Re: [PATCH v7 02/20] x86/virt/tdx: Detect TDX during kernel boot

[Sathyanarayanan Kuppuswamy]

On 11/20/22 4:26 PM, Kai Huang wrote:
> Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
> host and certain physical attacks.  A CPU-attested software module
> called 'the TDX module' runs inside a new isolated memory range as a
> trusted hypervisor to manage and run protected VMs.
> 
> Pre-TDX Intel hardware has support for a memory encryption architecture
> called MKTME.  The memory encryption hardware underpinning MKTME is also
> used for Intel TDX.  TDX ends up "stealing" some of the physical address
> space from the MKTME architecture for crypto-protection to VMs.  The
> BIOS is responsible for partitioning the "KeyID" space between legacy
> MKTME and TDX.  The KeyIDs reserved for TDX are called 'TDX private
> KeyIDs' or 'TDX KeyIDs' for short.
> 
> TDX doesn't trust the BIOS.  During machine boot, TDX verifies the TDX
> private KeyIDs are consistently and correctly programmed by the BIOS
> across all CPU packages before it enables TDX on any CPU core.  A valid
> TDX private KeyID range on BSP indicates TDX has been enabled by the
> BIOS, otherwise the BIOS is buggy.
> 
> The TDX module is expected to be loaded by the BIOS when it enables TDX,
> but the kernel needs to properly initialize it before it can be used to
> create and run any TDX guests.  The TDX module will be initialized at
> runtime by the user (i.e. KVM) on demand.
> 
> Add a new early_initcall(tdx_init) to do TDX early boot initialization.
> Only detect TDX private KeyIDs for now.  Some other early checks will
> follow up.  Also add a new function to report whether TDX has been
> enabled by BIOS (TDX private KeyID range is valid).  Kexec() will also
> need it to determine whether need to flush dirty cachelines that are
> associated with any TDX private KeyIDs before booting to the new kernel.
> 
> To start to support TDX, create a new arch/x86/virt/vmx/tdx/tdx.c for
> TDX host kernel support.  Add a new Kconfig option CONFIG_INTEL_TDX_HOST
> to opt-in TDX host kernel support (to distinguish with TDX guest kernel
> support).  So far only KVM is the only user of TDX.  Make the new config
> option depend on KVM_INTEL.
> 
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
> ---
> 
> v6 -> v7:
>  - No change.
> 
> v5 -> v6:
>  - Removed SEAMRR detection to make code simpler.
>  - Removed the 'default N' in the KVM_TDX_HOST Kconfig (Kirill).
>  - Changed to use 'obj-y' in arch/x86/virt/vmx/tdx/Makefile (Kirill).
> 
> 
> ---
>  arch/x86/Kconfig               | 12 +++++
>  arch/x86/Makefile              |  2 +
>  arch/x86/include/asm/tdx.h     |  7 +++
>  arch/x86/virt/Makefile         |  2 +
>  arch/x86/virt/vmx/Makefile     |  2 +
>  arch/x86/virt/vmx/tdx/Makefile |  2 +
>  arch/x86/virt/vmx/tdx/tdx.c    | 95 ++++++++++++++++++++++++++++++++++
>  arch/x86/virt/vmx/tdx/tdx.h    | 15 ++++++
>  8 files changed, 137 insertions(+)
>  create mode 100644 arch/x86/virt/Makefile
>  create mode 100644 arch/x86/virt/vmx/Makefile
>  create mode 100644 arch/x86/virt/vmx/tdx/Makefile
>  create mode 100644 arch/x86/virt/vmx/tdx/tdx.c
>  create mode 100644 arch/x86/virt/vmx/tdx/tdx.h
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 67745ceab0db..cced4ef3bfb2 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1953,6 +1953,18 @@ config X86_SGX
>  
>  	  If unsure, say N.
>  
> +config INTEL_TDX_HOST
> +	bool "Intel Trust Domain Extensions (TDX) host support"
> +	depends on CPU_SUP_INTEL
> +	depends on X86_64
> +	depends on KVM_INTEL
> +	help
> +	  Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
> +	  host and certain physical attacks.  This option enables necessary TDX
> +	  support in host kernel to run protected VMs.
> +
> +	  If unsure, say N.
> +
>  config EFI
>  	bool "EFI runtime service support"
>  	depends on ACPI
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 415a5d138de4..38d3e8addc5f 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -246,6 +246,8 @@ archheaders:
>  
>  libs-y  += arch/x86/lib/
>  
> +core-y += arch/x86/virt/
> +
>  # drivers-y are linked after core-y
>  drivers-$(CONFIG_MATH_EMULATION) += arch/x86/math-emu/
>  drivers-$(CONFIG_PCI)            += arch/x86/pci/
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index e9a3f4a6fba1..51c4222a13ae 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -98,5 +98,12 @@ static inline long tdx_kvm_hypercall(unsigned int nr, unsigned long p1,
>  	return -ENODEV;
>  }
>  #endif /* CONFIG_INTEL_TDX_GUEST && CONFIG_KVM_GUEST */
> +
> +#ifdef CONFIG_INTEL_TDX_HOST
> +bool platform_tdx_enabled(void);
> +#else	/* !CONFIG_INTEL_TDX_HOST */
> +static inline bool platform_tdx_enabled(void) { return false; }
> +#endif	/* CONFIG_INTEL_TDX_HOST */
> +
>  #endif /* !__ASSEMBLY__ */
>  #endif /* _ASM_X86_TDX_H */
> diff --git a/arch/x86/virt/Makefile b/arch/x86/virt/Makefile
> new file mode 100644
> index 000000000000..1e36502cd738
> --- /dev/null
> +++ b/arch/x86/virt/Makefile
> @@ -0,0 +1,2 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +obj-y	+= vmx/
> diff --git a/arch/x86/virt/vmx/Makefile b/arch/x86/virt/vmx/Makefile
> new file mode 100644
> index 000000000000..feebda21d793
> --- /dev/null
> +++ b/arch/x86/virt/vmx/Makefile
> @@ -0,0 +1,2 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +obj-$(CONFIG_INTEL_TDX_HOST)	+= tdx/
> diff --git a/arch/x86/virt/vmx/tdx/Makefile b/arch/x86/virt/vmx/tdx/Makefile
> new file mode 100644
> index 000000000000..93ca8b73e1f1
> --- /dev/null
> +++ b/arch/x86/virt/vmx/tdx/Makefile
> @@ -0,0 +1,2 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +obj-y += tdx.o
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> new file mode 100644
> index 000000000000..982d9c453b6b
> --- /dev/null
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -0,0 +1,95 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright(c) 2022 Intel Corporation.
> + *
> + * Intel Trusted Domain Extensions (TDX) support
> + */
> +
> +#define pr_fmt(fmt)	"tdx: " fmt
> +
> +#include <linux/types.h>
> +#include <linux/init.h>
> +#include <linux/printk.h>
> +#include <asm/msr-index.h>
> +#include <asm/msr.h>
> +#include <asm/tdx.h>
> +#include "tdx.h"
> +
> +static u32 tdx_keyid_start __ro_after_init;
> +static u32 tdx_keyid_num __ro_after_init;
> +
> +/*
> + * Detect TDX private KeyIDs to see whether TDX has been enabled by the
> + * BIOS.  Both initializing the TDX module and running TDX guest require
> + * TDX private KeyID.
> + *
> + * TDX doesn't trust BIOS.  TDX verifies all configurations from BIOS
> + * are correct before enabling TDX on any core.  TDX requires the BIOS
> + * to correctly and consistently program TDX private KeyIDs on all CPU
> + * packages.  Unless there is a BIOS bug, detecting a valid TDX private
> + * KeyID range on BSP indicates TDX has been enabled by the BIOS.  If
> + * there's such BIOS bug, it will be caught later when initializing the
> + * TDX module.
> + */
> +static int __init detect_tdx(void)
> +{
> +	int ret;
> +
> +	/*
> +	 * IA32_MKTME_KEYID_PARTIONING:
> +	 *   Bit [31:0]:	Number of MKTME KeyIDs.
> +	 *   Bit [63:32]:	Number of TDX private KeyIDs.
> +	 */
> +	ret = rdmsr_safe(MSR_IA32_MKTME_KEYID_PARTITIONING, &tdx_keyid_start,
> +			&tdx_keyid_num);
> +	if (ret)
> +		return -ENODEV;
> +
> +	if (!tdx_keyid_num)
> +		return -ENODEV;
> +
> +	/*
> +	 * KeyID 0 is for TME.  MKTME KeyIDs start from 1.  TDX private
> +	 * KeyIDs start after the last MKTME KeyID.
> +	 */
> +	tdx_keyid_start++;

It is not very clear why you increment tdx_keyid_start. What we read from
MSR_IA32_MKTME_KEYID_PARTITIONING is not the correct start keyid?

Also why is this global variable? At least in this patch, there seems to
be no use case.

> +
> +	pr_info("TDX enabled by BIOS. TDX private KeyID range: [%u, %u)\n",
> +			tdx_keyid_start, tdx_keyid_start + tdx_keyid_num);
> +
> +	return 0;
> +}
> +
> +static void __init clear_tdx(void)
> +{
> +	tdx_keyid_start = tdx_keyid_num = 0;
> +}
> +
> +static int __init tdx_init(void)
> +{
> +	if (detect_tdx())
> +		return -ENODEV;
> +
> +	/*
> +	 * Initializing the TDX module requires one TDX private KeyID.
> +	 * If there's only one TDX KeyID then after module initialization
> +	 * KVM won't be able to run any TDX guest, which makes the whole
> +	 * thing worthless.  Just disable TDX in this case.
> +	 */
> +	if (tdx_keyid_num < 2) {
> +		pr_info("Disable TDX as there's only one TDX private KeyID available.\n");
> +		goto no_tdx;
> +	}
> +
> +	return 0;
> +no_tdx:
> +	clear_tdx();
> +	return -ENODEV;
> +}
> +early_initcall(tdx_init);
> +
> +/* Return whether the BIOS has enabled TDX */
> +bool platform_tdx_enabled(void)
> +{
> +	return !!tdx_keyid_num;
> +}
> diff --git a/arch/x86/virt/vmx/tdx/tdx.h b/arch/x86/virt/vmx/tdx/tdx.h
> new file mode 100644
> index 000000000000..d00074abcb20
> --- /dev/null
> +++ b/arch/x86/virt/vmx/tdx/tdx.h
> @@ -0,0 +1,15 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _X86_VIRT_TDX_H
> +#define _X86_VIRT_TDX_H
> +
> +/*
> + * This file contains both macros and data structures defined by the TDX
> + * architecture and Linux defined software data structures and functions.
> + * The two should not be mixed together for better readability.  The
> + * architectural definitions come first.
> + */
> +
> +/* MSR to report KeyID partitioning between MKTME and TDX */
> +#define MSR_IA32_MKTME_KEYID_PARTITIONING	0x00000087
> +
> +#endif

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer