The 03/07/2023 15:00, Florian Weimer wrote: > * szabolcs: > > > changing/disabling the alt stack is not valid while a handler is > > executing on it. if we don't allow jumping out and back to an > > alt stack (swapcontext) then there can be only one alt stack > > live per thread and change/disable can do the shadow stack free. > > > > if jump back is allowed (linux even makes it race-free with > > SS_AUTODISARM) then the life-time of alt stack is extended > > beyond change/disable (jump back to an unregistered alt stack). > > > > to support jump back to an alt stack the requirements are > > > > 1) user has to manage an alt shadow stack together with the alt > > stack (requies user code change, not just libc). > > > > 2) kernel has to push a restore token on the thread shadow stack > > on signal entry (at least in case of alt shadow stack, and > > deal with corner cases around shadow stack overflow). > > We need to have a story for stackful coroutine switching as well, not > just for sigaltstack. I hope that we can use OpenJDK (Project Loom) and > QEMU as guinea pigs. If we have something that works for both, > hopefully that covers a broad range of scenarios. Userspace > coordination can eventually be handled by glibc; we can deallocate > alternate stacks on thread exit fairly easily (at least compared to the > current stack 8-). for stackful coroutines we just need a way to - allocate a shadow stack with a restore token on it. - switch to a target shadow stack with a restore token on it, while leaving behind a restore token on the old shadow stack. this is supported via map_shadow_stack syscall and the rstoressp, saveprevssp instruction pair. otoh there can be many alt shadow stacks per thread alive if we allow jump back (only one of them registered at a time) in fact they can be jumped to even from another thread, so their life-time is not tied to the thread (at least if we allow swapcontext across threads) so i think the libc cannot manage the alt shadow stacks, only user code can in the general case. and in case a signal runs on an alt shadow stack, the restore token can only be placed by the kernel on the old shadow stack. thanks.
