Re: [PATCH] x86/boot/KASLR: exclude EFI_BOOT_SERVICES_{CODE|DATA} from KASLR's choice

[Matt Fleming <matt@codeblueprint.co.uk>]

On Fri, 07 Jul, at 06:11:24AM, Naoya Horiguchi wrote:
> On Fri, Jul 07, 2017 at 11:07:59AM +0800, Baoquan He wrote:
> > On 07/06/17 at 03:57pm, Matt Fleming wrote:
> > > On Thu, 06 Jul, at 08:31:07AM, Naoya Horiguchi wrote:
> > > > +	for (i = 0; i < nr_desc; i++) {
> > > > +		md = (efi_memory_desc_t *)(pmap + (i * e->efi_memdesc_size));
> > > > +
> > > > +		/*
> > > > +		 * EFI_BOOT_SERVICES_{CODE|DATA} are avoided because boot
> > > > +		 * services regions could be accessed after ExitBootServices()
> > > > +		 * due to the workaround for buggy firmware.
> > > > +		 */
> > > > +		if (!(md->type == EFI_LOADER_CODE ||
> > > > +		      md->type == EFI_LOADER_DATA ||
> > > > +		      md->type == EFI_CONVENTIONAL_MEMORY))
> > > > +			continue;
> > > 
> > > Wouldn't it make more sense to *only* use EFI_CONVENTIONAL_MEMORY?
> > > 
> > > You can't re-use EFI_LOADER_* regions because the kaslr code is run so
> > > early in boot that you've no idea if data the kernel will need is in
> > > those EFI_LOADER_* regions.
> > > 
> > > For example, we pass struct setup_data objects inside of
> > > EFI_LOADER_DATA regions.
> > 
> > It doesn't matter because we have tried to avoid those memory setup_data
> > resides in in mem_avoid_overlap(). Here discarding EFI_LOADER_* could
> > discard the whole regions while setup_data could occupy small part of
> > them.
> 
> Hi Matt, Baoquan,
> 
> I added these three checks to accept any regions corresponding to
> E820_TYPE_RAM except EFI_BOOT_SERVICES_*, just thinking of that it's minimum
> surprising.  Baoquan gave a good justification on that, so I'll leave it
> as-is in next version.

I disagree. The least surprising option would be to use the region
type that everyone (boot loader, kernel, firmware) agrees is free:
EFI_CONVENTIONAL_MEMORY.