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]
