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 15128C4707B for ; Thu, 11 Jan 2024 12:04:43 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rNtn5-0003TI-Em; Thu, 11 Jan 2024 07:04:08 -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 1rNtmv-0003Ck-46 for qemu-devel@nongnu.org; Thu, 11 Jan 2024 07:03:57 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rNtmr-0007sf-Ig for qemu-devel@nongnu.org; Thu, 11 Jan 2024 07:03:56 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1704974631; 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=MnDCpR8iwHs29qPqwC1oRAoXUMrophlAnBaXqkXprn0=; b=Q5bi+YQKUnnMxDY9eOtjeIgaOZ803ewZ0M50uu3C9nek1I0VqG4ZabHYl8u9Q7pF0VYfPM cQbtq/sIjIbYI1eejz37EuGpw6TstwzTL3GdaRhOpZm0PfzulK6y52Sfrw0KPJ4zOpQRkd XpurYVTXtuKCTZKo3zzeWaZz4Jj3TKg= 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-687-hEKZ-eY0OWuNVq-QnZzcMg-1; Thu, 11 Jan 2024 07:03:50 -0500 X-MC-Unique: hEKZ-eY0OWuNVq-QnZzcMg-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (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 DF85C85A588; Thu, 11 Jan 2024 12:03:49 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.71]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 9D2F12166B33; Thu, 11 Jan 2024 12:03:49 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 8810821E6682; Thu, 11 Jan 2024 13:03:48 +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 1/2] meson: mitigate against ROP exploits with -fzero-call-used-regs In-Reply-To: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Tue, 9 Jan 2024 15:12:49 +0000") References: <20240103123414.2401208-1-berrange@redhat.com> <20240103123414.2401208-2-berrange@redhat.com> <87bk9ulfqo.fsf@pond.sub.org> Date: Thu, 11 Jan 2024 13:03:48 +0100 Message-ID: <87il40xejf.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 10.11.54.6 Received-SPF: pass client-ip=170.10.129.124; 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=-2.467, 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_H4=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: > On Tue, Jan 09, 2024 at 03:54:07PM +0100, Markus Armbruster wrote: >> Daniel P. Berrang=C3=A9 writes: >>=20 >> > To quote wikipedia: >> > >> > "Return-oriented programming (ROP) is a computer security exploit >> > technique that allows an attacker to execute code in the presence >> > of security defenses such as executable space protection and code >> > signing. >> > >> > In this technique, an attacker gains control of the call stack to >> > hijack program control flow and then executes carefully chosen >> > machine instruction sequences that are already present in the >> > machine's memory, called "gadgets". Each gadget typically ends in >> > a return instruction and is located in a subroutine within the >> > existing program and/or shared library code. Chained together, >> > these gadgets allow an attacker to perform arbitrary operations >> > on a machine employing defenses that thwart simpler attacks." >> > >> > QEMU is by no means perfect with an ever growing set of CVEs from >> > flawed hardware device emulation, which could potentially be >> > exploited using ROP techniques. >> > >> > Since GCC 11 there has been a compiler option that can mitigate >> > against this exploit technique: >> > >> > -fzero-call-user-regs >> > >> > To understand it refer to these two resources: >> > >> > https://www.jerkeby.se/newsletter/posts/rop-reduction-zero-call-use= r-regs/ >> > https://gcc.gnu.org/pipermail/gcc-patches/2020-August/552262.html >> > >> > I used two programs to scan qemu-system-x86_64 for ROP gadgets: >> > >> > https://github.com/0vercl0k/rp >> > https://github.com/JonathanSalwan/ROPgadget >> > >> > When asked to find 8 byte gadgets, the 'rp' tool reports: >> > >> > A total of 440278 gadgets found. >> > You decided to keep only the unique ones, 156143 unique gadgets foun= d. >> > >> > While the ROPgadget tool reports: >> > >> > Unique gadgets found: 353122 >> > >> > With the --ropchain argument, the latter attempts to use the found >> > gadgets to product a chain that can execute arbitrary syscalls. With >> > current QEMU it succeeds in this task, which is an undesirable >> > situation. >> > >> > With QEMU modified to use -fzero-call-user-regs=3Dused-gpr the 'rp' to= ol >> > reports >> > >> > A total of 528991 gadgets found. >> > You decided to keep only the unique ones, 121128 unique gadgets foun= d. >> > >> > This is 22% fewer unique gadgets >> > >> > While the ROPgadget tool reports: >> > >> > Unique gadgets found: 328605 >> > >> > This is 7% fewer unique gadgets. Crucially though, despite this more >> > modest reduction, the ROPgadget tool is no longer able to identify a >> > chain of gadgets for executing arbitrary syscalls. It fails at the >> > very first step, unable to find gadgets for populating registers for >> > a future syscall. Having said that, more advanced tools do still >> > manage to put together a viable ROP chain. >> > >> > Also this only takes into account QEMU code. QEMU links to many 3rd >> > party shared libraries and ideally all of them would be compiled with >> > this same hardening. That becomes a distro policy question though. >> > >> > In terms of performance impact, TCG was used as an evaluation test >> > case. We're not interested in protecting TCG since it isn't designed >> > to provide a security barrier, but it is performance sensitive code, >> > so useful as a guide to how other areas of QEMU might be impacted. >> > With the -fzero-call-user-regs=3Dused-gpr argument present, using the >> > real world test of booting a linux kernel and having init immediately >> > poweroff, there is a ~1% slow down in performance under TCG. The QEMU >> > binary size also grows by approximately 1%. >> > >> > By comparison, using the more aggressive -fzero-call-user-regs=3Dall, >> > results in a slowdown of over 25% in TCG, which is clearly not an >> > acceptable impact, and a binary size increase of 5%. >> > >> > Considering that 'used-gpr' succesfully stopped ROPgadget assembling >> > a chain, this more targetted protection is a justifiable hardening >> > / performance tradeoff. >>=20 >> Have you also considered 'used-arg'? > > No, not in any detail. I was mostly guided by the writeup here: > > https://www.jerkeby.se/newsletter/posts/rop-reduction-zero-call-user-re= gs/ > > which indicates Linux chose 'used-gpr'. I figured if Kees Cook > decide that was a good tradeoff for Linux, we might as well follow > it. Makes sense. > 'used-gpr' will target any general purpose registers > that are used in a method. 'used-arg' will taget any registers > used for parameters. IIUC, this makes 'used-gpr' be a slightly > stronger protection as it covers register usage even for things > which aren't args. The docs lead me to suspect it will *not* cover non-gpr registers that are used for passing arguments. Requires a calling convention that can pass arguments in non-gpr registers, such as floating-point and vector registers. I figure these are less useful for exploits than gprs. Thanks! [...]