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

David Hildenbrand <david@xxxxxxxxxx> · Mon, 19 Jun 2023 14:12:18 +0200

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@xxxxxxxxx>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---

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@xxxxxxxxxx>

--
Cheers,

David / dhildenb







