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

[Tom Lendacky]

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@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(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

>