Re: [PATCH v5 32/32] x86/mm: Add support to make use of Secure Memory Encryption

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

On 5/19/2017 6:27 AM, Borislav Petkov wrote:
> On Tue, Apr 18, 2017 at 04:22:23PM -0500, Tom Lendacky wrote:
>> Add support to check if SME has been enabled and if memory encryption
>> should be activated (checking of command line option based on the
>> configuration of the default state).  If memory encryption is to be
>> activated, 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/mm/mem_encrypt.c |   83 +++++++++++++++++++++++++++++++++++++++++++--
>>   2 files changed, 80 insertions(+), 4 deletions(-)
> 
> ...
> 
>> +unsigned long __init sme_enable(struct boot_params *bp)
>>   {
>> +	const char *cmdline_ptr, *cmdline_arg, *cmdline_on, *cmdline_off;
>> +	unsigned int eax, ebx, ecx, edx;
>> +	unsigned long me_mask;
>> +	bool active_by_default;
>> +	char buffer[16];
>> +	u64 msr;
>> +
>> +	/* Check for the SME support leaf */
>> +	eax = 0x80000000;
>> +	ecx = 0;
>> +	native_cpuid(&eax, &ebx, &ecx, &edx);
>> +	if (eax < 0x8000001f)
>> +		goto out;
>> +
>> +	/*
>> +	 * Check for the SME feature:
>> +	 *   CPUID Fn8000_001F[EAX] - Bit 0
>> +	 *     Secure Memory Encryption support
>> +	 *   CPUID Fn8000_001F[EBX] - Bits 5:0
>> +	 *     Pagetable bit position used to indicate encryption
>> +	 */
>> +	eax = 0x8000001f;
>> +	ecx = 0;
>> +	native_cpuid(&eax, &ebx, &ecx, &edx);
>> +	if (!(eax & 1))
>> +		goto out;
> 
> <---- newline here.
> 
>> +	me_mask = 1UL << (ebx & 0x3f);
>> +
>> +	/* Check if SME is enabled */
>> +	msr = __rdmsr(MSR_K8_SYSCFG);
>> +	if (!(msr & MSR_K8_SYSCFG_MEM_ENCRYPT))
>> +		goto out;
>> +
>> +	/*
>> +	 * Fixups have not been applied to phys_base yet, so we must obtain
>> +	 * the address to the SME command line option data in the following
>> +	 * way.
>> +	 */
>> +	asm ("lea sme_cmdline_arg(%%rip), %0"
>> +	     : "=r" (cmdline_arg)
>> +	     : "p" (sme_cmdline_arg));
>> +	asm ("lea sme_cmdline_on(%%rip), %0"
>> +	     : "=r" (cmdline_on)
>> +	     : "p" (sme_cmdline_on));
>> +	asm ("lea sme_cmdline_off(%%rip), %0"
>> +	     : "=r" (cmdline_off)
>> +	     : "p" (sme_cmdline_off));
>> +
>> +	if (IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT))
>> +		active_by_default = true;
>> +	else
>> +		active_by_default = false;
>> +
>> +	cmdline_ptr = (const char *)((u64)bp->hdr.cmd_line_ptr |
>> +				     ((u64)bp->ext_cmd_line_ptr << 32));
>> +
>> +	cmdline_find_option(cmdline_ptr, cmdline_arg, buffer, sizeof(buffer));
>> +
>> +	if (strncmp(buffer, cmdline_on, sizeof(buffer)) == 0)
>> +		sme_me_mask = me_mask;
> 
> Why doesn't simply
> 
> 	if (!strncmp(buffer, "on", 2))
> 		...
> 
> work?

In this case we're running identity mapped and the "on" constant ends up
as kernel address (0xffffffff81...) which results in a segfault.

Thanks,
Tom

> 

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