On Fri, 2018-06-08 at 08:01 -0700, Andy Lutomirski wrote: > On Fri, Jun 8, 2018 at 7:53 AM Florian Weimer <fweimer@xxxxxxxxxx> wrote: > > > > On 06/07/2018 10:53 PM, Andy Lutomirski wrote: > > > On Thu, Jun 7, 2018 at 12:47 PM Florian Weimer <fweimer@xxxxxxxxxx> wrote: > > >> > > >> On 06/07/2018 08:21 PM, Andy Lutomirski wrote: > > >>> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote: > > >>>> > > >>>> When fork() specifies CLONE_VM but not CLONE_VFORK, the child > > >>>> needs a separate program stack and a separate shadow stack. > > >>>> This patch handles allocation and freeing of the thread shadow > > >>>> stack. > > >>> > > >>> Aha -- you're trying to make this automatic. I'm not convinced this > > >>> is a good idea. The Linux kernel has a long and storied history of > > >>> enabling new hardware features in ways that are almost entirely > > >>> useless for userspace. > > >>> > > >>> Florian, do you have any thoughts on how the user/kernel interaction > > >>> for the shadow stack should work? > > >> > > >> I have not looked at this in detail, have not played with the emulator, > > >> and have not been privy to any discussions before these patches have > > >> been posted, however … > > >> > > >> I believe that we want as little code in userspace for shadow stack > > >> management as possible. One concern I have is that even with the code > > >> we arguably need for various kinds of stack unwinding, we might have > > >> unwittingly built a generic trampoline that leads to full CET bypass. > > > > > > I was imagining an API like "allocate a shadow stack for the current > > > thread, fail if the current thread already has one, and turn on the > > > shadow stack". glibc would call clone and then call this ABI pretty > > > much immediately (i.e. before making any calls from which it expects > > > to return). > > > > Ahh. So you propose not to enable shadow stack enforcement on the new > > thread even if it is enabled for the current thread? For the cases > > where CLONE_VM is involved? > > > > It will still need a new assembler wrapper which sets up the shadow > > stack, and it's probably required to disable signals. > > > > I think it should be reasonable safe and actually implementable. But > > the benefits are not immediately obvious to me. > > Doing it this way would have been my first incliniation. It would > avoid all the oddities of the kernel magically creating a VMA when > clone() is called, guessing the shadow stack size, etc. But I'm okay > with having the kernel do it automatically, too. HJ wanted to add a arch_prctl that allocates a new shadow stack and switches to it. That was mainly for swapcontext. Perhaps we can also use that for threads? HJ, can you comment on this? > I think it would be > very nice to have a way for user code to find out the size of the > shadow stack and change it, though. (And relocate it, but maybe > that's impossible. The CET documentation doesn't have a clear > description of the shadow stack layout.) The shadow stack is vm_mmap'ed from memory and does not have any special layout. We can add a arch_prctl to find out shadow stack's address and size. > > > > > We definitely want strong enough user control that tools like CRIU can > > > continue to work. I haven't looked at the SDM recently enough to > > > remember for sure, but I'm reasonably confident that user code can > > > learn the address of its own shadow stack. If nothing else, CRIU > > > needs to be able to restore from a context where there's a signal on > > > the stack and the signal frame contains a shadow stack pointer. > > > > CRIU also needs the shadow stack *contents*, which shouldn't be directly > > available to the process. So it needs very special interfaces anyway. > > True. I proposed in a different email that ptrace() have full control > of the shadow stack (read, write, lock, unlock, etc). PTRACE can do PTRACE_POKEDATA on shadow stack. We can add lock/unlock. > > > > Does CRIU implement MPX support? > > Dunno. But given that MPX seems to be dying, I'm not sure it matters. > > --Andy
