Re: [PATCH 26/26] x86, pkeys: Documentation

[Dave Hansen]

On 10/01/2015 11:23 PM, Ingo Molnar wrote:
>> > Also, how do we do mprotect_pkey and say "don't change the key"?
> So if we start managing keys as a resource (i.e. alloc/free up to 16 of them), and 
> provide APIs for user-space to do all that, then user-space is not supposed to 
> touch keys it has not allocated for itself - just like it's not supposed to write 
> to fds it has not opened.

I like that.  It gives us at least a "soft" indicator to userspace about
what keys it should or shouldn't be using.

> Such an allocation method can still 'mess up', and if the kernel allocates a key 
> for its purposes it should not assume that user-space cannot change it, but at 
> least for non-buggy code there's no interaction and it would work out fine.

Yeah.  It also provides a clean interface so that future hardware could
enforce enforce kernel "ownership" of a key which could protect against
even buggy code.

So, we add a pair of syscalls,

	unsigned long sys_alloc_pkey(unsigned long flags??)
	unsigned long sys_free_pkey(unsigned long pkey)

keep the metadata in the mm, and then make sure that userspace allocated
it before it is allowed to do an mprotect_pkey() with it.

mprotect_pkey(add, flags, pkey)
{
	if (!(mm->pkeys_allocated & (1 << pkey))
		return -EINVAL;
}

That should be pretty easy to implement.  The only real overhead is the
16 bits we need to keep in the mm somewhere.
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