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

[Lee Jones <lee@kernel.org>]

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?

-- 
Lee Jones [李琼斯]