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

[Lee Jones <lee@kernel.org>]

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]