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

[Andy Lutomirski]

On Thu, Jun 7, 2018 at 1:07 PM Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
>
> On Thu, Jun 07, 2018 at 11:30:34AM -0700, 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()?
> >
> > 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.
>
> For sure it might cause problems for CRIU since we have
> similar definitions for this structure inside our code.
> That said if kernel is about to modify the structures it
> should keep backward compatibility at least if a user
> passes previous version of a structure @ssp should be
> set to something safe by the kernel itself.
>
> I didn't read the whole series of patches in details
> yet, hopefully will be able tomorrow. Thanks Andy for
> CC'ing!

We have uc_flags.  It might be useful to carve out some of the flag
space (24 bits?) to indicate something like the *size* of sigcontext
and teach the kernel that new sigcontext fields should only be parsed
on sigreturn() if the size is large enough.