Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

["Yu, Yu-cheng" <yu-cheng.yu@intel.com>]

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().