[PATCH v2 1/2] meson: mitigate against ROP exploits with -fzero-call-used-regs

["Daniel P. Berrangé" <berrange@redhat.com>]

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.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 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)
 
-- 
2.43.0