On Tue, 2018-06-12 at 10:24 -0700, Yu-cheng Yu wrote: > On Tue, 2018-06-12 at 09:31 -0700, Andy Lutomirski wrote: > > On Tue, Jun 12, 2018 at 9:24 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote: > > > > > > On Tue, 2018-06-12 at 09:00 -0700, Andy Lutomirski wrote: > > > > On Tue, Jun 12, 2018 at 8:06 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> 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()? > > > > > > > > > > > > > > > > > > Balbir Singh. > > > > > > > > > > Yes. > > > > > > > > > > > > > I think we're going to need some provision to add an alternate signal > > > > stack to handle the case where the shadow stack overflows. > > > > > > The shadow stack stores only return addresses; its consumption will not > > > exceed a percentage of (program stack size + sigaltstack size) before > > > those overflow. When that happens, there is usually very little we can > > > do. So we set a default shadow stack size that supports certain nested > > > calls and allow sys admin to adjust it. > > > > > > > Of course there's something you can do: add a sigaltstack-like stack > > switching mechanism. Have a reserve shadow stack and, when a signal > > is delivered (possibly guarded by other conditions like "did the > > shadow stack overflow"), switch to a new shadow stack and maybe write > > a special token to the new shadow stack that says "signal delivery > > jumped here and will restore to the previous shadow stack and > > such-and-such address on return". > > If (shstk size == (stack size + sigaltstack size)), then shstk will not > overflow before program stack overflows and sigaltstack also overflows. > > Let me think about this. The reserve shadow stack will help only when the shstk overflows but signal stack/sigaltstack still has room and we can deliver a signal. If the shstk is large enough to cover any nested calls that will overflow both the program stack and sigaltstack then we don't need a reserve shstk. We can estimate how big the shstk needs to be; in the worst case it should not be greater than (program stack size + sigaltstack size). The default shstk size we choose pass all signal tests in GLIBC. In case there is a need to increase it for a very large RLIMIT_STACK or very large sigaltstack, the sys admin can increase the default shstk size. Yu-cheng
