[PATCH v8 1/5] x86/tdx: Add TDX Guest attestation interface driver

Kuppuswamy Sathyanarayanan
2022-06-09T02:52:52Z

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

One usage example is, when a TD guest uses encrypted drive and if the
decryption keys required to access the drive are stored in a secure 3rd
party keyserver, the key server can use attestation to verify TD's
trustworthiness and release the decryption keys to the TD.

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 attestation 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)
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. Also, the REPORTDATA used
in TDREPORT generation can possibly come from attestation service to
uniquely verify the Quote (like per instance verification). In such
case, since REPORTDATA is a secret, using sysfs to share it is insecure
compared to sending it via IOCTL.

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 | 118 ++++++++++++++++++++++++++++++++
arch/x86/include/uapi/asm/tdx.h | 42 ++++++++++++
3 files changed, 161 insertions(+), 1 deletion(-)
create mode 100644 arch/x86/coco/tdx/attest.c
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..24db0bad4923
--- /dev/null
+++ b/arch/x86/coco/tdx/attest.c
@@ -0,0 +1,118 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * attest.c - TDX guest attestation interface driver.
+ *
+ * Implements user interface to trigger attestation process.
+ *
+ * Copyright (C) 2022 Intel Corporation
+ *
+ */
+
+#define pr_fmt(fmt) "x86/tdx: attest: " fmt
+
+#include <linux/miscdevice.h>
+#include <linux/mm.h>
+#include <linux/io.h>
+#include <asm/tdx.h>
+#include <uapi/asm/tdx.h>
+
+#define DRIVER_NAME "tdx-attest"
+
+/* TDREPORT module call leaf ID */
+#define TDX_GET_REPORT 4
+
+static struct miscdevice miscdev;
+
+static long tdx_get_report(void __user *argp)
+{
+ void *reportdata = NULL, *tdreport = NULL;
+ long ret;
+
+ /* Allocate buffer space for REPORTDATA */
+ reportdata = kmalloc(TDX_REPORTDATA_LEN, GFP_KERNEL);
+ if (!reportdata)
+ return -ENOMEM;
+
+ /* Allocate buffer space for TDREPORT */
+ tdreport = kmalloc(TDX_REPORT_LEN, GFP_KERNEL);
+ if (!tdreport) {
+ ret = -ENOMEM;
+ goto out;
+ }
+
+ /* Copy REPORTDATA from the user buffer */
+ if (copy_from_user(reportdata, argp, TDX_REPORTDATA_LEN)) {
+ ret = -EFAULT;
+ goto out;
+ }
+
+ /*
+ * 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), 0, 0, NULL);
+ if (ret) {
+ pr_debug("TDREPORT TDCALL failed, status:%lx\n", ret);
+ ret = -EIO;
+ goto out;
+ }
+
+ /* Copy TDREPORT back to the user buffer */
+ if (copy_to_user(argp, tdreport, TDX_REPORT_LEN))
+ ret = -EFAULT;
+
+out:
+ kfree(reportdata);
+ kfree(tdreport);
+ return ret;
+}
+
+static long tdx_attest_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_attest_fops = {
+ .owner = THIS_MODULE,
+ .unlocked_ioctl = tdx_attest_ioctl,
+ .llseek = no_llseek,
+};
+
+static int __init tdx_attestation_init(void)
+{
+ int ret;
+
+ /* Make sure we are in a valid TDX platform */
+ if (!cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
+ return -EIO;
+
+ miscdev.name = DRIVER_NAME;
+ miscdev.minor = MISC_DYNAMIC_MINOR;
+ miscdev.fops = &tdx_attest_fops;
+
+ ret = misc_register(&miscdev);
+ if (ret) {
+ pr_err("misc device registration failed\n");
+ return ret;
+ }
+
+ return 0;
+}
+device_initcall(tdx_attestation_init)
diff --git a/arch/x86/include/uapi/asm/tdx.h b/arch/x86/include/uapi/asm/tdx.h
new file mode 100644
index 000000000000..8b57dea67eab
--- /dev/null
+++ b/arch/x86/include/uapi/asm/tdx.h
@@ -0,0 +1,42 @@
+/* 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.
+ *
+ * @reportdata : User-defined 64-Byte REPORTDATA to be included into
+ * TDREPORT. Typically it can be some nonce provided by
+ * attestation service, so the generated TDREPORT can be
+ * uniquely verified.
+ * @tdreport : TDREPORT output from TDCALL[TDG.MR.REPORT] of size
+ * TDX_REPORT_LEN.
+ *
+ * Used in TDX_CMD_GET_REPORT IOCTL request.
+ */
+struct tdx_report_req {
+ union {
+ __u8 reportdata[TDX_REPORTDATA_LEN];
+ __u8 tdreport[TDX_REPORT_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, struct tdx_report_req)
+
+#endif /* _UAPI_ASM_X86_TDX_H */
--
2.25.1