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

Tom Lendacky
2017-05-30T14:39:10Z

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

>