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]
