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bh=L+FXkunLWNbjs+6ivcqg+LYsLnsE2akCmTddYQd9QYs=; b=NSwWWK7I2Y1GW/IcJM/pHxF5zXnvRM0JOU4A+xJUsGaAWhQ5/LzJCsICqKwVCj/LEUuXp7 Dty88bjauu52s+8DbbTqGJLbbw5xlA+lLZUD5JR7jvQREZFV0JDhQ6S81pgUDrSfoD0ECo 0xQIxXxg8C1hSh9FDLrfTpp7EYi/jcg= X-MC-Unique: nr_1k4ycMJGIz-oVTHlj0g-1 From: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= To: qemu-devel@nongnu.org Cc: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Paolo Bonzini , =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= , Thomas Huth , =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= Subject: [PATCH 1/2] meson: mitigate against ROP exploits with -fzero-call-used-regs Date: Thu, 5 Oct 2023 18:38:11 +0100 Message-ID: <20231005173812.966264-2-berrange@redhat.com> In-Reply-To: <20231005173812.966264-1-berrange@redhat.com> References: <20231005173812.966264-1-berrange@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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 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+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1696527572575100008 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-reg= s/ 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=3Dused-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=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. Signed-off-by: Daniel P. Berrang=C3=A9 Reviewed-by: Thomas Huth --- meson.build | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/meson.build b/meson.build index 20ceeb8158..2003ca1ba4 100644 --- a/meson.build +++ b/meson.build @@ -435,6 +435,17 @@ if get_option('fuzzing') endif endif =20 +# Check further flags that make QEMU more robust against malicious parties + +hardening_flags =3D [ + # 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=3Dused-gpr', +] + +qemu_common_flags +=3D cc.get_supported_arguments(hardening_flags) + add_global_arguments(qemu_common_flags, native: false, language: all_langu= ages) add_global_link_arguments(qemu_ldflags, native: false, language: all_langu= ages) =20 --=20 2.41.0 From nobody Sun May 19 21:02:37 2024 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass(p=none dis=none) header.from=redhat.com ARC-Seal: i=1; a=rsa-sha256; t=1696527570; cv=none; d=zohomail.com; s=zohoarc; b=B7b+hTa5Fl0dqIG2UvWCjKIz72REuUMnMx0n2q25G5HhakbASqmxOyMe0TNxBU8UQfWvu7kfHbVXQ9aHeZC3kHGvhvlbOSc4taMTpNK2nTNgslIObR/V9SWW/HxtZlqHmHhZTwq8JbUhZCBArKW9FjboZp7nOg91Rf8tO6g0rxM= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; 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Thu, 5 Oct 2023 17:38:21 +0000 (UTC) Received: from toolbox.redhat.com (unknown [10.42.28.40]) by smtp.corp.redhat.com (Postfix) with ESMTP id E0368215670B; Thu, 5 Oct 2023 17:38:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1696527505; 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=hcHPM7M/4ZJp7mm+BUX+SXLoPxjf+QOV+TW04XDdcrg=; b=SZY31qmUIAFjvvIiTUlSR91s1GDqP5yv1/NXkdFctQdVmWuRY04Fu9TgiKBXUpi47kfV4z yjzFKh0hAJ0y9Y/y96XAmflVBzWLYw2cPwgBt75itEyJ/w8nN+bH9wyiLW5ea3lmOF08Iw k4RF6UdN2dTcWzoan8juCjNgEBnH8GQ= X-MC-Unique: 6DVU2UZDO6O9LoIcSeqPhw-1 From: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= To: qemu-devel@nongnu.org Cc: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Paolo Bonzini , =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= , Thomas Huth , =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= Subject: [PATCH 2/2] meson: mitigate against use of uninitialize stack for exploits Date: Thu, 5 Oct 2023 18:38:12 +0100 Message-ID: <20231005173812.966264-3-berrange@redhat.com> In-Reply-To: <20231005173812.966264-1-berrange@redhat.com> References: <20231005173812.966264-1-berrange@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.6 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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 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+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1696527572578100009 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 Reviewed-by: Thomas Huth --- meson.build | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/meson.build b/meson.build index 2003ca1ba4..19faea8d30 100644 --- a/meson.build +++ b/meson.build @@ -442,6 +442,11 @@ 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-var-auto-init=3Dzero', ] =20 qemu_common_flags +=3D cc.get_supported_arguments(hardening_flags) --=20 2.41.0