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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 86F65C46CD2 for ; Tue, 9 Jan 2024 14:49:26 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rNDPS-0005kC-K9; Tue, 09 Jan 2024 09:48:54 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rNDPQ-0005jo-L6 for qemu-devel@nongnu.org; Tue, 09 Jan 2024 09:48:52 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rNDPO-0002M8-Px for qemu-devel@nongnu.org; Tue, 09 Jan 2024 09:48:52 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1704811727; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=f0kkdzu6O2et3bnqvUGLEJulAQ/TCEAMYLMu0Bx5OP4=; b=MLHLeHViktS5L3SmiwthRQqZz7eLSFk0zigjJZBG+HU35+ofSHZ4BNlVfhwBOxAyI9eSOo 35kTDZfBssx+wAqEzhbQmmhyljaNB6Hyr1cT8VFymh41HURqOHtC9GmTOMsnceVN4T/lvc xOvWbXweL5rQjJ7OdR3GcEDqOsfV59w= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-696-OMh3mXgoODy0KI73SJH6uQ-1; Tue, 09 Jan 2024 09:48:44 -0500 X-MC-Unique: OMh3mXgoODy0KI73SJH6uQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id C861A836F8A; Tue, 9 Jan 2024 14:48:43 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.71]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A83132026D66; Tue, 9 Jan 2024 14:48:43 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id A506F21E6691; Tue, 9 Jan 2024 15:48:42 +0100 (CET) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: qemu-devel@nongnu.org, Thomas Huth , Paolo Bonzini , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Subject: Re: [PATCH v2 2/2] meson: mitigate against use of uninitialize stack for exploits In-Reply-To: <20240103123414.2401208-3-berrange@redhat.com> ("Daniel P. =?utf-8?Q?Berrang=C3=A9=22's?= message of "Wed, 3 Jan 2024 12:34:14 +0000") References: <20240103123414.2401208-1-berrange@redhat.com> <20240103123414.2401208-3-berrange@redhat.com> Date: Tue, 09 Jan 2024 15:48:42 +0100 Message-ID: <87frz6lfzp.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -45 X-Spam_score: -4.6 X-Spam_bar: ---- X-Spam_report: (-4.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.493, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Daniel P. Berrang=C3=A9 writes: > When variables are used without being initialized, there is potential > to take advantage of data that was pre-existing on the stack from an > earlier call, to drive an exploit. > > It is good practice to always initialize variables, and the compiler > can warn about flaws when -Wuninitialized is present. This warning, > however, is by no means foolproof with its output varying depending > on compiler version and which optimizations are enabled. > > The -ftrivial-auto-var-init option can be used to tell the compiler > to always initialize all variables. This increases the security and > predictability of the program, closing off certain attack vectors, > reducing the risk of unsafe memory disclosure. > > While the option takes several possible values, using 'zero' is > considered to be the option that is likely to lead to semantically > correct or safe behaviour[1]. eg sizes/indexes are not likely to > lead to out-of-bounds accesses when initialized to zero. Pointers > are less likely to point something useful if initialized to zero. > > Even with -ftrivial-auto-var-init=3Dzero set, GCC will still issue > warnings with -Wuninitialized if it discovers a problem, so we are > not loosing diagnostics for developers, just hardening runtime > behaviour and making QEMU behave more predictably in case of hitting > bad codepaths. > > [1] https://lists.llvm.org/pipermail/cfe-dev/2020-April/065221.html > Signed-off-by: Daniel P. Berrang=C3=A9 > --- > meson.build | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/meson.build b/meson.build > index eaa20d241d..efc1b4dd14 100644 > --- a/meson.build > +++ b/meson.build > @@ -440,6 +440,11 @@ hardening_flags =3D [ > # upon its return. This makes it harder to assemble > # ROP gadgets into something usable > '-fzero-call-used-regs=3Dused-gpr', > + > + # Initialize all stack variables to zero. This makes > + # it harder to take advantage of uninitialized stack > + # data to drive exploits > + '-ftrivial-auto-var-init=3Dzero', > ] >=20=20 > qemu_common_flags +=3D cc.get_supported_arguments(hardening_flags) Have you tried to throw in -Wtrivial-auto-var-init? Documentation, for your convenience: =E2=80=98-Wtrivial-auto-var-init=E2=80=99 Warn when =E2=80=98-ftrivial-auto-var-init=E2=80=99 cannot initialize = the automatic variable. A common situation is an automatic variable that is declared between the controlling expression and the first case label of a =E2=80=98switch=E2=80=99 statement.