Re: [PATCH 3/6] x86/efi: Map RAM into the identity page table for mixed mode

Matt Fleming
2015-11-12T19:45:16Z

On Thu, 12 Nov, at 07:01:26PM, Borislav Petkov wrote:
> On Thu, Nov 12, 2015 at 03:40:20PM +0000, Matt Fleming wrote:
> > We are relying on the pre-existing mappings in 'trampoline_pgd' when
> > accessing function arguments in the EFI mixed mode thunking code.
> >
> > Instead let's map memory explicitly so that things will continue to
> > work when we move to a separate page table in the future.
> >
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
> > Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
> > ---
> > arch/x86/platform/efi/efi_64.c | 20 ++++++++++++++++++++
> > 1 file changed, 20 insertions(+)
> >
> > diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> > index c8b58ac47b77..634536034e32 100644
> > --- a/arch/x86/platform/efi/efi_64.c
> > +++ b/arch/x86/platform/efi/efi_64.c
> > @@ -144,6 +144,7 @@ void efi_sync_low_kernel_mappings(void)
> > int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
> > {
> > unsigned long pfn, text;
> > + efi_memory_desc_t *md;
> > struct page *page;
> > unsigned npages;
> > pgd_t *pgd;
> > @@ -200,6 +201,25 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
> >
> > efi_scratch.phys_stack += PAGE_SIZE; /* stack grows down */
> >
> > + /*
> > + * Map all of RAM so that we can access arguments in the 1:1
>
> Why "all of RAM"?

Because the pointers to arguments we pass to the thunk code can point
anywhere in RAM.

> > + * mapping when making EFI runtime calls.
> > + */
> > + for_each_efi_memory_desc(&memmap, md) {
> > + if (md->type != EFI_CONVENTIONAL_MEMORY &&
> > + md->type != EFI_LOADER_DATA &&
> > + md->type != EFI_LOADER_CODE)
>
> That's mapping all those EFI_* types...

Hmm? This is mapping only those regions that are usable by the kernel
for general allocations. See the table in setup_e820() in the EFI boot
stub code - these types map to E820_RAM.