Daniel P. Berrangé <[email protected]> writes: > On Tue, Jan 09, 2024 at 03:54:07PM +0100, Markus Armbruster wrote: >> Daniel P. Berrangé <[email protected]> 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! [...]
