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

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

On Tue, Jan 09, 2024 at 03:54:07PM +0100, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> 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 <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)
> 

With regards,
Daniel
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