Re: [PATCH v2 5/6] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver

[Hans de Goede]

<hit send to soon, now with a complete p.s. section>

Hi,

On 3/31/22 00:18, Kuppuswamy Sathyanarayanan wrote:
> TDX guest supports encrypted disk as root or secondary drives.
> Decryption keys required to access such drives are usually maintained
> by 3rd party key servers. Attestation is required by 3rd party key
> servers to get the key for an encrypted disk volume, or possibly other
> encrypted services. Attestation is used to prove to the key server that
> the TD guest is running in a valid TD and the kernel and virtual BIOS
> and other environment are secure.
> 
> During the boot process various components before the kernel accumulate
> hashes in the TDX module, which can then combined into a report. This
> would typically include a hash of the bios, bios configuration, boot
> loader, command line, kernel, initrd.  After checking the hashes the
> key server will securely release the keys.
> 
> The actual details of the attestation protocol depend on the particular
> key server configuration, but some parts are common and need to
> communicate with the TDX module.
> 
> This communication is implemented in the attestation driver.
> 
> The supported steps 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.
> 
> Attestation agent uses IOCTLs implemented by the attestation driver to
> complete the various steps of the attestation process.
> 
> 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.
> 
> TDX_CMD_GEN_QUOTE IOCTL can be used to create an computation on the
> host, but TDX assumes that the host is able to deal with malicious
> guest flooding it anyways.
> 
> The interaction with the TDX module is like a RPM protocol here. There
> are several operations (get tdreport, get quote) that need to input a
> blob, and then output another blob. 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
> here. There is one ioctl per operation, that takes the input blob and
> returns the output blob, and as well as auxiliary ioctls to return the
> blob lengths. The ioctls are documented in the header file. 
> 
> [Chenyi Qiang: Proposed struct tdx_gen_quote for passing user buffer]
> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Acked-by: Hans de Goede <hdegoede@xxxxxxxxxx>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> ---
>  drivers/platform/x86/intel/Kconfig            |   1 +
>  drivers/platform/x86/intel/Makefile           |   1 +
>  drivers/platform/x86/intel/tdx/Kconfig        |  13 +
>  drivers/platform/x86/intel/tdx/Makefile       |   3 +
>  .../platform/x86/intel/tdx/intel_tdx_attest.c | 230 ++++++++++++++++++
>  include/uapi/misc/tdx.h                       |  42 ++++
>  6 files changed, 290 insertions(+)
>  create mode 100644 drivers/platform/x86/intel/tdx/Kconfig
>  create mode 100644 drivers/platform/x86/intel/tdx/Makefile
>  create mode 100644 drivers/platform/x86/intel/tdx/intel_tdx_attest.c
>  create mode 100644 include/uapi/misc/tdx.h
> 
> diff --git a/drivers/platform/x86/intel/Kconfig b/drivers/platform/x86/intel/Kconfig
> index 8e65086bb6c8..a2ed17d67052 100644
> --- a/drivers/platform/x86/intel/Kconfig
> +++ b/drivers/platform/x86/intel/Kconfig
> @@ -12,6 +12,7 @@ source "drivers/platform/x86/intel/pmt/Kconfig"
>  source "drivers/platform/x86/intel/speed_select_if/Kconfig"
>  source "drivers/platform/x86/intel/telemetry/Kconfig"
>  source "drivers/platform/x86/intel/wmi/Kconfig"
> +source "drivers/platform/x86/intel/tdx/Kconfig"
>  
>  config INTEL_HID_EVENT
>  	tristate "Intel HID Event"
> diff --git a/drivers/platform/x86/intel/Makefile b/drivers/platform/x86/intel/Makefile
> index 35f2066578b2..27a6c6c5a83f 100644
> --- a/drivers/platform/x86/intel/Makefile
> +++ b/drivers/platform/x86/intel/Makefile
> @@ -11,6 +11,7 @@ obj-$(CONFIG_INTEL_SKL_INT3472)		+= int3472/
>  obj-$(CONFIG_INTEL_PMC_CORE)		+= pmc/
>  obj-$(CONFIG_INTEL_PMT_CLASS)		+= pmt/
>  obj-$(CONFIG_INTEL_SPEED_SELECT_INTERFACE) += speed_select_if/
> +obj-$(CONFIG_INTEL_TDX_GUEST)		+= tdx/
>  obj-$(CONFIG_INTEL_TELEMETRY)		+= telemetry/
>  obj-$(CONFIG_INTEL_WMI)			+= wmi/
>  
> diff --git a/drivers/platform/x86/intel/tdx/Kconfig b/drivers/platform/x86/intel/tdx/Kconfig
> new file mode 100644
> index 000000000000..853e3a34c889
> --- /dev/null
> +++ b/drivers/platform/x86/intel/tdx/Kconfig
> @@ -0,0 +1,13 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +#
> +# X86 TDX Platform Specific Drivers
> +#
> +
> +config INTEL_TDX_ATTESTATION
> +	tristate "Intel TDX attestation driver"
> +	depends on INTEL_TDX_GUEST
> +	help
> +	  The TDX attestation driver provides IOCTL interfaces to the user to
> +	  request TDREPORT from the TDX module or request quote from the VMM
> +	  or to get quote buffer size. It is mainly used to get secure disk
> +	  decryption keys from the key server.
> diff --git a/drivers/platform/x86/intel/tdx/Makefile b/drivers/platform/x86/intel/tdx/Makefile
> new file mode 100644
> index 000000000000..124d6b7b20a0
> --- /dev/null
> +++ b/drivers/platform/x86/intel/tdx/Makefile
> @@ -0,0 +1,3 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +
> +obj-$(CONFIG_INTEL_TDX_ATTESTATION)	+= intel_tdx_attest.o
> diff --git a/drivers/platform/x86/intel/tdx/intel_tdx_attest.c b/drivers/platform/x86/intel/tdx/intel_tdx_attest.c
> new file mode 100644
> index 000000000000..0bf78d30e057
> --- /dev/null
> +++ b/drivers/platform/x86/intel/tdx/intel_tdx_attest.c
> @@ -0,0 +1,230 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * intel_tdx_attest.c - TDX guest attestation interface driver.
> + *
> + * Implements user interface to trigger attestation process and
> + * read the TD Quote result.
> + *
> + * Copyright (C) 2021-2022 Intel Corporation
> + *
> + * Author:
> + *     Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> + */
> +
> +#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/jiffies.h>
> +#include <linux/io.h>
> +#include <asm/apic.h>
> +#include <asm/tdx.h>
> +#include <asm/irq_vectors.h>
> +#include <uapi/misc/tdx.h>
> +
> +/* Used in Quote memory allocation */
> +#define QUOTE_SIZE			(2 * PAGE_SIZE)
> +/* Used in Get Quote request memory allocation */
> +#define GET_QUOTE_MAX_SIZE		(4 * PAGE_SIZE)
> +/* Get Quote timeout in msec */
> +#define GET_QUOTE_TIMEOUT		(5000)
> +
> +/* Mutex to synchronize attestation requests */
> +static DEFINE_MUTEX(attestation_lock);
> +/* Completion object to track attestation status */
> +static DECLARE_COMPLETION(attestation_done);
> +/* Buffer used to copy report data in attestation handler */
> +static u8 report_data[TDX_REPORT_DATA_LEN] __aligned(64);
> +/* Data pointer used to get TD Quote data in attestation handler */
> +static void *tdquote_data;
> +/* Data pointer used to get TDREPORT data in attestation handler */
> +static void *tdreport_data;
> +/* DMA handle used to allocate and free tdquote DMA buffer */
> +dma_addr_t tdquote_dma_handle;
> +
> +static void attestation_callback_handler(void)
> +{
> +	complete(&attestation_done);
> +}
> +
> +static long tdx_attest_ioctl(struct file *file, unsigned int cmd,
> +			     unsigned long arg)
> +{
> +	void __user *argp = (void __user *)arg;
> +	struct tdx_gen_quote tdquote_req;
> +	long ret = 0;
> +
> +	mutex_lock(&attestation_lock);
> +
> +	switch (cmd) {
> +	case TDX_CMD_GET_TDREPORT:
> +		if (copy_from_user(report_data, argp, TDX_REPORT_DATA_LEN)) {
> +			ret = -EFAULT;
> +			break;
> +		}
> +
> +		/* Generate TDREPORT_STRUCT */
> +		if (tdx_mcall_tdreport(tdreport_data, report_data)) {
> +			ret = -EIO;
> +			break;
> +		}
> +
> +		if (copy_to_user(argp, tdreport_data, TDX_TDREPORT_LEN))
> +			ret = -EFAULT;
> +		break;
> +	case TDX_CMD_GEN_QUOTE:
> +		reinit_completion(&attestation_done);
> +
> +		/* Copy TDREPORT data from user buffer */
> +		if (copy_from_user(&tdquote_req, argp, sizeof(struct tdx_gen_quote))) {
> +			ret = -EFAULT;
> +			break;
> +		}
> +
> +		if (tdquote_req.len <= 0 || tdquote_req.len > GET_QUOTE_MAX_SIZE) {
> +			ret = -EINVAL;
> +			break;
> +		}
> +
> +		if (copy_from_user(tdquote_data, tdquote_req.buf, tdquote_req.len)) {
> +			ret = -EFAULT;
> +			break;
> +		}
> +
> +		/* Submit GetQuote Request */
> +		if (tdx_hcall_get_quote(tdquote_data, GET_QUOTE_MAX_SIZE)) {
> +			ret = -EIO;
> +			break;
> +		}
> +
> +		/* Wait for attestation completion */
> +		ret = wait_for_completion_interruptible_timeout(
> +				&attestation_done,
> +				msecs_to_jiffies(GET_QUOTE_TIMEOUT));
> +		if (ret <= 0) {
> +			ret = -EIO;
> +			break;
> +		}
> +
> +		/* ret will be positive if completed. */
> +		ret = 0;
> +
> +		if (copy_to_user(tdquote_req.buf, tdquote_data, tdquote_req.len))
> +			ret = -EFAULT;
> +
> +		break;
> +	case TDX_CMD_GET_QUOTE_SIZE:
> +		ret = put_user(QUOTE_SIZE, (u64 __user *)argp);
> +		break;
> +	default:
> +		pr_err("cmd %d not supported\n", cmd);
> +		break;
> +	}
> +
> +	mutex_unlock(&attestation_lock);
> +
> +	return ret;
> +}
> +
> +static const struct file_operations tdx_attest_fops = {
> +	.owner		= THIS_MODULE,
> +	.unlocked_ioctl	= tdx_attest_ioctl,
> +	.llseek		= no_llseek,
> +};
> +
> +static struct miscdevice tdx_attest_device = {
> +	.minor          = MISC_DYNAMIC_MINOR,
> +	.name           = "tdx-attest",
> +	.fops           = &tdx_attest_fops,
> +};
> +
> +static int __init tdx_attest_init(void)
> +{
> +	dma_addr_t handle;
> +	long ret = 0;
> +
> +	mutex_lock(&attestation_lock);
> +
> +	ret = misc_register(&tdx_attest_device);
> +	if (ret) {
> +		pr_err("misc device registration failed\n");
> +		mutex_unlock(&attestation_lock);
> +		return ret;
> +	}

Why not do this as the last thing of the probe?

That will avoid the need to unregister this again in all
the error-exit paths and also fixes a possible deadlock.

Right now you possibly have:

1. probe() locks attestation_lock
2. probe() registers misc-device
3. userspace calls tdx_attest_ioctl
4. tdx_attest_ioctl blocks waiting for attestastion_lock
5. Something goes wrong in probe, probe calls
   misc_deregister()
6. misc_deregister waits for the ioctl to finish
7. deadlock

I'm not sure about 6, but if 6 does not happen then
instead we now have tdx_attest_ioctl running
after the misc_deregister, with tdquote_data and
tdreport_data as NULL, or pointing to free-ed memory
leading to various crash scenarios.

TL;DR: you must always delay registering any
interfaces for userspace until your code is
ready to deal with userspace calls.

Regards,

Hans

p.s.

As I mentioned with v1:

I really know very little about TDX.
I assume the rest of the series will be reviewed by someone
with more detailed knowledge of TDX as such I believe it would be good
if the platform/x86 patch is also reviewed as part of that.

Since the platform/x86 patch depends on the other patches I believe
it is also best if the entire series is merged in one go by the x86/tip
maintainers here is my ack for this:

Acked-by: Hans de Goede <hdegoede@xxxxxxxxxx>





> +
> +	/*
> +	 * tdreport_data needs to be 64-byte aligned.
> +	 * Full page alignment is more than enough.
> +	 */
> +	tdreport_data = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, 0);
> +	if (!tdreport_data) {
> +		ret = -ENOMEM;
> +		goto failed;
> +	}
> +
> +	ret = dma_set_coherent_mask(tdx_attest_device.this_device,
> +				    DMA_BIT_MASK(64));
> +	if (ret) {
> +		pr_err("dma set coherent mask failed\n");
> +		goto failed;
> +	}
> +
> +	/* Allocate DMA buffer to get TDQUOTE data from the VMM */
> +	tdquote_data = dma_alloc_coherent(tdx_attest_device.this_device,
> +					  GET_QUOTE_MAX_SIZE, &handle,
> +					  GFP_KERNEL | __GFP_ZERO);
> +	if (!tdquote_data) {
> +		ret = -ENOMEM;
> +		goto failed;
> +	}
> +
> +	tdquote_dma_handle =  handle;
> +
> +	/* Register attestation event notify handler */
> +	tdx_setup_ev_notify_handler(attestation_callback_handler);
> +
> +	mutex_unlock(&attestation_lock);
> +
> +	pr_debug("module initialization success\n");
> +
> +	return 0;
> +
> +failed:
> +	if (tdreport_data)
> +		free_pages((unsigned long)tdreport_data, 0);
> +
> +	misc_deregister(&tdx_attest_device);
> +
> +	mutex_unlock(&attestation_lock);
> +
> +	pr_debug("module initialization failed\n");
> +
> +	return ret;
> +}
> +
> +static void __exit tdx_attest_exit(void)
> +{
> +	mutex_lock(&attestation_lock);
> +
> +	dma_free_coherent(tdx_attest_device.this_device, GET_QUOTE_MAX_SIZE,
> +			  tdquote_data, tdquote_dma_handle);
> +	free_pages((unsigned long)tdreport_data, 0);
> +	misc_deregister(&tdx_attest_device);
> +	/* Unregister attestation event notify handler */
> +	tdx_remove_ev_notify_handler();
> +	mutex_unlock(&attestation_lock);
> +	pr_debug("module is successfully removed\n");
> +}
> +
> +module_init(tdx_attest_init);
> +module_exit(tdx_attest_exit);
> +
> +MODULE_AUTHOR("Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>");
> +MODULE_DESCRIPTION("TDX attestation driver");
> +MODULE_LICENSE("GPL");
> diff --git a/include/uapi/misc/tdx.h b/include/uapi/misc/tdx.h
> new file mode 100644
> index 000000000000..839b9a220022
> --- /dev/null
> +++ b/include/uapi/misc/tdx.h
> @@ -0,0 +1,42 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +#ifndef _UAPI_MISC_TDX_H
> +#define _UAPI_MISC_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
> +
> +/* 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
> + * 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.
> + */
> +#define TDX_CMD_GET_TDREPORT		_IOWR('T', 0x01, __u64)
> +
> +/*
> + * TDX_CMD_GEN_QUOTE IOCTL is used to request TD QUOTE from the VMM. User
> + * should pass TD report data of size TDX_TDREPORT_LEN bytes via user input
> + * buffer of quote size. Once IOCTL is successful quote data is copied back to
> + * the user buffer.
> + */
> +#define TDX_CMD_GEN_QUOTE		_IOR('T', 0x02, __u64)
> +
> +/*
> + * TDX_CMD_GET_QUOTE_SIZE IOCTL is used to get the TD Quote size info in bytes.
> + * This will be used for determining the input buffer allocation size when
> + * using TDX_CMD_GEN_QUOTE IOCTL.
> + */
> +#define TDX_CMD_GET_QUOTE_SIZE		_IOR('T', 0x03, __u64)
> +
> +struct tdx_gen_quote {
> +	void *buf __user;
> +	size_t len;
> +};
> +
> +#endif /* _UAPI_MISC_TDX_H */