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

[Tom Lendacky]

On 4/18/2017 4:22 PM, 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@xxxxxxx>
> ---
>  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(void)
> +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];

So it turns out that when KASLR is enabled (CONFIG_RAMDOMIZE_BASE=y)
the stack-protector support causes issues with this function because
it is called so early. I can get past it by adding:

CFLAGS_mem_encrypt.o := $(nostackp)

in the arch/x86/mm/Makefile, but that obviously eliminates the support
for the whole file.  Would it be better to split out the sme_enable()
and other boot routines into a separate file or just apply the
$(nostackp) to the whole file?

Thanks,
Tom

> +	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;
> +	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;
> +	else if (strncmp(buffer, cmdline_off, sizeof(buffer)) == 0)
> +		sme_me_mask = 0;
> +	else
> +		sme_me_mask = active_by_default ? me_mask : 0;
> +
> +out:
>  	return sme_me_mask;
>  }
>
> @@ -543,9 +618,9 @@ unsigned long sme_get_me_mask(void)
>
>  #else	/* !CONFIG_AMD_MEM_ENCRYPT */
>
> -void __init sme_encrypt_kernel(void)	{ }
> -unsigned long __init sme_enable(void)	{ return 0; }
> +void __init sme_encrypt_kernel(void)			{ }
> +unsigned long __init sme_enable(struct boot_params *bp)	{ return 0; }
>
> -unsigned long sme_get_me_mask(void)	{ return 0; }
> +unsigned long sme_get_me_mask(void)			{ return 0; }
>
>  #endif	/* CONFIG_AMD_MEM_ENCRYPT */