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

[Lee Jones <lee@kernel.org>]

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?

>         }
>  }
> 
> If so, I'll take it for a spin right now.

-- 
Lee Jones [李琼斯]