Re: [RFC v5 12/38] mm: ability to disable execute permission on a key at creation

On Tue, Jul 11, 2017 at 02:57:30PM -0700, Dave Hansen wrote:

On 07/11/2017 02:51 PM, Ram Pai wrote:

quoted

On Wed, Jul 12, 2017 at 07:29:37AM +1000, Benjamin Herrenschmidt wrote:

quoted

On Tue, 2017-07-11 at 11:11 -0700, Dave Hansen wrote:

quoted

On 07/05/2017 02:21 PM, Ram Pai wrote:

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Currently sys_pkey_create() provides the ability to disable read
and write permission on the key, at  creation. powerpc  has  the
hardware support to disable execute on a pkey as well.This patch
enhances the interface to let disable execute  at  key  creation
time. x86 does  not  allow  this.  Hence the next patch will add
ability  in  x86  to  return  error  if  PKEY_DISABLE_EXECUTE is
specified.

That leads to the question... How do you tell userspace.

(apologies if I missed that in an existing patch in the series)

How do we inform userspace of the key capabilities ? There are at least
two things userspace may want to know already:

 - What protection bits are supported for a key

the userspace is the one which allocates the keys and enables/disables the
protection bits on the key. the kernel is just a facilitator. Now if the
use space wants to know the current permissions on a given key, it can
just read the AMR/PKRU register on powerpc/intel respectively.

Let's say I want to execute-disable a region.  Can I use protection
keys?  Do I do

	pkey_mprotect(... PKEY_DISABLE_EXECUTE);

and assume that the -EINVAL is because PKEY_DISABLE_EXECUTE is
unsupported, or do I do:

#ifdef __ppc__
	pkey = pkey_aloc();
	pkey_mprotect(... PKEY_DISABLE_EXECUTE);
#else
	mprotect();
#endif

on ppc you could do either
pkey = pkey_alloc(..,PKEY_DISABLE_EXECUTE);
pkey_mprotect(...,pkey);

or you can just do the x86 way
mprotect();

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 - How many keys exist

There is no standard way of finding this other than trying to allocate
as many till you fail.  A procfs or sysfs file can be added to expose
this information.

It's also dynamic.  On x86, you lose a key if you've used the
execute-only support.  We also reserve the right to steal more in the
future if we want.

total number of key supported on the architecture is a constant.
How many are reserved by the architecture is also probably known in
advance.
Now how many does the kernel use to reserve for itself is something
the kernel knows too and hence can expose it, though the information
may change dynamically as the kernel reserves and releases the key
based on its internal needs. 

So i think we can expose this informaton through procfs/sysfs and let
the application decide how it wants to use the information.

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 - Which keys are available for use by userspace. On PowerPC, the
kernel can reserve some keys for itself, so can the hypervisor. In
fact, they do.

this information can be exposed through /proc or /sysfs

I am sure there will be more demands and requirements as applications
start leveraging these feature.

For 5 bits, I think just having someone run pkey_alloc() in a loop is
fine.  I don't think we really need to enumerate it in some other way.

-- 
Ram Pai