Re: Is _PAGE_PROTNONE set only for user mappings?

[Dave Hansen]

On 5/29/22 03:32, Hyeonggon Yoo wrote:
> On Mon, May 16, 2022 at 07:04:32AM -0700, Dave Hansen wrote:
> Writing code I'm a bit confused:
> commit d1440b23c922d8 ("x86/mm: Factor out pageattr
> _PAGE_GLOBAL setting") says:
> 
> 	"This unconditional setting of _PAGE_GLOBAL is a problem when we have
> 	PTI and non-PTI and we want some areas to have _PAGE_GLOBAL and some
> 	not." 
> 
> Is this this sentence not valid anymore in PTI,
> and just unconditionally setting _PAGE_GLOBAL would be okay in kernel
> side regardless of PTI?

I believe it's still valid.

IIRC, there are three cases:

1. No KPTI.  All kernel mappings are _PAGE_GLOBAL.  Basically, for
   present mappings, if _PAGE_USER is clear, _PAGE_GLOBAL is set.
2. KPTI with PCID hardware support (or in a few other cases): The kernel
   image is mostly non-global.  Anything mapped into userspace *is*
   marked global, like entry text.
3. KPTI without PCIDs: Basically case #2, but with more of the kernel
   image left global.

So, not only are there different KPTI modes, there a different pars of
the kernel that require different _PAGE_GLOBAL behavior.

pti_kernel_image_global_ok() in arch/x86/mm/pti.c explains it pretty well.