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 5E7A5C46CD2 for ; Tue, 9 Jan 2024 15:13:46 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rNDmo-00033Y-LV; Tue, 09 Jan 2024 10:13:02 -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 1rNDmm-00033P-DX for qemu-devel@nongnu.org; Tue, 09 Jan 2024 10:13:00 -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 1rNDmi-0004oi-IS for qemu-devel@nongnu.org; Tue, 09 Jan 2024 10:13:00 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1704813174; h=from:from:reply-to: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=r6Uw16NrmnAM8qrnTlSt0XBsmAW4jzKIu/srpB35UKU=; b=hIJu74sjr4BcE2f6JbRbqZiKazVSQbXnsw2ahn/cYlj5eaxnQitXBcUHDtsN/IupTrb5dn JmVWFjVTQsZdtR+kp8Fvw8wpk/GVpinX7dIBdw1n4iBv6C42QGfANI1aFBqOAokCPoGUE5 9W2OeK+7NnpMgFHNYIwekK4RV1gk9Rw= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-50-_WfWA6N4PS-CsTDs19KQlA-1; Tue, 09 Jan 2024 10:12:53 -0500 X-MC-Unique: _WfWA6N4PS-CsTDs19KQlA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (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 08F703813F3A; Tue, 9 Jan 2024 15:12:53 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.86]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CA55D1121312; Tue, 9 Jan 2024 15:12:51 +0000 (UTC) Date: Tue, 9 Jan 2024 15:12:49 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Markus Armbruster 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 Message-ID: References: <20240103123414.2401208-1-berrange@redhat.com> <20240103123414.2401208-2-berrange@redhat.com> <87bk9ulfqo.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87bk9ulfqo.fsf@pond.sub.org> User-Agent: Mutt/2.2.10 (2023-03-25) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.3 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Tue, Jan 09, 2024 at 03:54:07PM +0100, Markus Armbruster wrote: > Daniel P. Berrangé writes: > > > 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-user-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 found. > > > > 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=used-gpr the 'rp' tool > > reports > > > > A total of 528991 gadgets found. > > You decided to keep only the unique ones, 121128 unique gadgets found. > > > > 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=used-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=all, > > 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. > > 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-regs/ 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. '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. > > > Reviewed-by: Thomas Huth > > Signed-off-by: Daniel P. Berrangé > > --- > > meson.build | 11 +++++++++++ > > 1 file changed, 11 insertions(+) > > > > diff --git a/meson.build b/meson.build > > index 6c77d9687d..eaa20d241d 100644 > > --- a/meson.build > > +++ b/meson.build > > @@ -433,6 +433,17 @@ if get_option('fuzzing') > > endif > > endif > > > > +# Check further flags that make QEMU more robust against malicious parties > > + > > +hardening_flags = [ > > + # Zero out registers used during a function call > > + # upon its return. This makes it harder to assemble > > + # ROP gadgets into something usable > > + '-fzero-call-used-regs=used-gpr', > > +] > > + > > +qemu_common_flags += cc.get_supported_arguments(hardening_flags) > > + > > add_global_arguments(qemu_common_flags, native: false, language: all_languages) > > add_global_link_arguments(qemu_ldflags, native: false, language: all_languages) > With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|