[PATCH v9 1/6] x86/tdx: Add TDX Guest attestation interface driver

Kuppuswamy Sathyanarayanan
2022-07-28T03:44:39Z

In TDX guest, attestation is used to verify the trustworthiness of a TD
to other entities before provisioning secrets to the TD.

During the TD launch, the initial contents and configuration of the TD
are recorded by the Intel TDX module in build time measurement register
(MRTD). It is a SHA384 digest created using data from TD private pages(
including TD firmware) and the configuration of the TD.

After TD build, run-time measurement registers (RTMRs) can be used by
the guest TD software to extend the TD measurements. TDX supports 4
RTMR registers, and TDG.MR.RTMR.EXTEND TDCALL is used to update the
RTMR registers securely. RTMRs are mainly used to record measurements
related to sections like the kernel image, command line parameters,
initrd, ACPI tables, firmware data, configuration firmware volume (CFV)
of TDVF, etc. For more details, please refer to TDX Virtual Firmware
design specification, sec titled "TD Measurement".

At TD runtime, the Intel TDX module reuses the Intel SGX attestation
infrastructure to provide support for attesting to these measurements
as described below.

The attestation process consists of two steps: TDREPORT generation and
Quote generation.

TDREPORT (TDREPORT_STRUCT) is a fixed-size data structure generated by
the TDX module which contains TD-specific information (such as TD
measurements), platform security version, and the MAC to protect the
integrity of the TDREPORT. The TD kernel uses TDCALL[TDG.MR.REPORT] to
get the TDREPORT from the TDX module. A user-provided 64-Byte
REPORTDATA is used as input and included in the TDREPORT. Typically it
can be some nonce provided by attestation service so the TDREPORT can
be verified uniquely. More details about TDREPORT can be found in
Intel TDX Module specification, section titled "TDG.MR.REPORT Leaf".

TDREPORT can only be verified on local platform as the MAC key is bound
to the platform. To support remote verification of the TDREPORT, TDX
leverages Intel SGX Quote Enclave (QE) to verify the TDREPORT locally
and convert it to a remote verifiable Quote.

After getting the TDREPORT, the second step of the attestation process
is to send it to the QE to generate the Quote. TDX doesn't support SGX
inside the TD, so the QE can be deployed in the host, or in another
legacy VM with SGX support. How to send the TDREPORT to QE and receive
the Quote is implementation and deployment specific.

Implement a basic TD guest misc driver to allow TD userspace to get the
TDREPORT. The TD userspace attestation software can get the TDREPORT
and then choose whatever communication channel available (i.e. vsock
or hypercall) to send the TDREPORT to QE and receive the Quote.

Also note that explicit access permissions are not enforced in this
driver because the quote and measurements are not a secret. However
the access permissions of the device node can be used to set any
desired access policy. The udev default is usually root access
only.

Operations like getting TDREPORT or Quote generation involves sending
a blob of data as input and getting another blob of data as output. It
was considered to use a sysfs interface for this, but it doesn't fit
well into the standard sysfs model for configuring values. It would be
possible to do read/write on files, but it would need multiple file
descriptors, which would be somewhat messy. IOCTLs seems to be the best
fitting and simplest model for this use case. This is similar to AMD
SEV platform, which also uses IOCTL interface to support attestation.

Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Acked-by: Wander Lairson Costa <wander@redhat.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
---
arch/x86/coco/tdx/Makefile | 2 +-
arch/x86/coco/tdx/attest.c | 81 +++++++++++++++++++++++++++++++++
arch/x86/coco/tdx/tdx.c | 55 ++++++++++++++++++++++
arch/x86/coco/tdx/tdx.h | 9 ++++
arch/x86/include/uapi/asm/tdx.h | 51 +++++++++++++++++++++
5 files changed, 197 insertions(+), 1 deletion(-)
create mode 100644 arch/x86/coco/tdx/attest.c
create mode 100644 arch/x86/coco/tdx/tdx.h
create mode 100644 arch/x86/include/uapi/asm/tdx.h

diff --git a/arch/x86/coco/tdx/Makefile b/arch/x86/coco/tdx/Makefile
index 46c55998557d..d2db3e6770e5 100644
--- a/arch/x86/coco/tdx/Makefile
+++ b/arch/x86/coco/tdx/Makefile
@@ -1,3 +1,3 @@
# SPDX-License-Identifier: GPL-2.0

-obj-y += tdx.o tdcall.o
+obj-y += tdx.o tdcall.o attest.o
diff --git a/arch/x86/coco/tdx/attest.c b/arch/x86/coco/tdx/attest.c
new file mode 100644
index 000000000000..46a2f3612753
--- /dev/null
+++ b/arch/x86/coco/tdx/attest.c
@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * attest.c - TDX attestation feature support.
+ *
+ * Implements attestation related IOCTL handlers.
+ *
+ * Copyright (C) 2022 Intel Corporation
+ *
+ */
+
+#include <linux/mm.h>
+#include <linux/io.h>
+#include <asm/tdx.h>
+
+#include "tdx.h"
+
+/* TDREPORT module call leaf ID */
+#define TDX_GET_REPORT 4
+
+long tdx_get_report(void __user *argp)
+{
+ u8 *reportdata = NULL, *tdreport = NULL;
+ struct tdx_report_req req;
+ long ret;
+
+ /* Copy request struct from the user buffer */
+ if (copy_from_user(&req, argp, sizeof(req)))
+ return -EFAULT;
+
+ /*
+ * Per TDX Module 1.0 specification, section titled
+ * "TDG.MR.REPORT", REPORTDATA and TDREPORT length
+ * is fixed as TDX_REPORTDATA_LEN and TDX_REPORT_LEN.
+ */
+ if (req.rpd_len != TDX_REPORTDATA_LEN || req.tdr_len != TDX_REPORT_LEN)
+ return -EINVAL;
+
+ /* Allocate kernel buffers for REPORTDATA and TDREPORT */
+ reportdata = kzalloc(req.rpd_len, GFP_KERNEL);
+ if (!reportdata) {
+ ret = -ENOMEM;
+ goto report_failed;
+ }
+
+ tdreport = kzalloc(req.tdr_len, GFP_KERNEL);
+ if (!tdreport) {
+ ret = -ENOMEM;
+ goto report_failed;
+ }
+
+
+ /* Copy REPORTDATA from user to kernel buffer */
+ if (copy_from_user(reportdata, (void *)req.reportdata, req.rpd_len)) {
+ ret = -EFAULT;
+ goto report_failed;
+ }
+
+ /*
+ * Generate TDREPORT using "TDG.MR.REPORT" TDCALL.
+ *
+ * Get the TDREPORT using REPORTDATA as input. Refer to
+ * section 22.3.3 TDG.MR.REPORT leaf in the TDX Module 1.0
+ * Specification for detailed information.
+ */
+ ret = __tdx_module_call(TDX_GET_REPORT, virt_to_phys(tdreport),
+ virt_to_phys(reportdata), req.subtype,
+ 0, NULL);
+ if (ret) {
+ ret = -EIO;
+ goto report_failed;
+ }
+
+ /* Copy TDREPORT data back to the user buffer */
+ if (copy_to_user((void *)req.tdreport, tdreport, req.tdr_len))
+ ret = -EFAULT;
+
+report_failed:
+ kfree(reportdata);
+ kfree(tdreport);
+ return ret;
+}
diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 928dcf7a20d9..205f98f42da8 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -5,6 +5,9 @@
#define pr_fmt(fmt) "tdx: " fmt

#include <linux/cpufeature.h>
+#include <linux/miscdevice.h>
+#include <linux/mm.h>
+#include <linux/io.h>
#include <asm/coco.h>
#include <asm/tdx.h>
#include <asm/vmx.h>
@@ -12,6 +15,8 @@
#include <asm/insn-eval.h>
#include <asm/pgtable.h>

+#include "tdx.h"
+
/* TDX module Call Leaf IDs */
#define TDX_GET_INFO 1
#define TDX_GET_VEINFO 3
@@ -34,6 +39,10 @@
#define VE_GET_PORT_NUM(e) ((e) >> 16)
#define VE_IS_IO_STRING(e) ((e) & BIT(4))

+#define DRIVER_NAME "tdx-guest"
+
+static struct miscdevice tdx_misc_dev;
+
/*
* Wrapper for standard use of __tdx_hypercall with no output aside from
* return code.
@@ -775,3 +784,49 @@ void __init tdx_early_init(void)

pr_info("Guest detected\n");
}
+
+static long tdx_guest_ioctl(struct file *file, unsigned int cmd,
+ unsigned long arg)
+{
+ void __user *argp = (void __user *)arg;
+ long ret = -EINVAL;
+
+ switch (cmd) {
+ case TDX_CMD_GET_REPORT:
+ ret = tdx_get_report(argp);
+ break;
+ default:
+ pr_debug("cmd %d not supported\n", cmd);
+ break;
+ }
+
+ return ret;
+}
+
+static const struct file_operations tdx_guest_fops = {
+ .owner = THIS_MODULE,
+ .unlocked_ioctl = tdx_guest_ioctl,
+ .llseek = no_llseek,
+};
+
+static int __init tdx_guest_init(void)
+{
+ int ret;
+
+ /* Make sure we are in a valid TDX platform */
+ if (!cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
+ return -EIO;
+
+ tdx_misc_dev.name = DRIVER_NAME;
+ tdx_misc_dev.minor = MISC_DYNAMIC_MINOR;
+ tdx_misc_dev.fops = &tdx_guest_fops;
+
+ ret = misc_register(&tdx_misc_dev);
+ if (ret) {
+ pr_err("misc device registration failed\n");
+ return ret;
+ }
+
+ return 0;
+}
+device_initcall(tdx_guest_init)
diff --git a/arch/x86/coco/tdx/tdx.h b/arch/x86/coco/tdx/tdx.h
new file mode 100644
index 000000000000..67231d6d1027
--- /dev/null
+++ b/arch/x86/coco/tdx/tdx.h
@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __X86_COCO_TDX_H__
+#define __X86_COCO_TDX_H__
+
+#include <uapi/asm/tdx.h>
+
+long tdx_get_report(void __user *argp);
+
+#endif /* __X86_COCO_TDX_H__ */
diff --git a/arch/x86/include/uapi/asm/tdx.h b/arch/x86/include/uapi/asm/tdx.h
new file mode 100644
index 000000000000..20db8081772b
--- /dev/null
+++ b/arch/x86/include/uapi/asm/tdx.h
@@ -0,0 +1,51 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_ASM_X86_TDX_H
+#define _UAPI_ASM_X86_TDX_H
+
+#include <linux/types.h>
+#include <linux/ioctl.h>
+
+/* Length of the REPORTDATA used in TDG.MR.REPORT TDCALL */
+#define TDX_REPORTDATA_LEN 64
+
+/* Length of TDREPORT used in TDG.MR.REPORT TDCALL */
+#define TDX_REPORT_LEN 1024
+
+/**
+ * struct tdx_report_req: Get TDREPORT using REPORTDATA as input.
+ *
+ * @subtype : Subtype of TDREPORT (fixed as 0 by TDX Module
+ * specification, but added a parameter to handle
+ * future extension).
+ * @reportdata : User-defined REPORTDATA to be included into
+ * TDREPORT. Typically it can be some nonce
+ * provided by attestation service, so the
+ * generated TDREPORT can be uniquely verified.
+ * @rpd_len : Length of the REPORTDATA (fixed as 64 bytes by
+ * the TDX Module specification, but parameter is
+ * added to handle future extension).
+ * @tdreport : TDREPORT output from TDCALL[TDG.MR.REPORT].
+ * @tdr_len : Length of the TDREPORT (fixed as 1024 bytes by
+ * the TDX Module specification, but a parameter
+ * is added to accommodate future extension).
+ *
+ * Used in TDX_CMD_GET_REPORT IOCTL request.
+ */
+struct tdx_report_req {
+ __u8 subtype;
+ __u64 reportdata;
+ __u32 rpd_len;
+ __u64 tdreport;
+ __u32 tdr_len;
+};
+
+/*
+ * TDX_CMD_GET_REPORT - Get TDREPORT using TDCALL[TDG.MR.REPORT]
+ *
+ * Return 0 on success, -EIO on TDCALL execution failure, and
+ * standard errno on other general error cases.
+ *
+ */
+#define TDX_CMD_GET_REPORT _IOWR('T', 0x01, __u64)
+
+#endif /* _UAPI_ASM_X86_TDX_H */
--
2.25.1