Re: [PATCH v11 02/20] x86/virt/tdx: Detect TDX during kernel boot

[David Hildenbrand <david@redhat.com>]

On 04.06.23 16:27, 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 by
> the KVM subsystem when KVM wants to use TDX.
> 
> Add a new early_initcall(tdx_init) to detect the TDX by detecting TDX
> private KeyIDs.  Also add a function to report whether TDX is enabled by
> the BIOS.  Similar to AMD SME, kexec() will use it to determine whether
> cache flush is needed.
> 
> The TDX module itself requires one TDX KeyID as the 'TDX global KeyID'
> to protect its metadata.  Each TDX guest also needs a TDX KeyID for its
> own protection.  Just use the first TDX KeyID as the global KeyID and
> leave the rest for TDX guests.  If no TDX KeyID is left for TDX guests,
> disable TDX as initializing the TDX module alone is useless.
> 
> 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 uses TDX.  Make the new config option depend
> on KVM_INTEL.
> 
> Signed-off-by: Kai Huang <kai.huang@intel.com>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> 
> v10 -> v11 (David):
>   - "host kernel" -> "the host kernel"
>   - "protected VM" -> "confidential VM".
>   - Moved setting tdx_global_keyid to the end of tdx_init().
> 
> v9 -> v10:
>   - No change.
> 
> v8 -> v9:
>   - Moved MSR macro from local tdx.h to <asm/msr-index.h> (Dave).
>   - Moved reserving the TDX global KeyID from later patch to here.
>   - Changed 'tdx_keyid_start' and 'nr_tdx_keyids' to
>     'tdx_guest_keyid_start' and 'tdx_nr_guest_keyids' to represent KeyIDs
>     can be used by guest. (Dave)
>   - Slight changelog update according to above changes.
> 
> v7 -> v8: (address Dave's comments)
>   - Improved changelog:
>      - "KVM user" -> "The TDX module will be initialized by KVM when ..."
>      - Changed "tdx_int" part to "Just say what this patch is doing"
>      - Fixed the last sentence of "kexec()" paragraph
>    - detect_tdx() -> record_keyid_partitioning()
>    - Improved how to calculate tdx_keyid_start.
>    - tdx_keyid_num -> nr_tdx_keyids.
>    - Improved dmesg printing.
>    - Add comment to clear_tdx().
> 
> 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/msr-index.h |  3 ++
>   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      | 92 ++++++++++++++++++++++++++++++++
>   8 files changed, 122 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
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 53bab123a8ee..191587f75810 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1952,6 +1952,18 @@ 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 the host kernel to run confidential 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 b39975977c03..ec0e71d8fa30 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -252,6 +252,8 @@ 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/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 3aedae61af4f..6d8f15b1552c 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -523,6 +523,9 @@
>   #define MSR_RELOAD_PMC0			0x000014c1
>   #define MSR_RELOAD_FIXED_CTR0		0x00001309
>   
> +/* KeyID partitioning between MKTME and TDX */
> +#define MSR_IA32_MKTME_KEYID_PARTITIONING	0x00000087
> +
>   /*
>    * AMD64 MSRs. Not complete. See the architecture manual for a more
>    * complete list.
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index 25fd6070dc0b..4dfe2e794411 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -94,5 +94,12 @@ 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..2d91e7120c90
> --- /dev/null
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -0,0 +1,92 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright(c) 2023 Intel Corporation.
> + *
> + * Intel Trusted Domain Extensions (TDX) support
> + */
> +
> +#define pr_fmt(fmt)	"tdx: " fmt
> +
> +#include <linux/types.h>
> +#include <linux/cache.h>
> +#include <linux/init.h>
> +#include <linux/errno.h>
> +#include <linux/printk.h>
> +#include <asm/msr-index.h>
> +#include <asm/msr.h>
> +#include <asm/tdx.h>
> +
> +static u32 tdx_global_keyid __ro_after_init;
> +static u32 tdx_guest_keyid_start __ro_after_init;
> +static u32 tdx_nr_guest_keyids __ro_after_init;
> +
> +static int __init record_keyid_partitioning(u32 *tdx_keyid_start,
> +					    u32 *nr_tdx_keyids)
> +{
> +	u32 _nr_mktme_keyids, _tdx_keyid_start, _nr_tdx_keyids;
> +	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, &_nr_mktme_keyids,
> +			&_nr_tdx_keyids);
> +	if (ret)
> +		return -ENODEV;
> +
> +	if (!_nr_tdx_keyids)
> +		return -ENODEV;
> +
> +	/* TDX KeyIDs start after the last MKTME KeyID. */
> +	_tdx_keyid_start = _nr_mktme_keyids + 1;
> +
> +	*tdx_keyid_start = _tdx_keyid_start;
> +	*nr_tdx_keyids = _nr_tdx_keyids;
> +
> +	return 0;
> +}
> +
> +static int __init tdx_init(void)
> +{
> +	u32 tdx_keyid_start, nr_tdx_keyids;
> +	int err;
> +
> +	err = record_keyid_partitioning(&tdx_keyid_start, &nr_tdx_keyids);
> +	if (err)
> +		return err;
> +
> +	pr_info("BIOS enabled: private KeyID range [%u, %u)\n",
> +			tdx_keyid_start, tdx_keyid_start + nr_tdx_keyids);
> +
> +	/*
> +	 * The TDX module itself requires one 'global KeyID' to protect
> +	 * its metadata.  If there's only one TDX KeyID, there won't be
> +	 * any left for TDX guests thus there's no point to enable TDX
> +	 * at all.
> +	 */
> +	if (nr_tdx_keyids < 2) {
> +		pr_info("initialization failed: too few private KeyIDs available.\n");
> +		goto no_tdx;
> +	}
> +
> +	/*
> +	 * Just use the first TDX KeyID as the 'global KeyID' and
> +	 * leave the rest for TDX guests.
> +	 */
> +	tdx_global_keyid = tdx_keyid_start;
> +	tdx_guest_keyid_start = ++tdx_keyid_start;
> +	tdx_nr_guest_keyids = --nr_tdx_keyids;

tdx_guest_keyid_start = tdx_keyid_start + 1;
tdx_nr_guest_keyids = nr_tdx_keyids - 1;

Easier to get, because the modified values are unused.

I'd probably avoid the "tdx" terminology in the local variables 
("keid_start", "nr_keyids") to give a better hint what the global 
variables are (tdx_*), but just a personal preference.



Apart from that,

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb