Re: [PATCH v3 2/2] x86/efi: clean up dead code around efi_reserve_boot_services()

Baoquan He
2017-07-26T01:13:43Z

On 07/26/17 at 12:12am, Naoya Horiguchi wrote:
> On Mon, Jul 24, 2017 at 02:20:44PM +0100, Matt Fleming wrote:
> > On Mon, 10 Jul, at 02:51:36PM, Naoya Horiguchi wrote:
> > > EFI_BOOT_SERVICES_{CODE|DATA} regions never overlap the kernel now,
> > > so we can clean up the check in efi_reserve_boot_services().
> > >
> > > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > > ---
> > > arch/x86/platform/efi/quirks.c | 23 +----------------------
> > > 1 file changed, 1 insertion(+), 22 deletions(-)
> >
> > Is this true for kernels not using KASLR?
>
> Thank you for pointing out this. It's not true depending on memmap layout.
> If a firmware does not define the memory around the kernel address
> (0x1000000 or CONFIG_PHYSICAL_START) as EFI_BOOT_SERVICES_*, no overlap
> happens. That's true in my testing server, but I don't think that we can
> expect it generally.
>
> So I think of adding some assertion in the patch 1/2 to detect this overlap
> in extract_kernel() even for no KASLR case.

EFI_BOOT_SERVICES_* memory are collected as e820 region of
E820_TYPE_RAM, how can we guarantee kernel won't use them after jumping
into the running kernel whether KASLR enabled or not? We can only wish
that EFI firmware engineer don't put EFI_BOOT_SERVICES_* far from
0x1000000 where normal kernel is loaded.