Re: [PATCH v11 1/3] x86/tdx: Add TDX Guest attestation interface driver

Sathyanarayanan Kuppuswamy
2022-08-28T01:38:35Z

Hi,

On 8/27/22 12:33 AM, Greg Kroah-Hartman wrote:
> On Fri, Aug 26, 2022 at 08:06:36AM -0700, Kuppuswamy Sathyanarayanan wrote:
>> Attestation is used to verify the TDX guest trustworthiness to other
>> entities before provisioning secrets to the guest. For example, a key
>> server may request for attestation before releasing the encryption keys
>> to mount the encrypted rootfs or secondary drive.
>>
>> During the TDX guest launch, the initial contents (including the
>> firmware image) and configuration of the guest are recorded by the
>> Intel TDX module in build time measurement register (MRTD). After TDX
>> guest is created, run-time measurement registers (RTMRs) can be used by
>> the guest software to extend the 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 complete details, please refer to TDX Virtual
>> Firmware design specification, sec titled "TD Measurement".
>>
>> At TDX guest 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 guest-specific information (such as build
>> and boot measurements), platform security version, and the MAC to
>> protect the integrity of the TDREPORT. The guest 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
>> the TDREPORT can be found in Intel TDX Module specification, section
>> titled "TDG.MR.REPORT Leaf".
>>
>> TDREPORT by design can only be verified on the 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 guest, so the QE can be deployed in the host, or in another
>> legacy VM with SGX support. QE checks the integrity of TDREPORT and if
>> it is valid, a certified quote signing key is used to sign the Quote.
>> How to send the TDREPORT to QE and receive the Quote is implementation
>> and deployment specific.
>>
>> Implement a basic guest misc driver to allow userspace to get the
>> TDREPORT. After getting TDREPORT, the userspace attestation software
>> can 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.
>>
>> Any distribution enabling TDX is also expected to need attestation. So
>> enable it by default with TDX guest support.
>>
>> 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>
>> ---
>>
>> Changes since v10:
>> * Replaced TD/TD Guest usage with TDX Guest or Guest.
>> * Removed unnecessary comments.
>> * Added more validation to user input in tdx_get_report().
>> * Used u64_to_user_ptr when reading user u64 pointers.
>> * Fixed commit log as per review comments.
>>
>> Changes since v9:
>> * Dropped the cover letter. Since this patch set only adds
>> TDREPORT support, the commit log itself has all the required details.
>> * Dropped the Quote support and event IRQ support as per Dave's
>> review suggestion.
>> * Dropped attest.c and moved its contents to tdx.c
>> * Updated commit log and comments to reflect latest changes.
>>
>> Changes since v8:
>> * Please refer to https://lore.kernel.org/all/ \
>> 20220728034420.648314-1-sathyanarayanan.kuppuswamy@linux.intel.com/
>>
>> arch/x86/coco/tdx/tdx.c | 114 ++++++++++++++++++++++++++++++++
>> arch/x86/include/uapi/asm/tdx.h | 51 ++++++++++++++
>> 2 files changed, 165 insertions(+)
>> create mode 100644 arch/x86/include/uapi/asm/tdx.h
>>
>> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
>> index 928dcf7a20d9..0888bdf93a4e 100644
>> --- a/arch/x86/coco/tdx/tdx.c
>> +++ b/arch/x86/coco/tdx/tdx.c
>> @@ -5,16 +5,21 @@
>> #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>
>> #include <asm/insn.h>
>> #include <asm/insn-eval.h>
>> #include <asm/pgtable.h>
>> +#include <uapi/asm/tdx.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 +39,10 @@
>> #define VE_GET_PORT_NUM(e) ((e) >> 16)
>> #define VE_IS_IO_STRING(e) ((e) & BIT(4))
>>
>> +#define DRIVER_NAME "tdx-guest"
>
> KBUILD_MODNAME?

It is only used to set the tdx_misc_dev name and we want to use "tdx-guest"
as the interface name. So KBUILD_MODNAME will not work. Since userspace
also needs this device name, I will rename it as TDX_GUEST_DEVICE and move
it to arch/x86/include/uapi/asm/tdx.h.

>
>> +static struct miscdevice tdx_misc_dev;
>
> Why not actually set the values here, instead of in the code down below?

Fine with me. I will move this above tdx_guest_init() and initialize it
directly.

+static struct miscdevice tdx_misc_dev = {
+ .name = TDX_GUEST_DEVICE,
+ .minor = MISC_DYNAMIC_MINOR,
+ .fops = &tdx_guest_fops,
+};

>
>> +
>> /*
>> * Wrapper for standard use of __tdx_hypercall with no output aside from
>> * return code.
>> @@ -775,3 +784,108 @@ void __init tdx_early_init(void)
>>
>> pr_info("Guest detected\n");
>> }
>> +
>> +static long tdx_get_report(void __user *argp)
>> +{
>> + u8 *reportdata, *tdreport;
>> + struct tdx_report_req req;
>> + long ret;
>> +
>> + if (copy_from_user(&req, argp, sizeof(req)))
>> + return -EFAULT;
>> +
>> + /*
>> + * Per TDX Module 1.0 specification, section titled
>> + * "TDG.MR.REPORT", REPORTDATA length is fixed as
>> + * TDX_REPORTDATA_LEN, TDREPORT length is fixed as
>> + * TDX_REPORT_LEN, and TDREPORT subtype is fixed as
>> + * 0. Also check for valid user pointers.
>> + */
>> + if (!req.reportdata || !req.tdreport || req.subtype ||
>> + req.rpd_len != TDX_REPORTDATA_LEN ||
>> + req.tdr_len != TDX_REPORT_LEN)
>> + return -EINVAL;
>> +
>> + reportdata = kmalloc(req.rpd_len, GFP_KERNEL);
>> + if (!reportdata)
>> + return -ENOMEM;
>> +
>> + tdreport = kzalloc(req.tdr_len, GFP_KERNEL);
>> + if (!tdreport) {
>> + kfree(reportdata);
>> + return -ENOMEM;
>> + }
>> +
>> + if (copy_from_user(reportdata, u64_to_user_ptr(req.reportdata),
>> + req.rpd_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), req.subtype,
>> + 0, NULL);
>> + if (ret) {
>> + ret = -EIO;
>> + goto out;
>> + }
>> +
>> + if (copy_to_user(u64_to_user_ptr(req.tdreport), tdreport, req.tdr_len))
>> + ret = -EFAULT;
>> +
>> +out:
>> + kfree(reportdata);
>> + kfree(tdreport);
>> + return ret;
>> +}
>> +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;
>
> Wrong error value for an invalid ioctl.
>
> You all know this, how did this slip through?
>
> :(

Sorry, I missed it. I will fix it in next version.

>
>> + }
>> +
>> + 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;
>> +
>> + 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;
>
> Again, set this in the definition above, and you will be fine.

Agreed. I will move it.

>
>> +
>> + 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/include/uapi/asm/tdx.h b/arch/x86/include/uapi/asm/tdx.h
>> new file mode 100644
>> index 000000000000..c1667b20fe20
>> --- /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;
>
> Unaligned data? That's really how the chip works? That feels very odd.
>
> Or do you require the hole here that the compiler added? If so, you
> can't always rely on that, properly pad things out.

This is not related to hardware. This struct is only used in user/kernel ABI.
So we don't need to care about alignment. In tdx_get_report(), we read
these values and pass it in format suitable for TDX_GET_REPORT TDCALL.

>
>
>> + __u32 rpd_len;
>> + __u64 tdreport;
>> + __u32 tdr_len;
>
> Again, check for holes.
>
> And as this comes directly from hardware, what is the endian-ness of
> these values?

It does not come from the hardware. This ABI allows userspace to
get the TDREPORT from the TDX Module.. After request completion, the kernel
will copy the TDREPORT data to user buffer specified in tdx_report_req.tdreport.

>
> thanks,
>
> greg k-h

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer