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. > Also, I have a couple of other questions after reading the > documentation some more: > > 1. Why on Earth does INCSSP only take an 8-bit number of frames to > skip? It seems to me that code that calls setjmp() and then calls > longjmp() while nested more than 256 function call levels will crash. GLIBC takes care of more than 256 functions calls. > 2. The mnemonic RSTORSSP makes no sense to me. RSTORSSP is a stack > *switch* operation not a stack *restore* operation, unless I'm > seriously misunderstanding. The intention is to switch shadow stacks with tokens. RSTORSSP restores to a previous shadow stack address from a restore token. > 3. Is there anything resembling clear documentation of the format of > the shadow stack? That is, what types of values might be found on the > shadow stack and what do they all mean? Only return addresses and restore tokens can be on a user-mode shadow stack. The restore token has the incoming shadow stack address plus one bit indicating 64/32-bit mode. I will put this into Documentation/x86/intel_cet.txt. > > 4. Usually Intel doesn't submit upstream Linux patches for ISA > extensions until the ISA is documented for real. CET does not appear > to be documented for real. Could Intel kindly release something that > at least claims to be authoritative documentation? > > --Andy
