From 043eaa0f0c8618382b2dba2f2e4fe762215b2e29 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= Date: Wed, 3 Jan 2024 12:34:13 +0000 Subject: [PATCH] meson: mitigate against ROP exploits with -fzero-call-used-regs MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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' successfully stopped ROPgadget assembling a chain, this more targeted protection is a justifiable hardening / performance tradeoff. Reviewed-by: Thomas Huth Signed-off-by: "Daniel P. Berrangé" Message-ID: <20240103123414.2401208-2-berrange@redhat.com> Signed-off-by: Thomas Huth --- meson.build | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/meson.build b/meson.build index 38deb9363c..1bda391de6 100644 --- a/meson.build +++ b/meson.build @@ -552,6 +552,17 @@ if get_option('cfi') add_global_link_arguments(cfi_flags, native: false, language: all_languages) 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)