Re: x86: pgtable / kaslr initialisation (OOB) help

[Lee Jones]

On Wed, 14 Jun 2023, Dave Hansen wrote:

> On 6/14/23 08:26, Lee Jones wrote:
> > On Wed, 14 Jun 2023, Lee Jones wrote:
> > 
> >> On Wed, 14 Jun 2023, Lee Jones wrote:
> >>
> >>> Thanks for chiming in Dave.  I hoped you would.
> >>>
> >>> On Wed, 14 Jun 2023, Dave Hansen wrote:
> >>>
> >>>> On 6/14/23 07:37, Lee Jones wrote:
> >>>>> Still unsure how we (the kernel) can/should write to an area of memory
> >>>>> that does not belong to it.  Should we allocate enough memory
> >>>>> (2*PAGE_SIZE? rather than 8-Bytes) for trampoline_pgd_entry to consume
> >>>>> in a more sane way?
> >>>>
> >>>> No.
> >>>>
> >>>> I think this:
> >>>>
> >>>>                 set_pgd(&trampoline_pgd_entry,
> >>>>                         __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> >>>>
> >>>> is bogus-ish.  set_pgd() wants to operate on a pgd_t inside a pgd
> >>>> *PAGE*.  But it's just being pointed at a single  _entry_.  The address
> >>>> of 'trampoline_pgd_entry' in your case  also just (unfortunately)
> >>>> happens to pass the:
> >>>>
> >>>> 	__pti_set_user_pgtbl -> pgdp_maps_userspace()
> >>>>
> >>>> test.  I _think_ we want these to just be something like:
> >>>>
> >>>> 	trampoline_pgd_entry = __pgd(_KERNPG_TABLE |
> >>>> 				     __pa(p4d_page_tramp);
> >>>>
> >>>> That'll keep us away from all of the set_pgd()-induced nastiness.
> >>>
> >>> Okay.  Is this what you're suggesting?
> >>>
> >>> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c                 v
> >>> index d336bb0cb38b..803595c7dcc8 100644
> >>> --- a/arch/x86/mm/kaslr.c
> >>> +++ b/arch/x86/mm/kaslr.c
> >>> @@ -176,7 +176,7 @@ void __meminit init_trampoline_kaslr(void)
> >>>                 set_pgd(&trampoline_pgd_entry,
> >>>                         __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> >>>         } else {
> >>> -               set_pgd(&trampoline_pgd_entry,
> >>> -                       __pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
> >>> +               trampoline_pgd_entry =
> >>> +                       __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp);
> >>
> >> Note the change of *.page_tramp here.
> >>
> >>   s/pud/p4d/
> >>
> >> I'm assuming that too was intentional?
> > 
> > Never mind.  I can see that p4d_page_tramp is local to the if() segment.
> > 
> > While we're at it, does the if() segment look correct to you:
> > 
> >   if (pgtable_l5_enabled()) {
> >         p4d_page_tramp = alloc_low_page();
> > 
> >         p4d_tramp = p4d_page_tramp + p4d_index(paddr);
> > 
> >         set_p4d(p4d_tramp,
> >                 __p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
> > 
> >         set_pgd(&trampoline_pgd_entry,
> >                 __pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> >   } else {
> >         trampoline_pgd_entry =
> >                 __pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
> >   }
> > 
> >  - pud_page_tramp is being passed to set_p4d()
> >  - p4d_page_tramp is being passed to set_pgd()
> > 
> > Should those be the other way around, or am I missing the point?
> 
> You're missing the point. :)

Super, thanks for the explanation.
 
> PGDs are always set up to point to the physical address of the thing at
> one lower level than them.  A page is allocated for that level when
> 5-level paging is in play.  No page is needed when it is not in play.
> 
> The pattern is _almost_ always
> 
> 	pgd = ... __pa(p4d);
> 
> In other words, point the PGD at the physical address of a p4d.  But
> things get funky on systems without p4ds, thus the special casing here.
> 
> Does the (completely untested) attached patch fix your problem?

I just submitted a (tested) patch.

It doesn't cover the if() segment though.  I'll do so and resubmit.

> ---
> 
>  b/arch/x86/mm/kaslr.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff -puN arch/x86/mm/kaslr.c~trampoline_pgd_entry arch/x86/mm/kaslr.c
> --- a/arch/x86/mm/kaslr.c~trampoline_pgd_entry	2023-06-14 08:54:08.685554094 -0700
> +++ b/arch/x86/mm/kaslr.c	2023-06-14 08:55:36.077089793 -0700
> @@ -172,10 +172,10 @@ void __meminit init_trampoline_kaslr(voi
>  		set_p4d(p4d_tramp,
>  			__p4d(_KERNPG_TABLE | __pa(pud_page_tramp)));
>  
> -		set_pgd(&trampoline_pgd_entry,
> -			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
> +		trampoline_pgd_entry =
> +			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp));
>  	} else {
> -		set_pgd(&trampoline_pgd_entry,
> -			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
> +		trampoline_pgd_entry =
> +		       	__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
>  	}
>  }
> _


-- 
Lee Jones [李琼斯]