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

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Date:   Tue, 8 Sep 2020 10:57:25 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Dave Martin <Dave.Martin@....com>, "H.J. Lu" <hjl.tools@...il.com>,
        Florian Weimer <fweimer@...hat.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 9/8/20 10:50 AM, Yu, Yu-cheng wrote:
> What about this:
> 
> - Do not add any new syscall or arch_prctl for creating a new shadow stack.
> 
> - Add a new arch_prctl that can turn an anonymous mapping to a shadow
> stack mapping.
> 
> This allows the application to do whatever is necessary.  It can even
> allow GDB or JIT code to create or fix a call stack.

Fine with me.  But, it's going to effectively be

	arch_prctl(PR_CONVERT_TO_SHS..., addr, len);

when it could just as easily be:

	madvise(addr, len, MADV_SHSTK...);

Or a new syscall.  The only question in my mind is whether we want to do
something generic that we can use for other similar things in the
future, like:

	madvise2(addr, len, flags, MADV2_SHSTK...);

I don't really feel strongly about it, though.  Could you please share
your logic on why you want a prctl() as opposed to a whole new syscall?