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Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
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>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        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>,
        Eugene Syromiatnikov <esyr@redhat.com>,
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        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
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Message-ID: <5979c58d-a6e3-d14d-df92-72cdeb97298d@intel.com>
Date:   Wed, 9 Sep 2020 15:08:45 -0700
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On 9/8/2020 11:25 AM, Yu, Yu-cheng wrote:
> 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().

After looking at this more, I found the changes are more similar to 
mprotect() than madvise().  We are going to change an anonymous mapping 
to a read-only mapping, and add the VM_SHSTK flag to it.  Would an 
x86-specific mprotect(PROT_SHSTK) make more sense?

One alternative would be requiring a read-only mapping for 
madvise(MADV_SHSTK).  But that is inconvenient for the application.

Yu-cheng