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);
}
}
If so, I'll take it for a spin right now.
--
Lee Jones [李琼斯]
