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

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Date:   Thu, 16 Jul 2020 17:20:02 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, kernel@...labora.com,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Paul Gofman <gofmanp@...il.com>,
        Kees Cook <keescook@...omium.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v4 1/2] kernel: Implement selective syscall userspace redirection

On Thu, Jul 16, 2020 at 12:31 PM Gabriel Krisman Bertazi
<krisman@...labora.com> 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.