On Tue, Jan 09, 2024 at 03:54:07PM +0100, Markus Armbruster wrote: > Daniel P. Berrangé <berra...@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 <th...@redhat.com> > > Signed-off-by: Daniel P. Berrangé <berra...@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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
