[PATCH 1/1 v2] x86: pkey-mprotect must allow pkey-0

[Dave Hansen]

On 03/14/2018 12:46 AM, Ram Pai wrote:
> Once an address range is associated with an allocated pkey, it cannot be
> reverted back to key-0. There is no valid reason for the above behavior.  On
> the contrary applications need the ability to do so.

I'm trying to remember why we cared in the first place. :)

Could you add that to the changelog, please?

> @@ -92,7 +92,8 @@ int mm_pkey_alloc(struct mm_struct *mm)
>  static inline
>  int mm_pkey_free(struct mm_struct *mm, int pkey)
>  {
> -	if (!mm_pkey_is_allocated(mm, pkey))
> +	/* pkey 0 is special and can never be freed */
> +	if (!pkey || !mm_pkey_is_allocated(mm, pkey))
>  		return -EINVAL;

If an app was being really careful, couldn't it free up all of the
implicitly-pkey-0-assigned memory so that it is not in use at all?  In
that case, we might want to allow this.

On the other hand, nobody is likely to _ever_ actually do this so this
is good shoot-yourself-in-the-foot protection.