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

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Date:   Tue, 8 Sep 2020 11:25:20 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...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 9/8/2020 10:57 AM, Dave Hansen wrote:
> 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?
> 

A new syscall is more intrusive, I think.  When creating a new shadow 
stack, the kernel also installs a restore token on the top of the new 
shadow stack, and it is somewhat x86-specific.  So far no other arch's 
need this.

Yes, madvise is better if the kernel only needs to change the mapping. 
The application itself can create the restore token before calling 
madvise().