On Mon, 2018-06-18 at 14:44 -0700, Andy Lutomirski wrote: > On Sat, Jun 16, 2018 at 8:16 PM Balbir Singh <bsingharora@xxxxxxxxx> wrote: > > > > On Thu, 2018-06-14 at 07:56 -0700, Yu-cheng Yu wrote: > > > On Thu, 2018-06-14 at 11:07 +1000, Balbir Singh wrote: > > > > On Tue, 2018-06-12 at 08:03 -0700, Yu-cheng Yu wrote: > > > > > On Tue, 2018-06-12 at 20:56 +1000, Balbir Singh wrote: > > > > > > > > > > > > On 08/06/18 00:37, Yu-cheng Yu wrote: > > > > > > > This series introduces CET - Shadow stack > > > > > > > > > > > > > > At the high level, shadow stack is: > > > > > > > > > > > > > > Allocated from a task's address space with vm_flags VM_SHSTK; > > > > > > > Its PTEs must be read-only and dirty; > > > > > > > Fixed sized, but the default size can be changed by sys admin. > > > > > > > > > > > > > > For a forked child, the shadow stack is duplicated when the next > > > > > > > shadow stack access takes place. > > > > > > > > > > > > > > For a pthread child, a new shadow stack is allocated. > > > > > > > > > > > > > > The signal handler uses the same shadow stack as the main program. > > > > > > > > > > > > > > > > > > > Even with sigaltstack()? > > > > > > > > > > > > > > > > Yes. > > > > > > > > I am not convinced that it would work, as we switch stacks, oveflow might > > > > be an issue. I also forgot to bring up setcontext(2), I presume those > > > > will get new shadow stacks > > > > > > Do you mean signal stack/sigaltstack overflow or swapcontext in a signal > > > handler? > > > > > > > I meant any combination of that. If there is a user space threads implementation that uses sigaltstack for switching threads > > > > Anyone who does that is nuts. The whole point of user space threads > is speed, and signals are very slow. For userspace threads to work, > we need an API to allocate new shadow stacks, and we need to use the > extremely awkwardly defined RSTORSSP stuff to switch. (I assume this > is possible on an ISA level. The docs are bad, and the mnemonics for > the relevant instructions are nonsensical.) The whole point was to ensure we don't break applications/code that work today. I think as long as there is a shadow stack allocated corresponding to the user space stack and we can Restore SSP as we switch things should be fine. Balbir Singh.
