Re: [PATCH v9 41/43] virt: Add SEV-SNP guest driver

Borislav Petkov
2022-02-06T22:39:54Z

On Fri, Jan 28, 2022 at 11:18:02AM -0600, Brijesh Singh wrote:
> SEV-SNP specification provides the guest a mechanism to communicate with
^
The

> the PSP without risk from a malicious hypervisor who wishes to read, alter,
> drop or replay the messages sent. The driver uses snp_issue_guest_request()
> to issue GHCB SNP_GUEST_REQUEST or SNP_EXT_GUEST_REQUEST NAE events to
> submit the request to PSP.
>
> The PSP requires that all communication should be encrypted using key
> specified through the platform_data.
>
> The userspace can use SNP_GET_REPORT ioctl() to query the guest

s/The u/U/

> attestation report.
>
> See SEV-SNP spec section Guest Messages for more details.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> Documentation/virt/coco/sevguest.rst | 81 ++++
> drivers/virt/Kconfig | 3 +
> drivers/virt/Makefile | 1 +
> drivers/virt/coco/sevguest/Kconfig | 12 +
> drivers/virt/coco/sevguest/Makefile | 2 +
> drivers/virt/coco/sevguest/sevguest.c | 605 ++++++++++++++++++++++++++
> drivers/virt/coco/sevguest/sevguest.h | 98 +++++
> include/uapi/linux/sev-guest.h | 50 +++
> 8 files changed, 852 insertions(+)
> create mode 100644 Documentation/virt/coco/sevguest.rst
> create mode 100644 drivers/virt/coco/sevguest/Kconfig
> create mode 100644 drivers/virt/coco/sevguest/Makefile
> create mode 100644 drivers/virt/coco/sevguest/sevguest.c
> create mode 100644 drivers/virt/coco/sevguest/sevguest.h
> create mode 100644 include/uapi/linux/sev-guest.h
>
> diff --git a/Documentation/virt/coco/sevguest.rst b/Documentation/virt/coco/sevguest.rst
> new file mode 100644
> index 000000000000..47ef3b0821d5
> --- /dev/null
> +++ b/Documentation/virt/coco/sevguest.rst

Adding documentation is all fine and good but you need to link that file
into the TOC somewhere so that it is visible when the documentation gets
generated... I guess something like this:

diff --git a/Documentation/virt/index.rst b/Documentation/virt/index.rst
index edea7fea95a8..40ad0d20032e 100644
--- a/Documentation/virt/index.rst
+++ b/Documentation/virt/index.rst
@@ -13,6 +13,7 @@ Linux Virtualization Support
guest-halt-polling
ne_overview
acrn/index
+ coco/sevguest

.. only:: html and subproject

And once you do, do "make htmldocs" to see whether it complains about
some formatting issues or other errors etc.

/me goes and does it...

Ah, here we go:

Documentation/virt/coco/sevguest.rst:48: WARNING: Inline emphasis start-string without end-string.
Documentation/virt/coco/sevguest.rst:51: WARNING: Inline emphasis start-string without end-string.
Documentation/virt/coco/sevguest.rst:55: WARNING: Inline emphasis start-string without end-string.
Documentation/virt/coco/sevguest.rst:57: WARNING: Definition list ends without a blank line; unexpected unindent.

There's something it doesn't like about the struct. Yeah, when I look at
the html output, it is all weird and not monospaced...

> @@ -0,0 +1,81 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +===================================================================
> +The Definitive SEV Guest API Documentation
> +===================================================================
> +
> +1. General description
> +======================
> +
> +The SEV API is a set of ioctls that are used by the guest or hypervisor
> +to get or set certain aspect of the SEV virtual machine. The ioctls belong
^
a

> +to the following classes:
> +
> + - Hypervisor ioctls: These query and set global attributes which affect the
> + whole SEV firmware. These ioctl are used by platform provision tools.

"provisioning" maybe?

> +
> + - Guest ioctls: These query and set attributes of the SEV virtual machine.
> +
> +2. API description
> +==================
> +
> +This section describes ioctls that can be used to query or set SEV guests.
^^^^^^^^^^^^^^^

That sounds weird.

> +For each ioctl, the following information is provided along with a
> +description:
> +
> + Technology:
> + which SEV technology provides this ioctl. sev, sev-es, sev-snp or all.
^^^^^^^^^^^^^^^^^^^^

Marketing is going to scold you - those need to be in all caps. :-P

> +
> + Type:
> + hypervisor or guest. The ioctl can be used inside the guest or the
> + hypervisor.
> +
> + Parameters:
> + what parameters are accepted by the ioctl.
> +
> + Returns:
> + the return value. General error numbers (ENOMEM, EINVAL)

Those are negative: -ENOMEM, -EINVAL

> + are not detailed, but errors with specific meanings are.
> +
> +The guest ioctl should be issued on a file descriptor of the /dev/sev-guest device.
> +The ioctl accepts struct snp_user_guest_request. The input and output structure is
> +specified through the req_data and resp_data field respectively. If the ioctl fails
> +to execute due to a firmware error, then fw_err code will be set otherwise the
> +fw_err will be set to 0xff.

fw_err is u64. What does 0xff mean? Everything above the least
significant byte is reserved 0?

> diff --git a/drivers/virt/coco/sevguest/Kconfig b/drivers/virt/coco/sevguest/Kconfig
> new file mode 100644
> index 000000000000..07ab9ec6471c
> --- /dev/null
> +++ b/drivers/virt/coco/sevguest/Kconfig
> @@ -0,0 +1,12 @@
> +config SEV_GUEST
> + tristate "AMD SEV Guest driver"
> + default y

Definitely not. We don't enable drivers by default unless they're
ubiquitous.

> + depends on AMD_MEM_ENCRYPT && CRYPTO_AEAD2
> + help
> + SEV-SNP firmware provides the guest a mechanism to communicate with
> + the PSP without risk from a malicious hypervisor who wishes to read,
> + alter, drop or replay the messages sent. The driver provides
> + userspace interface to communicate with the PSP to request the
> + attestation report and more.
> +
> + If you choose 'M' here, this module will be called sevguest.

...

> +static bool is_vmpck_empty(struct snp_guest_dev *snp_dev)
> +{
> + char zero_key[VMPCK_KEY_LEN] = {0};
> +
> + if (snp_dev->vmpck)
> + return memcmp(snp_dev->vmpck, zero_key, VMPCK_KEY_LEN) == 0;

return !memcmp(...);
> +
> + return true;
> +}
> +
> +static void snp_disable_vmpck(struct snp_guest_dev *snp_dev)
> +{
> + memzero_explicit(snp_dev->vmpck, VMPCK_KEY_LEN);
> + snp_dev->vmpck = NULL;
> +}
> +
> +static inline u64 __snp_get_msg_seqno(struct snp_guest_dev *snp_dev)
> +{
> + u64 count;
> +
> + lockdep_assert_held(&snp_cmd_mutex);
> +
> + /* Read the current message sequence counter from secrets pages */
> + count = *snp_dev->os_area_msg_seqno;
> +
> + return count + 1;
> +}
> +
> +/* Return a non-zero on success */
> +static u64 snp_get_msg_seqno(struct snp_guest_dev *snp_dev)
> +{
> + u64 count = __snp_get_msg_seqno(snp_dev);
> +
> + /*
> + * The message sequence counter for the SNP guest request is a 64-bit
> + * value but the version 2 of GHCB specification defines a 32-bit storage
> + * for it. If the counter exceeds the 32-bit value then return zero.
> + * The caller should check the return value, but if the caller happens to
> + * not check the value and use it, then the firmware treats zero as an
> + * invalid number and will fail the message request.
> + */
> + if (count >= UINT_MAX) {
> + pr_err_ratelimited("SNP guest request message sequence counter overflow\n");

How does error message help the user? Is she supposed to reboot the
machine or so?

Because it sounds to me like if this goes over 32-bit, this driver stops
working. So what resets those sequence numbers?

> + return 0;
> + }
> +
> + return count;
> +}
> +
> +static void snp_inc_msg_seqno(struct snp_guest_dev *snp_dev)
> +{
> + /*
> + * The counter is also incremented by the PSP, so increment it by 2
> + * and save in secrets page.
> + */
> + *snp_dev->os_area_msg_seqno += 2;
> +}
> +
> +static inline struct snp_guest_dev *to_snp_dev(struct file *file)
> +{
> + struct miscdevice *dev = file->private_data;
> +
> + return container_of(dev, struct snp_guest_dev, misc);
> +}
> +
> +static struct snp_guest_crypto *init_crypto(struct snp_guest_dev *snp_dev, u8 *key, size_t keylen)
> +{
> + struct snp_guest_crypto *crypto;
> +
> + crypto = kzalloc(sizeof(*crypto), GFP_KERNEL_ACCOUNT);
> + if (!crypto)
> + return NULL;
> +
> + crypto->tfm = crypto_alloc_aead("gcm(aes)", 0, 0);
> + if (IS_ERR(crypto->tfm))
> + goto e_free;
> +
> + if (crypto_aead_setkey(crypto->tfm, key, keylen))
> + goto e_free_crypto;
> +
> + crypto->iv_len = crypto_aead_ivsize(crypto->tfm);
> + if (crypto->iv_len < 12) {
> + dev_err(snp_dev->dev, "IV length is less than 12.\n");

And? < 12 is bad? Make that error message more user-friendly pls.

> + goto e_free_crypto;
> + }
> +
> + crypto->iv = kmalloc(crypto->iv_len, GFP_KERNEL_ACCOUNT);
> + if (!crypto->iv)
> + goto e_free_crypto;
> +
> + if (crypto_aead_authsize(crypto->tfm) > MAX_AUTHTAG_LEN) {
> + if (crypto_aead_setauthsize(crypto->tfm, MAX_AUTHTAG_LEN)) {
> + dev_err(snp_dev->dev, "failed to set authsize to %d\n", MAX_AUTHTAG_LEN);
> + goto e_free_crypto;
> + }
> + }
> +
> + crypto->a_len = crypto_aead_authsize(crypto->tfm);
> + crypto->authtag = kmalloc(crypto->a_len, GFP_KERNEL_ACCOUNT);
> + if (!crypto->authtag)
> + goto e_free_crypto;
> +
> + return crypto;
> +
> +e_free_crypto:
> + crypto_free_aead(crypto->tfm);
> +e_free:
> + kfree(crypto->iv);
> + kfree(crypto->authtag);
> + kfree(crypto);

The order of those free calls needs to be the opposite of the kmallocs
above.

> +
> + return NULL;
> +}

...

> +static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, int msg_ver,
> + u8 type, void *req_buf, size_t req_sz, void *resp_buf,
> + u32 resp_sz, __u64 *fw_err)
> +{
> + unsigned long err;
> + u64 seqno;
> + int rc;
> +
> + /* Get message sequence and verify that its a non-zero */
> + seqno = snp_get_msg_seqno(snp_dev);
> + if (!seqno)
> + return -EIO;
> +
> + memset(snp_dev->response, 0, sizeof(*snp_dev->response));

sizeof(struct snp_guest_msg)

> +
> + /* Encrypt the userspace provided payload */
> + rc = enc_payload(snp_dev, seqno, msg_ver, type, req_buf, req_sz);
> + if (rc)
> + return rc;
> +
> + /* Call firmware to process the request */
> + rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
> + if (fw_err)
> + *fw_err = err;
> +
> + if (rc)
> + return rc;
> +
> + rc = verify_and_dec_payload(snp_dev, resp_buf, resp_sz);
> + if (rc) {
> + /*
> + * The verify_and_dec_payload() will fail only if the hypervisor is
> + * actively modifying the message header or corrupting the encrypted payload.
> + * This hints that hypervisor is acting in a bad faith. Disable the VMPCK so that
> + * the key cannot be used for any communication. The key is disabled to ensure
> + * that AES-GCM does not use the same IV while encrypting the request payload.
> + */

Put that comment over the function call.

> + dev_alert(snp_dev->dev,
> + "Detected unexpected decode failure, disabling the vmpck_id %d\n",
> + vmpck_id);
> + snp_disable_vmpck(snp_dev);
> + return rc;
> + }
> +
> + /* Increment to new message sequence after payload decryption was successful. */
> + snp_inc_msg_seqno(snp_dev);
> +
> + return 0;
> +}
> +
> +static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_ioctl *arg)
> +{
> + struct snp_guest_crypto *crypto = snp_dev->crypto;
> + struct snp_report_req req = {0};
> + struct snp_report_resp *resp;
> + int rc, resp_len;

lockdep_assert_held(&snp_cmd_mutex);

> + if (!arg->req_data || !arg->resp_data)
> + return -EINVAL;
> +
> + /* Copy the request payload from userspace */
> + if (copy_from_user(&req, (void __user *)arg->req_data, sizeof(req)))
> + return -EFAULT;
> +
> + /*
> + * The intermediate response buffer is used while decrypting the
> + * response payload. Make sure that it has enough space to cover the
> + * authtag.
> + */
> + resp_len = sizeof(resp->data) + crypto->a_len;
> + resp = kzalloc(resp_len, GFP_KERNEL_ACCOUNT);
> + if (!resp)
> + return -ENOMEM;
> +
> + /* Issue the command to get the attestation report */
> + rc = handle_guest_request(snp_dev, SVM_VMGEXIT_GUEST_REQUEST, arg->msg_version,
> + SNP_MSG_REPORT_REQ, &req, sizeof(req), resp->data,
> + resp_len, &arg->fw_err);
> + if (rc)
> + goto e_free;
> +
> + /* Copy the response payload to userspace */
> + if (copy_to_user((void __user *)arg->resp_data, resp, sizeof(*resp)))
> + rc = -EFAULT;
> +
> +e_free:
> + kfree(resp);
> + return rc;
> +}
> +
> +static long snp_guest_ioctl(struct file *file, unsigned int ioctl, unsigned long arg)
> +{
> + struct snp_guest_dev *snp_dev = to_snp_dev(file);
> + void __user *argp = (void __user *)arg;
> + struct snp_guest_request_ioctl input;
> + int ret = -ENOTTY;
> +
> + if (copy_from_user(&input, argp, sizeof(input)))
> + return -EFAULT;
> +
> + input.fw_err = 0xff;
> +
> + /* Message version must be non-zero */
> + if (!input.msg_version)
> + return -EINVAL;
> +
> + mutex_lock(&snp_cmd_mutex);

That mutex probably is to be held while issuing SNP commands but then
you hold it here already for...

> +
> + /* Check if the VMPCK is not empty */
> + if (is_vmpck_empty(snp_dev)) {

... this here which is not really a SNP command issuing.

Should that mutex be grabbed only around handle_guest_request() above or
is it supposed to protect more stuff?

> + dev_err_ratelimited(snp_dev->dev, "VMPCK is disabled\n");
> + mutex_unlock(&snp_cmd_mutex);
> + return -ENOTTY;
> + }
> +
> + switch (ioctl) {
> + case SNP_GET_REPORT:
> + ret = get_report(snp_dev, &input);
> + break;
> + default:
> + break;
> + }
> +
> + mutex_unlock(&snp_cmd_mutex);
> +
> + if (input.fw_err && copy_to_user(argp, &input, sizeof(input)))
> + return -EFAULT;
> +
> + return ret;
> +}
> +
> +static void free_shared_pages(void *buf, size_t sz)
> +{
> + unsigned int npages = PAGE_ALIGN(sz) >> PAGE_SHIFT;
> +
> + if (!buf)
> + return;
> +
> + /* If fail to restore the encryption mask then leak it. */

Useless comment - the error message says the same.

> + if (WARN_ONCE(set_memory_encrypted((unsigned long)buf, npages),
> + "Failed to restore encryption mask (leak it)\n"))
> + return;
> +
> + __free_pages(virt_to_page(buf), get_order(sz));
> +}
> +
> +static void *alloc_shared_pages(size_t sz)
> +{
> + unsigned int npages = PAGE_ALIGN(sz) >> PAGE_SHIFT;
> + struct page *page;
> + int ret;
> +
> + page = alloc_pages(GFP_KERNEL_ACCOUNT, get_order(sz));
> + if (IS_ERR(page))
> + return NULL;
> +
> + ret = set_memory_decrypted((unsigned long)page_address(page), npages);
> + if (ret) {
> + pr_err("SEV-SNP: failed to mark page shared, ret=%d\n", ret);

Use pr_fmt, grep the tree for an example how and drop "SEV-SNP:" - that
prefix should be "sevguest:".

> + __free_pages(page, get_order(sz));
> + return NULL;
> + }
> +
> + return page_address(page);
> +}

...

> +static int __init snp_guest_probe(struct platform_device *pdev)
> +{
> + struct snp_secrets_page_layout *layout;
> + struct snp_guest_platform_data *data;
> + struct device *dev = &pdev->dev;
> + struct snp_guest_dev *snp_dev;
> + struct miscdevice *misc;
> + int ret;
> +
> + if (!dev->platform_data)
> + return -ENODEV;
> +
> + data = (struct snp_guest_platform_data *)dev->platform_data;
> + layout = (__force void *)ioremap_encrypted(data->secrets_gpa, PAGE_SIZE);
> + if (!layout)
> + return -ENODEV;
> +
> + ret = -ENOMEM;
> + snp_dev = devm_kzalloc(&pdev->dev, sizeof(struct snp_guest_dev), GFP_KERNEL);
> + if (!snp_dev)
> + goto e_fail;
> +
> + ret = -EINVAL;
> + snp_dev->vmpck = get_vmpck(vmpck_id, layout, &snp_dev->os_area_msg_seqno);
> + if (!snp_dev->vmpck) {
> + dev_err(dev, "invalid vmpck id %d\n", vmpck_id);
> + goto e_fail;
> + }
> +
> + /* Verify that VMPCK is not zero. */
> + if (is_vmpck_empty(snp_dev)) {
> + dev_err(dev, "vmpck id %d is null\n", vmpck_id);
> + goto e_fail;
> + }
> +
> + platform_set_drvdata(pdev, snp_dev);
> + snp_dev->dev = dev;
> + snp_dev->layout = layout;
> +
> + /* Allocate the shared page used for the request and response message. */
> + snp_dev->request = alloc_shared_pages(sizeof(struct snp_guest_msg));
> + if (!snp_dev->request)
> + goto e_fail;
> +
> + snp_dev->response = alloc_shared_pages(sizeof(struct snp_guest_msg));
> + if (!snp_dev->response)
> + goto e_fail;
> +
> + ret = -EIO;
> + snp_dev->crypto = init_crypto(snp_dev, snp_dev->vmpck, VMPCK_KEY_LEN);
> + if (!snp_dev->crypto)
> + goto e_fail;
> +
> + misc = &snp_dev->misc;
> + misc->minor = MISC_DYNAMIC_MINOR;
> + misc->name = DEVICE_NAME;
> + misc->fops = &snp_guest_fops;
> +
> + /* initial the input address for guest request */
> + snp_dev->input.req_gpa = __pa(snp_dev->request);
> + snp_dev->input.resp_gpa = __pa(snp_dev->response);
> +
> + ret = misc_register(misc);
> + if (ret)
> + goto e_fail;
> +
> + dev_info(dev, "Initialized SEV-SNP guest driver (using vmpck_id %d)\n", vmpck_id);
> + return 0;
> +
> +e_fail:
> + iounmap(layout);
> + free_shared_pages(snp_dev->request, sizeof(struct snp_guest_msg));
> + free_shared_pages(snp_dev->response, sizeof(struct snp_guest_msg));

The freeing needs to happen in the opposite order of the allocations.
Audit all gotos.

> +
> + return ret;
> +}
> +
> +static int __exit snp_guest_remove(struct platform_device *pdev)
> +{
> + struct snp_guest_dev *snp_dev = platform_get_drvdata(pdev);
> +
> + free_shared_pages(snp_dev->request, sizeof(struct snp_guest_msg));
> + free_shared_pages(snp_dev->response, sizeof(struct snp_guest_msg));
> + deinit_crypto(snp_dev->crypto);
> + misc_deregister(&snp_dev->misc);
> +
> + return 0;
> +}
> +
> +static struct platform_driver snp_guest_driver = {
> + .remove = __exit_p(snp_guest_remove),
> + .driver = {
> + .name = "snp-guest",
> + },
> +};
> +
> +module_platform_driver_probe(snp_guest_driver, snp_guest_probe);
> +
> +MODULE_AUTHOR("Brijesh Singh <brijesh.singh@amd.com>");
> +MODULE_LICENSE("GPL");
> +MODULE_VERSION("1.0.0");
> +MODULE_DESCRIPTION("AMD SNP Guest Driver");
> diff --git a/drivers/virt/coco/sevguest/sevguest.h b/drivers/virt/coco/sevguest/sevguest.h
> new file mode 100644
> index 000000000000..cfa76cf8a21a
> --- /dev/null
> +++ b/drivers/virt/coco/sevguest/sevguest.h
> @@ -0,0 +1,98 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2021 Advanced Micro Devices, Inc.
> + *
> + * Author: Brijesh Singh <brijesh.singh@amd.com>
> + *
> + * SEV-SNP API spec is available at https://developer.amd.com/sev
> + */
> +
> +#ifndef __LINUX_SEVGUEST_H_

I guess VIRT_SEVGUEST_H is better fitting.

> +#define __LINUX_SEVGUEST_H_

...

--
Regards/Gruss,
Boris.