Re: Is _PAGE_PROTNONE set only for user mappings?

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Mon, May 16, 2022 at 07:04:32AM -0700, Dave Hansen wrote:
> On 5/12/22 22:33, Hyeonggon Yoo wrote:
> > Thanks Mel, and IIUC nor does do_kern_addr_fault() in arch/x86/mm/fault.c
> > expect _PAGE_PROTNONE instead of _PAGE_GLOBAL. I want to make it clear
> > in the code that _PAGE_PROTNONE is only used for user mappings.
> > 
> > How does below look?
> > 
> > diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> > index 40497a9020c6..f8d02b91a90c 100644
> > --- a/arch/x86/include/asm/pgtable_types.h
> > +++ b/arch/x86/include/asm/pgtable_types.h
> > @@ -35,7 +35,10 @@
> >  #define _PAGE_BIT_DEVMAP	_PAGE_BIT_SOFTW4
> >  
> >  /* If _PAGE_BIT_PRESENT is clear, we use these: */
> > -/* - if the user mapped it with PROT_NONE; pte_present gives true */
> > +/*
> > + * if the user mapped it with PROT_NONE; pte_present gives true
> > + * this is only used for user mappings (with _PAGE_BIT_USER)
> > + */
> >  #define _PAGE_BIT_PROTNONE	_PAGE_BIT_GLOBAL
> >  
> >  #define _PAGE_PRESENT	(_AT(pteval_t, 1) << _PAGE_BIT_PRESENT)
> > @@ -115,7 +118,8 @@
> >  #define _PAGE_DEVMAP	(_AT(pteval_t, 0))
> >  #endif
> >  
> > -#define _PAGE_PROTNONE	(_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)
> > +#define _PAGE_PROTNONE	((_AT(pteval_t, 1) << _PAGE_BIT_USER) | \
> > +			 (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE))
> >  
> >  /*
> >   * Set of bits not changed in pte_modify.  The pte's
> 
> I don't like the idea of _PAGE_BIT_USER being so implicit.  It is
> something kernel users should know explicitly that they are messing with.
>

Sounds reasonable. Better explicit than implicit.

> I was thinking of something more along the lines of taking the
> set_memory.c code and ensuring that it never sets (or even observes)
> _PAGE_BIT_GLOBAL on a _PAGE_USER mapping.

Yeah that would be a bit more explicit solution.

> There was also a question of
> if set_memory.c is ever used on userspace mappings.  It would be good to
> validate whether it's possible in-tree today and if not, enforce that
> _PAGE_USER PTEs should never even be observed with set_memory.c.

Simply adding dump_stack() tells me my kernel on my machine does not use
set_memory.c for userspace mappings but Hmm I'll take a look.

> The arch/x86/mm/dump_pagetables.c code is also a reasonable place to put
> assumptions about the page tables since it walks *everything* when asked.

Thanks for that information!

Thanks,
Hyeonggon