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

[Andy Lutomirski]

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.

> +int cet_push_shstk(int ia32, unsigned long ssp, unsigned long val)
> +{
> +       if (val >= TASK_SIZE)
> +               return -EINVAL;

TASK_SIZE_MAX.  But I'm a bit unsure why you need this check at all.

> +int cet_restore_signal(unsigned long ssp)
> +{
> +       if (!current->thread.cet.shstk_enabled)
> +               return 0;
> +       return cet_set_shstk_ptr(ssp);
> +}

This will blow up if the shadow stack enabled state changes in a
signal handler.  Maybe we don't care.