Re: [PATCH 03/10] x86/cet: Signal handling for shadow stack

[Yu-cheng Yu]

On Thu, 2018-06-07 at 20:58 +0200, Florian Weimer wrote:
> On 06/07/2018 08:30 PM, Andy Lutomirski wrote:
> > On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> >>
> >> Set and restore shadow stack pointer for signals.
> > 
> > How does this interact with siglongjmp()?
> 
> We plan to use some unused signal mask bits in the jump buffer (we have 
> a lot of those in glibc for some reason) to store the shadow stack pointer.
> 
> > This patch makes me extremely nervous due to the possibility of ABI
> > issues and CRIU breakage.
> > 
> >> diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
> >> index 844d60eb1882..6c8997a0156a 100644
> >> --- a/arch/x86/include/uapi/asm/sigcontext.h
> >> +++ b/arch/x86/include/uapi/asm/sigcontext.h
> >> @@ -230,6 +230,7 @@ struct sigcontext_32 {
> >>          __u32                           fpstate; /* Zero when no FPU/extended context */
> >>          __u32                           oldmask;
> >>          __u32                           cr2;
> >> +       __u32                           ssp;
> >>   };
> >>
> >>   /*
> >> @@ -262,6 +263,7 @@ struct sigcontext_64 {
> >>          __u64                           trapno;
> >>          __u64                           oldmask;
> >>          __u64                           cr2;
> >> +       __u64                           ssp;
> >>
> >>          /*
> >>           * fpstate is really (struct _fpstate *) or (struct _xstate *)
> >> @@ -320,6 +322,7 @@ struct sigcontext {
> >>          struct _fpstate __user          *fpstate;
> >>          __u32                           oldmask;
> >>          __u32                           cr2;
> >> +       __u32                           ssp;
> > 
> > Is it actually okay to modify these structures like this?  They're
> > part of the user ABI, and I don't know whether any user code relies on
> > the size being constant.
> 
> Probably not.  Historically, these things have been tacked at the end of 
> the floating point state, see struct _xstate:
> 
>          /* New processor state extensions go here: */
> 
> However, I'm not sure if this is really ideal because I doubt that 
> everyone who needs the shadow stack pointer also wants to sacrifice 
> space for the AVX-512 save area (which is already a backwards 
> compatibility hazard).  Other architectures have variable offsets and 
> some TLV-style setup here.
> 
> Thanks,
> Florian

I will move 'ssp' to _xstate for now, and look for other ways.