Subject: Re: [RFC PATCH v3 20/20] x86: Add support to make use of Secure
 Memory Encryption
To: Borislav Petkov <bp@alien8.de>
References: <20161110003426.3280.2999.stgit@tlendack-t1.amdoffice.net>
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From: Tom Lendacky <thomas.lendacky@amd.com>
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Date: Tue, 29 Nov 2016 12:48:17 -0600
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On 11/26/2016 2:47 PM, Borislav Petkov wrote:
> On Wed, Nov 09, 2016 at 06:38:38PM -0600, Tom Lendacky wrote:
>> This patch adds the support to check if SME has been enabled and if the
>> mem_encrypt=on command line option is set. If both of these conditions
>> are true, then the encryption mask is set and the kernel is encrypted
>> "in place."
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  arch/x86/kernel/head_64.S          |    1 +
>>  arch/x86/kernel/mem_encrypt_init.c |   60 +++++++++++++++++++++++++++++++++++-
>>  arch/x86/mm/mem_encrypt.c          |    2 +
>>  3 files changed, 62 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
>> index e8a7272..c225433 100644
>> --- a/arch/x86/kernel/head_64.S
>> +++ b/arch/x86/kernel/head_64.S
>> @@ -100,6 +100,7 @@ startup_64:
>>  	 * to include it in the page table fixups.
>>  	 */
>>  	push	%rsi
>> +	movq	%rsi, %rdi
>>  	call	sme_enable
>>  	pop	%rsi
>>  	movq	%rax, %r12
>> diff --git a/arch/x86/kernel/mem_encrypt_init.c b/arch/x86/kernel/mem_encrypt_init.c
>> index 7bdd159..c94ceb8 100644
>> --- a/arch/x86/kernel/mem_encrypt_init.c
>> +++ b/arch/x86/kernel/mem_encrypt_init.c
>> @@ -16,9 +16,14 @@
>>  #include <linux/mm.h>
>>  
>>  #include <asm/sections.h>
>> +#include <asm/processor-flags.h>
>> +#include <asm/msr.h>
>> +#include <asm/cmdline.h>
>>  
>>  #ifdef CONFIG_AMD_MEM_ENCRYPT
>>  
>> +static char sme_cmdline_arg[] __initdata = "mem_encrypt=on";
> 
> One more thing: just like we're adding an =on switch, we'd need an =off
> switch in case something's wrong with the SME code. IOW, if a user
> supplies "mem_encrypt=off", we do not encrypt.

Well, we can document "off", but if the exact string "mem_encrypt=on"
isn't specified on the command line then the encryption won't occur.
The cmdline_find_option_bool() function looks for the exact string and
isn't interpreting the value on the right side of the equal sign. So
omitting mem_encrypt=on or using mem_encrypt=off is the same.

Thanks,
Tom

> 
> Thanks.
>