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

[Florian Weimer]

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