Re: [RFC PATCH v4 15/28] Add support to access persistent memory in the clear

[Tom Lendacky <thomas.lendacky@amd.com>]

On 3/17/2017 5:58 PM, Elliott, Robert (Persistent Memory) wrote:
>
>
>> -----Original Message-----
>> From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-
>> owner@vger.kernel.org] On Behalf Of Tom Lendacky
>> Sent: Thursday, February 16, 2017 9:45 AM
>> Subject: [RFC PATCH v4 15/28] Add support to access persistent memory in
>> the clear
>>
>> Persistent memory is expected to persist across reboots. The encryption
>> key used by SME will change across reboots which will result in corrupted
>> persistent memory.  Persistent memory is handed out by block devices
>> through memory remapping functions, so be sure not to map this memory as
>> encrypted.
>
> The system might be able to save and restore the correct encryption key for a
> region of persistent memory, in which case it does need to be mapped as
> encrypted.

If the OS could get some indication that BIOS/UEFI has saved and
restored the encryption key, then it could be mapped encrypted.

>
> This might deserve a new EFI_MEMORY_ENCRYPTED attribute bit so the
> system firmware can communicate that information to the OS (in the
> UEFI memory map and the ACPI NFIT SPA Range structures).  It wouldn't
> likely ever be added to the E820h table - ACPI 6.1 already obsoleted the
> Extended Attribute for AddressRangeNonVolatile.

An attribute bit in some form would be a nice way to inform the OS that
the persistent memory can be mapped encrypted.

>
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  arch/x86/mm/ioremap.c |    2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
>> index b0ff6bc..c6cb921 100644
>> --- a/arch/x86/mm/ioremap.c
>> +++ b/arch/x86/mm/ioremap.c
>> @@ -498,6 +498,8 @@ static bool
>> memremap_should_map_encrypted(resource_size_t phys_addr,
>>  	case E820_TYPE_ACPI:
>>  	case E820_TYPE_NVS:
>>  	case E820_TYPE_UNUSABLE:
>> +	case E820_TYPE_PMEM:
>> +	case E820_TYPE_PRAM:
>>  		return false;
>>  	default:
>>  		break;
>
> E820_TYPE_RESERVED is also used to report persistent memory in
> some systems (patch 16 adds that for other reasons).
>
> You might want to intercept the persistent memory types in the
> efi_mem_type(phys_addr) switch statement earlier in the function
> as well.  https://lkml.org/lkml/2017/3/13/357 recently mentioned that
> "in qemu hotpluggable memory isn't put into E820," with the latest
> information only in the UEFI memory map.
>
> Persistent memory can be reported there as:
> * EfiReservedMemoryType type with the EFI_MEMORY_NV attribute
> * EfiPersistentMemory type with the EFI_MEMORY_NV attribute
>
> Even the UEFI memory map is not authoritative, though.  To really
> determine what is in these regions requires parsing the ACPI NFIT
> SPA Ranges structures.  Parts of the E820 or UEFI regions could be
> reported as volatile there and should thus be encrypted.

Thanks for the details on this. I'll take a closer look at this and
update the checks appropriately.

Thanks,
Tom

>
> ---
> Robert Elliott, HPE Persistent Memory
>
>
>

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