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Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Dave Martin <Dave.Martin@arm.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        Florian Weimer <fweimer@redhat.com>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
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        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Weijiang Yang <weijiang.yang@intel.com>
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 <46dffdfd-92f8-0f05-6164-945f217b0958@intel.com>
 <ed929729-4677-3d3b-6bfd-b379af9272b8@intel.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <6e1e22a5-1b7f-2783-351e-c8ed2d4893b8@intel.com>
Date:   Tue, 8 Sep 2020 11:25:20 -0700
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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().