Re: [RFC PATCH v2 11/20] mm: Access BOOT related data in the clear

[Tom Lendacky]

On 09/15/2016 04:57 AM, Matt Fleming wrote:
> On Wed, 14 Sep, at 09:20:44AM, Tom Lendacky wrote:
>> On 09/12/2016 11:55 AM, Andy Lutomirski wrote:
>>> On Aug 22, 2016 6:53 PM, "Tom Lendacky" <thomas.lendacky@xxxxxxx> wrote:
>>>>
>>>> BOOT data (such as EFI related data) is not encyrpted when the system is
>>>> booted and needs to be accessed as non-encrypted.  Add support to the
>>>> early_memremap API to identify the type of data being accessed so that
>>>> the proper encryption attribute can be applied.  Currently, two types
>>>> of data are defined, KERNEL_DATA and BOOT_DATA.
>>>
>>> What happens when you memremap boot services data outside of early
>>> boot?  Matt just added code that does this.
>>>
>>> IMO this API is not so great.  It scatters a specialized consideration
>>> all over the place.  Could early_memremap not look up the PA to figure
>>> out what to do?
>>
>> Yes, I could see if the PA falls outside of the kernel usable area and,
>> if so, remove the memory encryption attribute from the mapping (for both
>> early_memremap and memremap).
>>
>> Let me look into that, I would prefer something along that line over
>> this change.
> 
> So, the last time we talked about using the address to figure out
> whether to encrypt/decrypt you said,
> 
>  "I looked into this and this would be a large change also to parse
>   tables and build lists."
> 
> Has something changed that makes this approach easier?

The original idea of parsing the tables and building a list was
a large change.  This approach would be simpler by just checking if
the PA is outside the kernel usable area, and if so, removing the
encryption bit.

> 
> And again, you need to be careful with the EFI kexec code paths, since
> you've got a mixture of boot and kernel data being passed. In
> particular the EFI memory map is allocated by the firmware on first
> boot (BOOT_DATA) but by the kernel on kexec (KERNEL_DATA).
> 
> That's one of the reasons I suggested requiring the caller to decide
> on BOOT_DATA vs KERNEL_DATA - when you start looking at kexec the
> distinction isn't easily made.

Yeah, for kexec I think I'll need to make sure that everything looks
like it came from the BIOS/UEFI/bootloader.  If all of the kexec
pieces are allocated with un-encrypted memory, then the boot path
should remain the same.  That's the piece I need to investigate
further.

Thanks,
Tom

>