On Thu, Jun 22, 2023 at 2:28 AM szabolcs.nagy@xxxxxxx <szabolcs.nagy@xxxxxxx> wrote: > > The 06/21/2023 18:54, Edgecombe, Rick P wrote: > > On Wed, 2023-06-21 at 12:36 +0100, szabolcs.nagy@xxxxxxx wrote: > > > > The 06/20/2023 19:34, Edgecombe, Rick P wrote: > > > > > > I actually did a POC for this, but rejected it. The problem is, > > > > > > if > > > > > > there is a shadow stack overflow at that point then the kernel > > > > > > > > can't > > > > > > push the shadow stack token to the old stack. And shadow stack > > > > > > > > overflow > > > > > > is exactly the alt shadow stack use case. So it doesn't really > > > > > > > > solve > > > > > > the problem. > > > > > > > > the restore token in the alt shstk case does not regress anything > > > > but > > > > makes some use-cases work. > > > > > > > > alt shadow stack is important if code tries to jump in and out of > > > > signal handlers (dosemu does this with swapcontext) and for that a > > > > restore token is needed. > > > > > > > > alt shadow stack is important if the original shstk did not > > > > overflow > > > > but the signal handler would overflow it (small thread stack, huge > > > > sigaltstack case). > > > > > > > > alt shadow stack is also important for crash reporting on shstk > > > > overflow even if longjmp does not work then. longjmp to a > > > > makecontext > > > > stack would still work and longjmp back to the original stack can > > > > be > > > > made to mostly work by an altshstk option to overwrite the top > > > > entry > > > > with a restore token on overflow (this can break unwinding though). > > > > > > > > There was previously a request to create an alt shadow stack for the > > purpose of handling shadow stack overflow. So you are now suggesting to > > to exclude that and instead target a different use case for alt shadow > > stack? > > that is not what i said. > > > But I'm not sure how much we should change the ABI at this point since > > we are constrained by existing userspace. If you read the history, we > > may end up needing to deprecate the whole elf bit for this and other > > reasons. > > i'm not against deprecating the elf bit, but i think binary > marking will be difficult for this kind of feature no matter what > (code may be incompatible for complex runtime dependent reasons). > > > So should we struggle to find a way to grow the existing ABI without > > disturbing the existing userspace? Or should we start with something, > > finally, and see where we need to grow and maybe get a chance at a > > fresh start to grow it? > > > > Like, maybe 3 people will show up saying "hey, I *really* need to use > > shadow stack and longjmp from a ucontext stack", and no one says > > anything about shadow stack overflow. Then we know what to do. And > > maybe dosemu decides it doesn't need to implement shadow stack (highly > > likely I would think). Now that I think about it, AFAIU SS_AUTODISARM > > was created for dosemu, and the alt shadow stack patch adopted this > > behavior. So it's speculation that there is even a problem in that > > scenario. > > > > Or maybe people just enable WRSS for longjmp() and directly jump back > > to the setjmp() point. Do most people want fast setjmp/longjmp() at the > > cost of a little security? > > > > Even if, with enough discussion, we could optimize for all > > hypotheticals without real user feedback, I don't see how it helps > > users to hold shadow stack. So I think we should move forward with the > > current ABI. > > you may not get a second chance to fix a security feature. > it will be just disabled if it causes problems. *I* would use altshadowstack. I run a production system (that cares about correctness *and* performance, but that's not really relevant here -- SHSTK ought to be fast). And, if it crashes, I want to know why. So I handle SIGSEGV, etc so I have good logs if it crashes. And I want those same logs if I overflow the stack. That being said, I have no need for longjmp or siglongjmp for this. I use exit(2) to escape. For what it's worth, setjmp/longjmp is a bad API. The actual pattern that ought to work well (and that could be supported well by fancy compilers and non-C languages, as I understand it) is more like a function call that has two ways out. Like this (pseudo-C): void function(struct better_jmp_buf &buf, args...) { ... if (condition) better_long_jump(buf); // long jumps out! // could also pass buf to another function ... // could also return normally } better_call_with_jmp_buf(function, args); *This* could support altshadowstack just fine. And many users might be okay with the understanding that, if altshadowstack is on, you have to use a better long jump to get out (or a normal sigreturn or _exit). No one is getting an altshadowstack signal handler without code changes. siglongjmp() could support altshadowstack with help from the kernel, but we probably don't want to go there. --Andy