On Tue, Sep 1, 2020 at 10:23 AM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote: > > On 9/1/2020 3:28 AM, Dave Martin wrote: > > On Thu, Aug 27, 2020 at 06:26:11AM -0700, H.J. Lu wrote: > >> On Wed, Aug 26, 2020 at 12:57 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > >>> > >>> On 8/26/20 11:49 AM, Yu, Yu-cheng wrote: > >>>>> I would expect things like Go and various JITs to call it directly. > >>>>> > >>>>> If we wanted to be fancy and add a potentially more widely useful > >>>>> syscall, how about: > >>>>> > >>>>> mmap_special(void *addr, size_t length, int prot, int flags, int type); > >>>>> > >>>>> Where type is something like MMAP_SPECIAL_X86_SHSTK. Fundamentally, > >>>>> this is really just mmap() except that we want to map something a bit > >>>>> magical, and we don't want to require opening a device node to do it. > >>>> > >>>> One benefit of MMAP_SPECIAL_* is there are more free bits than MAP_*. > >>>> Does ARM have similar needs for memory mapping, Dave? > >>> > >>> No idea. > >>> > >>> But, mmap_special() is *basically* mmap2() with extra-big flags space. > >>> I suspect it will grow some more uses on top of shadow stacks. It could > >>> have, for instance, been used to allocate MPX bounds tables. > >> > >> There is no reason we can't use > >> > >> long arch_prctl (int, unsigned long, unsigned long, unsigned long, ..); > >> > >> for ARCH_X86_CET_MMAP_SHSTK. We just need to use > >> > >> syscall (SYS_arch_prctl, ARCH_X86_CET_MMAP_SHSTK, ...); > > > > > > For arm64 (and sparc etc.) we continue to use the regular mmap/mprotect > > family of calls. One or two additional arch-specific mmap flags are > > sufficient for now. > > > > Is x86 definitely not going to fit within those calls? > > That can work for x86. Andy, what if we create PROT_SHSTK, which can > been seen only from the user. Once in kernel, it is translated to > VM_SHSTK. One question for mremap/mprotect is, do we allow a normal > data area to become shadow stack? I'm unconvinced that we want to use a somewhat precious PROT_ or VM_ bit for this. Using a flag bit makes sense if we expect anyone to ever map an fd or similar as a shadow stack, but that seems a bit odd in the first place. To me, it seems more logical for a shadow stack to be a special sort of mapping with a special vm_ops, not a normal mapping with a special flag set. Although I realize that we want shadow stacks to work like anonymous memory with respect to fork(). Dave? --Andy
