Re: [PATCH v4 1/2] kernel: Implement selective syscall userspace redirection

[Andy Lutomirski <luto@xxxxxxxxxx>]

On Thu, Jul 16, 2020 at 12:31 PM Gabriel Krisman Bertazi
<krisman@xxxxxxxxxxxxx> wrote:
>

This is quite nice.  I have a few comments, though:

You mentioned rt_sigreturn().  Should this automatically exempt the
kernel-provided signal restorer on architectures (e.g. x86_32) that
provide one?

The amount of syscall entry wiring that arches need to do is IMO
already a bit out of hand.  Should we instead rename TIF_SECCOMP to
TIF_SYSCALL_INTERCEPTION and have one generic callback that handles
seccomp and this new thing?

> +int do_syscall_user_dispatch(struct pt_regs *regs)
> +{
> +       struct syscall_user_dispatch *sd = &current->syscall_dispatch;
> +       unsigned long ip = instruction_pointer(regs);
> +       char state;
> +
> +       if (likely(ip >= sd->dispatcher_start && ip <= sd->dispatcher_end))
> +               return 0;
> +
> +       if (likely(sd->selector)) {
> +               if (unlikely(__get_user(state, sd->selector)))
> +                       do_exit(SIGSEGV);
> +
> +               if (likely(state == 0))
> +                       return 0;
> +
> +               if (state != 1)
> +                       do_exit(SIGSEGV);

This seems a bit extreme and hard to debug if it ever happens.