linux-kernel - Re: [PATCH v11 25/25] x86/cet/shstk: Add arch

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Message-ID: <CALCETrUUKb6oyBmB3CSeVy1xT7mcnV=BD2eipAnKUhma7K3qKw@mail.gmail.com>
Date:   Fri, 28 Aug 2020 10:39:05 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "H.J. Lu" <hjl.tools@...il.com>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Dave Hansen <dave.hansen@...el.com>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack

On Fri, Aug 28, 2020 at 4:38 AM H.J. Lu <hjl.tools@...il.com> wrote:
>
> On Thu, Aug 27, 2020 at 11:24 PM Florian Weimer <fweimer@...hat.com> wrote:
> >
> > * H. J. Lu:
> >
> > > Can you think of ANY issues of passing more arguments to arch_prctl?
> >
> > On x32, the glibc arch_prctl system call wrapper only passes two
> > arguments to the kernel, and applications have no way of detecting that.
> > musl only passes two arguments on all architectures.  It happens to work
> > anyway with default compiler flags, but that's an accident.
>
> In the current glibc, there is no arch_prctl wrapper for i386.  There are
> arch_prctl wrappers with 2 arguments for x86-64 and x32.  But this isn't an
> issue for glibc since glibc is both the provider and the user of the new
> arch_prctl extension.  Besides,
>
> long syscall(long number, ...);
>
> is always available.

Userspace is probably full of tools and libraries that contain tables
of system calls and their signatures.  Think tracing, audit, container
management, etc.  I don't know how they will react to the addition of
new arguments.