On Thu, Jun 7, 2018 at 10:50 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote: > > On Thu, 2018-06-07 at 09:37 -0700, Andy Lutomirski wrote: > > On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote: > > > > > > This patch adds basic shadow stack enabling/disabling routines. > > > A task's shadow stack is allocated from memory with VM_SHSTK > > > flag set and read-only protection. The shadow stack is > > > allocated to a fixed size and that can be changed by the system > > > admin. > > > > How do threads work? Can a user program mremap() its shadow stack to > > make it bigger? > > A pthread's shadow stack is allocated/freed by the kernel. This patch > has the supporting routines that handle both non-pthread and pthread. > > In [PATCH 04/10] "Handle thread shadow stack", we allocate pthread > shadow stack in copy_thread_tls(), and free it in deactivate_mm(). > > If clone of a pthread fails, shadow stack is freed in > cet_disable_free_shstk() below (I will add more comments): > > If (Current thread existing) > Disable and free shadow stack > > If (Clone of a pthread fails) > Free the pthread shadow stack > > We block mremap, mprotect, madvise, and munmap on a vma that has > VM_SHSTK (in separate patches). Why? mremap() seems like a sensible way to enlarge a shadow stack. munmap() seems like a good way to get rid of one, and mmap() seems like a nice way to create a new shadow stack if one were needed (for green threads or similar).
