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

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Message-ID: <7c522f65-c5c8-9362-e1eb-d0765e3ea6c9@amd.com>
Date:   Tue, 30 May 2017 09:38:36 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>
CC:     <linux-arch@...r.kernel.org>, <linux-efi@...r.kernel.org>,
        <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <x86@...nel.org>, <kexec@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <kasan-dev@...glegroups.com>,
        <linux-mm@...ck.org>, <iommu@...ts.linux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Toshimitsu Kani <toshi.kani@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dave Young <dyoung@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v5 32/32] x86/mm: Add support to make use of Secure Memory
 Encryption

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@....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

>