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

Borislav Petkov
2017-05-19T11:27:17Z

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?

--
Regards/Gruss,
Boris.

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