021 15:12:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60738 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235289AbhJDTMs (ORCPT ); Mon, 4 Oct 2021 15:12:48 -0400 Received: from mail-yb1-xb36.google.com (mail-yb1-xb36.google.com [IPv6:2607:f8b0:4864:20::b36]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 111F0C061745 for ; Mon, 4 Oct 2021 12:10:59 -0700 (PDT) Received: by mail-yb1-xb36.google.com with SMTP id q189so22076178ybq.1 for ; Mon, 04 Oct 2021 12:10:59 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=TX98aLvs0jm488r8l6cPv4x2o09VgHdP6nIV9yzsoR8=; b=jrdny670eRSrixabawz8NM8mV1j3NcKBf3/5o2HmMYs5Pt/3jEDKejaagNLZuHD4eA XsA7lnk5rlDb255PMmTfbkU01FmcbAxBIYEQtM94T1iMAclIOyb2sv35ZTukC2pwU2GG ltnDtI/O33I3Ik/UGu+M8Yrc0A6fNj4jCPp0p3IE11EK9JyQmUG6php2mjyq7W1jvwzx hm4UYlfHmBM4ZggOpSlbBQ2TH3Rh8u5TAQjTysy9vuPEMDgSJTBc8GQ46Bm7g/nowXkw mNseks8sftxpTK7nsWq+M26M6JTRKKfc+IXpLEJJw6dXy7eJuoEmg5mbmgeSD8SJR7lw RJxQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=TX98aLvs0jm488r8l6cPv4x2o09VgHdP6nIV9yzsoR8=; b=tONi8jOti1ej9N/p2mgQSb/V6ooAtL0ydvtUSUAxxz4yzhrVoZr82An+6/gRNLvwOM DUQp5CGSQSJH6U/zk5WDkKs5h4F68ey41CrFEkhtrvE3qWm2/BlyV987S/XMHiwPpmtD Z0y6/LBBEc2FnG9ty/t3GuFnTXuBKiFNij1o8K78aa03DrJqt0LY6QYEqj9Mhrz6/YiQ YAvCgY+wecvaHvV5xUCsp/Ek68eVyj0XoQYPn/VLYJD1SheHPLOXj/vU2qg2vfE/fDOG ZhUwlbU/P7xEaNRKR3i4TlW0+05zPnTCWL7TWm+sjYoXhygIA05Tk/NjJLjakAQJiOuZ 5YcA== X-Gm-Message-State: AOAM532OTdldIerrFCwSvbHuwQyW/sND8qi72/MzItOvW4fbXw6FUpwx yjF76bp9k6rwleicxZTI/pDeMdHEt90nB6TFPMQxEA== X-Google-Smtp-Source: ABdhPJyvignQgUWVGULEBdeVmUDF7c5rAhbNIgSFqFjfcL8Oh9L1Nm3LtbDC/MhhvQjpl759ZDx6OBAK3+tEv7om3q8= X-Received: by 2002:a25:aaec:: with SMTP id t99mr17333011ybi.456.1633374657849; Mon, 04 Oct 2021 12:10:57 -0700 (PDT) MIME-Version: 1.0 References: <20210930180531.1190642-1-samitolvanen@google.com> <20210930180531.1190642-5-samitolvanen@google.com> In-Reply-To: From: Sami Tolvanen Date: Mon, 4 Oct 2021 12:10:46 -0700 Message-ID: Subject: Re: [PATCH v4 04/15] cfi: Add DEFINE_CFI_IMMEDIATE_RETURN_STUB To: Peter Zijlstra Cc: X86 ML , Kees Cook , Josh Poimboeuf , Nathan Chancellor , Nick Desaulniers , Sedat Dilek , linux-hardening@vger.kernel.org, LKML , llvm@lists.linux.dev Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org On Mon, Oct 4, 2021 at 6:50 AM Peter Zijlstra wrote: > > On Thu, Sep 30, 2021 at 11:05:20AM -0700, Sami Tolvanen wrote: > > This change introduces the DEFINE_CFI_IMMEDIATE_RETURN_STUB macro, > > which defines a stub function that immediately returns and when > > defined in the core kernel, always passes indirect call checking > > with CONFIG_CFI_CLANG. Note that this macro should only be used when > > a stub cannot be called using the correct function type. > > > diff --git a/include/linux/cfi.h b/include/linux/cfi.h > > index 879744aaa6e0..19f74af8eac2 100644 > > --- a/include/linux/cfi.h > > +++ b/include/linux/cfi.h > > @@ -20,6 +20,17 @@ extern void __cfi_check(uint64_t id, void *ptr, void *diag); > > #define __CFI_ADDRESSABLE(fn, __attr) \ > > const void *__cfi_jt_ ## fn __visible __attr = (void *)&fn > > > > +/* > > + * Defines a stub function that returns immediately, and when defined and > > + * referenced in the core kernel, always passes CFI checking. This should > > + * be used only for stubs that cannot be called using the correct function > > + * pointer type, which should be rare. > > + */ > > +#define DEFINE_CFI_IMMEDIATE_RETURN_STUB(fn) \ > > + void fn(void) { return; } \ > > + const void *__cfi_excl_ ## fn __visible \ > > + __section(".cfi_excluded_stubs") = (void *)&fn > > + > > #ifdef CONFIG_CFI_CLANG_SHADOW > > > > extern void cfi_module_add(struct module *mod, unsigned long base_addr); > > @@ -35,6 +46,8 @@ static inline void cfi_module_remove(struct module *mod, unsigned long base_addr > > #else /* !CONFIG_CFI_CLANG */ > > > > #define __CFI_ADDRESSABLE(fn, __attr) > > +#define DEFINE_CFI_IMMEDIATE_RETURN_STUB(fn) \ > > + void fn(void) { return; } > > > > #endif /* CONFIG_CFI_CLANG */ > > > > Why DEFINE_CFI_IMMEDIATE_RETURN_STUB() vs __no_cfi attribute that we can > stick on the relvant functions? To avoid accidentally creating useful gadgets for attackers. For example, while excluding an empty stub isn't necessarily ideal, allowing calls to a function that always returns zero would be worse. > Because I've got at least one more variant for you :-) See > kernel/static_call.c:__static_call_return0 Does __static_call_return0 ever get called indirectly on architectures that support static calls? If it's always patched into a direct call, the type mismatch isn't an issue. Sami