Re: [PATCH v4 1/3] x86/tdx: Add TDX Guest attestation interface driver

Kai Huang
2022-04-25T05:44:47Z

On Fri, 2022-04-22 at 16:34 -0700, Kuppuswamy Sathyanarayanan wrote:
> In TDX guest, attestation is used to verify the trustworthiness of a TD
> to other entities before making any secure communication.

Before provisioning secrets to the TD.

>
> One usage example is, when a TD guest uses encrypted drive and the
> decryption keys required to access the drive are stored in a secure
> 3rd party keyserver, TD guest can use the quote generated via the
> attestation process to prove its trustworthiness with keyserver and
> get access to the storage keys.

"The key server can use attestation to verify TD's trustworthiness and release
the decryption keys to the TD".

It is the key server who starts the attestation request, but not the TD.

>
> General steps involved in attestation process are,
>
>   1. TD guest generates the TDREPORT that contains version information
>      about the Intel TDX module, measurement of the TD, along with a
>      TD-specified nonce.
>   2. TD guest shares the TDREPORT with TD host via GetQuote hypercall
>      which is used by the host to generate a quote via quoting
>      enclave (QE).
>   3. Quote generation completion notification is sent to TD OS via
>      callback interrupt vector configured by TD using
>      SetupEventNotifyInterrupt hypercall.
>   4. After receiving the generated TDQUOTE, a remote verifier can be
>      used to verify the quote and confirm the trustworthiness of the
>      TD.

Let's separate TDREPORT generation and Quote generation, and get rid of details
of how to get Quote part for now. Detail of GetQuote belongs to the patch which
implements it. Here I think we should focus on explaining why we need such a
basic driver to allow userspace to get the TDREPORT.

Below is for your reference:

"
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 to contain the TD-specific informatin (such as TD
measurements), platform information such as the security version of the platform
and the TDX module and the MAC to protect the integrity of the TDREPORT. 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.

TDREPORT can only be verified locally as the MAC key is bound to the platform.
TDX attestation leverages Intel SGX Quote Enclave (QE) to verify the TDREPORT
locally and convert it to a remote verifiable Quote to support remote
attestation of the TDREPORT. After getting the TDREPORT, the second step of
attestation process is to send the TDREPORT to QE to generate the Quote.

How is the QE, or Quote Generation Service (QGS) in general, implemented and
deployed is implementation specific. As a result, how to send the TDREPORT to
QE/QGS also depends on QE/QGS implementation and the deployment. TDX doesn't
support SGX inside a TD, so the QE/QGS can be deployed in the host, or inside a
dedicated legacy VM.

A typical implementation is TD userspace attestation software gets the TDREPORT
from TD kernel, sends it to QE/QGS, and QE/QGS returns the Quote. The data and
data format that TD userspace attestation software sends to the QE/QGS is also
implementation specific, but not necessarily just the raw TDREPORT. TD
attestation software can use any available communication channel to talk to
QE/QGS, such as using vsock and tcp/ip.

To support the case that those communication channels are not directly available
to the TD, TDX also defines TDVMCALLs to allow TD to use TDVMCALL to ask VMM to
help with sending the TDREPORT and receiving the Quote. This support is
documented in the GHCI spec "5.4 TD attestation".

Implement a basic attestation driver to allow TD userspace to get the TDREPORT
so the attestation software can send it to the QE to generate a Quote for remote
verification.
"

>
> More details on above mentioned steps can be found in TDX Guest-Host
> Communication Interface (GHCI) for Intel TDX 1.0, section titled
> "TD attestation".
>
> To allow the attestation agent (user application) to implement this
> feature, add an IOCTL interface to get TDREPORT and TDQUOTE from the
> user space. Since attestation agent can also use methods like vosck or
> TCP/IP to get the TDQUOTE, adding an IOCTL interface for it is an
> optional feature. So to simplify the driver, first add support for
> TDX_CMD_GET_TDREPORT IOCTL. Support for TDQUOTE IOCTL will be added by
> follow-on patches.
>
> TDREPORT can be generated by sending a TDCALL with leaf ID as 0x04.
> More details about the TDREPORT TDCALL can be found in Intel Trust
> Domain Extensions (Intel TDX) Module specification, section titled
> "TDG.MR.REPORT Leaf". Add a wrapper function (tdx_mcall_tdreport())
> to get the TDREPORT from the TDX Module. This API will be used by the
> interface driver to request for TDREPORT.
>
> 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.
>
> 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>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> ---
> arch/x86/coco/tdx/Makefile | 2 +-
> arch/x86/coco/tdx/attest.c | 191 ++++++++++++++++++++++++++++++++
> arch/x86/coco/tdx/tdx.c | 45 ++++++++
> arch/x86/include/asm/tdx.h | 2 +
> arch/x86/include/uapi/asm/tdx.h | 23 ++++
> 5 files changed, 262 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..b776e81f6c20
> --- /dev/null
> +++ b/arch/x86/coco/tdx/attest.c
> @@ -0,0 +1,191 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * attest.c - TDX guest attestation interface driver.
> + *
> + * Implements user interface to trigger attestation process and
> + * read the TD Quote result.

Read TDREPORT. You can extend it in later patch if you want to call out
TDREPORT, Quote. But I don't think it's even necessary. Perhaps just say "TDX
attestation support" in general is enough.

> + *
> + * Copyright (C) 2022 Intel Corporation
> + *
> + */
> +
> +#define pr_fmt(fmt) "x86/tdx: attest: " fmt
> +
> +#include <linux/module.h>
> +#include <linux/miscdevice.h>
> +#include <linux/uaccess.h>
> +#include <linux/fs.h>
> +#include <linux/mm.h>
> +#include <linux/slab.h>
> +#include <linux/set_memory.h>
> +#include <linux/dma-mapping.h>
> +#include <linux/platform_device.h>
> +#include <linux/jiffies.h>
> +#include <linux/io.h>
> +#include <asm/apic.h>
> +#include <asm/tdx.h>
> +#include <asm/irq_vectors.h>
> +#include <uapi/asm/tdx.h>
> +
> +#define DRIVER_NAME "tdx-attest"
> +
> +static struct platform_device *pdev;
> +static struct miscdevice miscdev;
> +
> +static long tdx_get_tdreport(void __user *argp)
> +{
> + void *report_buf = NULL, *tdreport_buf = NULL;
> + long ret = 0, err;
> +
> + /* Allocate space for report data */
> + report_buf = kmalloc(TDX_REPORT_DATA_LEN, GFP_KERNEL);
> + if (!report_buf)
> + return -ENOMEM;
> +
> + /*
> + * Allocate space for TDREPORT buffer (1024-byte aligned).
> + * Full page alignment is more than enough.
> + */
> + tdreport_buf = (void *)get_zeroed_page(GFP_KERNEL);
> + if (!tdreport_buf) {
> + ret = -ENOMEM;
> + goto tdreport_failed;
> + }
> +
> + /* Copy report data to kernel buffer */
> + if (copy_from_user(report_buf, argp, TDX_REPORT_DATA_LEN)) {
> + ret = -EFAULT;
> + goto tdreport_failed;
> + }
> +
> + /* Generate TDREPORT using report data in report_buf */
> + err = tdx_mcall_tdreport(tdreport_buf, report_buf);
> + if (err) {
> + /* If failed, pass TDCALL error code back to user */
> + ret = put_user(err, (long __user *)argp);
> + ret = -EIO;
> + goto tdreport_failed;
> + }

If you want support this, I guess it's better to explicitly use some data
structure so userspace software can be very clear about what does this IOCTL do:

struct tdx_get_tdreport {
union {
/* Input: REPORTDATA for TDCALL[TDG.MR.REPORT] */
__u8 reportdata[TDX_REPORT_DATA_LEN];
/* Output when TDCALL[TDG.MR.REPORT] fails */
__u64 tdcall_err;
} buf;
};

And you need to explain in the comment saying -EIO means TDCALL failed, and the
error code is returned to userspace.

But I am not sure whether this is necessary. The spec says TDG.MR.REPORT can
return: TDX_OPERAND_BUSY, TDX_OPERAND_INVALID, TDX_SUCCESS.

TDX_OPERAND_INVALID basically happens when buffer alignment doesn't meet, GPA is
wrong, etc, so this case is kernel bug. I don't see there's a need to expose it
to userspace as userspace won't be able to do anything anyway.

The BUSY case is (if I understand correctly, I took a look at the SEAM module
code) basically there's another thread updating the TDMR (registers for runtime
measurement update). Since it seems current kernel doesn't support
TDG.MR.RTMR.EXTEND, TDX_OPERAND_BUSY should not happen either. Even considering
the support of TDG.MR.RTMR.EXTEND in the future, I think kernel should use some
mutex to serialize it with TDG.MR.REPORT?

The bottom line is even we want to report BUSY to userspace, we choose to
return -EBUSY to indicate this case. So basically I don't see the value of
exposing TDCALL error to userspace.

> +
> + /* Copy TDREPORT data back to user buffer */
> + if (copy_to_user(argp, tdreport_buf, TDX_TDREPORT_LEN))
> + ret = -EFAULT;
> +
> +tdreport_failed:
> + kfree(report_buf);
> + if (tdreport_buf)
> + free_pages((unsigned long)tdreport_buf, 0);
> +
> + return ret;
> +}
> +
> +static long tdx_attest_ioctl(struct file *file, unsigned int cmd,
> + unsigned long arg)
> +{
> + void __user *argp = (void __user *)arg;
> + long ret = 0;
> +
> + switch (cmd) {
> + case TDX_CMD_GET_TDREPORT:
> + ret = tdx_get_tdreport(argp);
> + break;
> + default:
> + pr_err("cmd %d not supported\n", cmd);
> + break;

Seems you are returning 0 in the default case.

> + }
> +
> + return ret;
> +}
> +
> +static const struct file_operations tdx_attest_fops = {
> + .owner = THIS_MODULE,
> + .unlocked_ioctl = tdx_attest_ioctl,
> + .llseek = no_llseek,
> +};
> +
> +static int tdx_attest_probe(struct platform_device *attest_pdev)
> +{
> + struct device *dev = &attest_pdev->dev;
> + long ret = 0;
> +
> + /* Only single device is allowed */
> + if (pdev)
> + return -EBUSY;
> +
> + pdev = attest_pdev;
> +
> + miscdev.name = DRIVER_NAME;
> + miscdev.minor = MISC_DYNAMIC_MINOR;
> + miscdev.fops = &tdx_attest_fops;
> + miscdev.parent = dev;
> +
> + ret = misc_register(&miscdev);
> + if (ret) {
> + pr_err("misc device registration failed\n");
> + goto failed;
> + }
> +
> + pr_debug("module initialization success\n");
> +
> + return 0;
> +
> +failed:
> + misc_deregister(&miscdev);
> +
> + pr_debug("module initialization failed\n");
> +
> + return ret;
> +}
> +
> +static int tdx_attest_remove(struct platform_device *attest_pdev)
> +{
> + misc_deregister(&miscdev);
> + pr_debug("module is successfully removed\n");
> + return 0;
> +}
> +
> +static struct platform_driver tdx_attest_driver = {
> + .probe = tdx_attest_probe,
> + .remove = tdx_attest_remove,
> + .driver = {
> + .name = DRIVER_NAME,
> + },
> +};
> +
> +static int __init tdx_attest_init(void)
> +{
> + int ret;
> +
> + /* Make sure we are in a valid TDX platform */
> + if (!cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
> + return -EIO;
> +
> + ret = platform_driver_register(&tdx_attest_driver);
> + if (ret) {
> + pr_err("failed to register driver, err=%d\n", ret);
> + return ret;
> + }
> +
> + pdev = platform_device_register_simple(DRIVER_NAME, -1, NULL, 0);
> + if (IS_ERR(pdev)) {
> + ret = PTR_ERR(pdev);
> + pr_err("failed to allocate device, err=%d\n", ret);
> + platform_driver_unregister(&tdx_attest_driver);
> + return ret;
> + }
> +
> + return 0;
> +}
> +
> +static void __exit tdx_attest_exit(void)
> +{
> + platform_device_unregister(pdev);
> + platform_driver_unregister(&tdx_attest_driver);
> +}
> +
> +module_init(tdx_attest_init);
> +module_exit(tdx_attest_exit);

Is there any particular reason to use platform driver and support it as a
module?

SGX driver uses misc_register() to register /dev/sgx_enclave during boot. Looks
it would be simpler.

> +
> +MODULE_AUTHOR("Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>");
> +MODULE_DESCRIPTION("TDX attestation driver");
> +MODULE_LICENSE("GPL");
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 03deb4d6920d..2a79ca92a52d 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -11,10 +11,12 @@
> #include <asm/insn.h>
> #include <asm/insn-eval.h>
> #include <asm/pgtable.h>
> +#include <asm/io.h>
>
> /* TDX module Call Leaf IDs */
> #define TDX_GET_INFO 1
> #define TDX_GET_VEINFO 3
> +#define TDX_GET_REPORT 4
> #define TDX_ACCEPT_PAGE 6
>
> /* TDX hypercall Leaf IDs */
> @@ -34,6 +36,10 @@
> #define VE_GET_PORT_NUM(e) ((e) >> 16)
> #define VE_IS_IO_STRING(e) ((e) & BIT(4))
>
> +/* TDX Module call error codes */
> +#define TDCALL_RETURN_CODE_MASK 0xffffffff00000000
> +#define TDCALL_RETURN_CODE(a) ((a) & TDCALL_RETURN_CODE_MASK)
> +
> /*
> * Wrapper for standard use of __tdx_hypercall with no output aside from
> * return code.
> @@ -98,6 +104,45 @@ static inline void tdx_module_call(u64 fn, u64 rcx, u64 rdx, u64 r8, u64 r9,
> panic("TDCALL %lld failed (Buggy TDX module!)\n", fn);
> }
>
> +/*
> + * tdx_mcall_tdreport() - Generate TDREPORT_STRUCT using TDCALL.
> + *
> + * @data : Address of 1024B aligned data to store
> + * TDREPORT_STRUCT.
> + * @reportdata : Address of 64B aligned report data
> + *
> + * return 0 on success or failure error number.
> + */
> +long tdx_mcall_tdreport(void *data, void *reportdata)

Change 'data' to be something more meaningful, i.e. tdreport?

> +{
> + u64 ret;
> +
> + /*
> + * Check for a valid TDX guest to ensure this API is only
> + * used by TDX guest platform. Also make sure "data" and
> + * "reportdata" pointers are valid.
> + */
> + if (!data || !reportdata || !cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
> + return -EINVAL;

Other TDCALL wrappers such as tdx_get_ve_info() doesn't have
X86_FEATURE_TDX_GUEST check. Why is it needed in this particular one?

> +
> + /*
> + * Pass the physical address of user generated report data
> + * and the physical address of output buffer to the TDX module
> + * to generate the TD report. Generated data contains
> + * measurements/configuration data of the TD guest. More info
> + * about ABI can be found in TDX 1.0 Module specification, sec
> + * titled "TDG.MR.REPORT".
> + */
> + ret = __tdx_module_call(TDX_GET_REPORT, virt_to_phys(data),
> + virt_to_phys(reportdata), 0, 0, NULL);
> +
> + if (ret)
> + return TDCALL_RETURN_CODE(ret);

If we don't expose TDCALL error to userspace, I don't think this is required?

> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(tdx_mcall_tdreport);

If you use SGX similar way to use misc_register() at boot but don't support the
driver as module, then you don't have to export this symbol.

> +
> static u64 get_cc_mask(void)
> {
> struct tdx_module_output out;
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index 020c81a7c729..a151f69dd6ef 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -67,6 +67,8 @@ void tdx_safe_halt(void);
>
> bool tdx_early_handle_ve(struct pt_regs *regs);
>
> +long tdx_mcall_tdreport(void *data, void *reportdata);
> +
> #else
>
> static inline void tdx_early_init(void) { };
> diff --git a/arch/x86/include/uapi/asm/tdx.h b/arch/x86/include/uapi/asm/tdx.h
> new file mode 100644
> index 000000000000..c21f9d6fe88b
> --- /dev/null
> +++ b/arch/x86/include/uapi/asm/tdx.h
> @@ -0,0 +1,23 @@
> +/* 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>
> +
> +/* Input report data length for TDX_CMD_GET_TDREPORT IOCTL request */
> +#define TDX_REPORT_DATA_LEN 64

I'd change to TDX_REPORTDATA_LEN to make it consistent with spec.

REPORT_DATA can be vague.

> +
> +/* Output TD report data length after TDX_CMD_GET_TDREPORT IOCTL execution */
> +#define TDX_TDREPORT_LEN 1024
> +
> +/*
> + * TDX_CMD_GET_TDREPORT IOCTL is used to get TDREPORT data from the TDX

"TDREPORT data" -> "TDREPORT", or "TDREPORT_STRUCT".

> + * Module. Users should pass report data of size TDX_REPORT_DATA_LEN bytes
> + * via user input buffer of size TDX_TDREPORT_LEN. Once IOCTL is successful
> + * TDREPORT data is copied to the user buffer. On failure, TDCALL error
> + * code is copied back to the user buffer.

As I commented above, I am not convinced we need to copy TDCALL error code back
to userspace. If we want to do that, we need to explicitly tell on what error
code (-EIO, i.e.), the TDCALL error code is copied back.

> + */
> +#define TDX_CMD_GET_TDREPORT _IOWR('T', 0x01, __u64)

I personally think it's better to define a structure to represent the argument
used by this IOCTL:

struct tdx_get_tdreport {
__u8 reportdata[TDX_REPORTDATA_LEN];
};

You might add some padding for future expending too.

Then we can have a clearer comment explaining what the reportdata. Below is
just for your reference. You may come up with better words.

"User-defined 64-Byte REPORTDATA to be included into the TDREPORT. Typically it
can be some nonce provided by attestation software so the generated TDREPORT can
be uniquely verified."

> +
> +#endif /* _UAPI_ASM_X86_TDX_H */