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

[Markus Armbruster]

Daniel P. Berrangé <berrange@redhat.com> writes:

> 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.

Makes sense.

> '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.

The docs lead me to suspect it will *not* cover non-gpr registers that
are used for passing arguments.  Requires a calling convention that can
pass arguments in non-gpr registers, such as floating-point and vector
registers.  I figure these are less useful for exploits than gprs.

Thanks!

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