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 X-Spam-Level: X-Spam-Status: No, score=-10.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3F5B7CA90AF for ; Tue, 12 May 2020 18:06:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1B86720720 for ; Tue, 12 May 2020 18:06:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589306760; bh=ez20Q+vbHatmoUrQcLugh4B8uaSdEsJghLxVdqu1yKk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=Jt6cU68r6dsc8mo9a/VzraI6Eyes/zeAANliczHsWmdmqaJnsQ+Ko++XgJd/eN5Tc bKGH3QMpos6OwKAUoqUyd/A7daUl3J1xwTF1KASFkDvTD83xhT9ggJGYXnqmNkouii dXL3C40PDVENyh/FEKIVG/o+dC1HGQsi3lK2g/Oo= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729944AbgELSF7 (ORCPT ); Tue, 12 May 2020 14:05:59 -0400 Received: from mail.kernel.org ([198.145.29.99]:52936 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728081AbgELSF7 (ORCPT ); Tue, 12 May 2020 14:05:59 -0400 Received: from embeddedor (unknown [189.207.59.248]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id D8AFD20673; Tue, 12 May 2020 18:05:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589306758; bh=ez20Q+vbHatmoUrQcLugh4B8uaSdEsJghLxVdqu1yKk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=SfnlaLpi8iGkdyKXHCoxYcGLnQAXHWsHurzFCPMp/79mAMkJzJekdMYpqj8X//u2b bxZ3YPRlvn3oXc8QyUMlmVKLx0JPIwDVzOoeZZAC8fAxHu5J3mhBByNviAL52XqZSp JIkFfyKrszv5BHbOkEsGfaT8eWEFgbcZvEb3B56Q= Date: Tue, 12 May 2020 13:10:32 -0500 From: "Gustavo A. R. Silva" To: Kees Cook Cc: "Gustavo A. R. Silva" , Peter Huewe , Jarkko Sakkinen , Jason Gunthorpe , Arnd Bergmann , Greg Kroah-Hartman , linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] tpm: eventlog: Replace zero-length array with flexible-array member Message-ID: <20200512181032.GG4897@embeddedor> References: <20200508163826.GA768@embeddedor> <202005112224.9EFD07F5@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <202005112224.9EFD07F5@keescook> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-integrity-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-integrity@vger.kernel.org On Mon, May 11, 2020 at 10:24:10PM -0700, Kees Cook wrote: > On Fri, May 08, 2020 at 11:38:26AM -0500, Gustavo A. R. Silva wrote: > > The current codebase makes use of the zero-length array language > > extension to the C90 standard, but the preferred mechanism to declare > > variable-length types such as these ones is a flexible array member[1][2], > > introduced in C99: > > > > struct foo { > > int stuff; > > struct boo array[]; > > }; > > > > By making use of the mechanism above, we will get a compiler warning > > in case the flexible array does not occur last in the structure, which > > will help us prevent some kind of undefined behavior bugs from being > > inadvertently introduced[3] to the codebase from now on. > > > > Also, notice that, dynamic memory allocations won't be affected by > > this change: > > > > "Flexible array members have incomplete type, and so the sizeof operator > > may not be applied. As a quirk of the original implementation of > > zero-length arrays, sizeof evaluates to zero."[1] > > > > sizeof(flexible-array-member) triggers a warning because flexible array > > members have incomplete type[1]. There are some instances of code in > > which the sizeof operator is being incorrectly/erroneously applied to > > zero-length arrays and the result is zero. Such instances may be hiding > > some bugs. So, this work (flexible-array member conversions) will also > > help to get completely rid of those sorts of issues. > > > > Also, the following issue shows up due to the flexible-array member > > having incomplete type[4]: > > > > drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_start’: > > drivers/char/tpm/eventlog/tpm2.c:54:46: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’} > > 54 | size = sizeof(struct tcg_pcr_event) - sizeof(event_header->event) > > | ^ > > drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_bios_measurements_next’: > > drivers/char/tpm/eventlog/tpm2.c:102:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’} > > 102 | sizeof(event_header->event) + event_header->event_size; > > | ^ > > drivers/char/tpm/eventlog/tpm2.c: In function ‘tpm2_binary_bios_measurements_show’: > > drivers/char/tpm/eventlog/tpm2.c:140:10: error: invalid application of ‘sizeof’ to incomplete type ‘u8[]’ {aka ‘unsigned char[]’} > > 140 | sizeof(event_header->event) + event_header->event_size; > > | ^ > > scripts/Makefile.build:266: recipe for target 'drivers/char/tpm/eventlog/tpm2.o' failed > > make[3]: *** [drivers/char/tpm/eventlog/tpm2.o] Error 1 > > > > As mentioned above: "Flexible array members have incomplete type, and > > so the sizeof operator may not be applied. As a quirk of the original > > implementation of zero-length arrays, sizeof evaluates to zero."[1] As > > in "sizeof(event_header->event) always evaluated to 0, so removing it > > has no effect". > > > > Lastly, make use of the struct_size() helper to deal with the > > flexible array member and its host structure. > > > > This issue was found with the help of Coccinelle. > > > > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html > > [2] https://github.com/KSPP/linux/issues/21 > > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour") > > [4] https://github.com/KSPP/linux/issues/43 > > > > Signed-off-by: Gustavo A. R. Silva > > Reviewed-by: Kees Cook > Thanks, Kees. -- Gustavo