path: root/drivers/virt/coco/sev-guest/sev-guest.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * AMD Secure Encrypted Virtualization (SEV) guest driver interface
 *
 * Copyright (C) 2021-2024 Advanced Micro Devices, Inc.
 *
 * Author: Brijesh Singh <brijesh.singh@amd.com>
 */

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/types.h>
#include <linux/mutex.h>
#include <linux/io.h>
#include <linux/platform_device.h>
#include <linux/miscdevice.h>
#include <linux/set_memory.h>
#include <linux/fs.h>
#include <linux/tsm.h>
#include <crypto/aead.h>
#include <linux/scatterlist.h>
#include <linux/psp-sev.h>
#include <linux/sockptr.h>
#include <linux/cleanup.h>
#include <linux/uuid.h>
#include <linux/configfs.h>
#include <uapi/linux/sev-guest.h>
#include <uapi/linux/psp-sev.h>

#include <asm/svm.h>
#include <asm/sev.h>

#define DEVICE_NAME	"sev-guest"
#define AAD_LEN		48
#define MSG_HDR_VER	1

#define SNP_REQ_MAX_RETRY_DURATION	(60*HZ)
#define SNP_REQ_RETRY_DELAY		(2*HZ)

#define SVSM_MAX_RETRIES		3

struct snp_guest_crypto {
	struct crypto_aead *tfm;
	u8 *iv, *authtag;
	int iv_len, a_len;
};

struct snp_guest_dev {
	struct device *dev;
	struct miscdevice misc;

	void *certs_data;
	struct snp_guest_crypto *crypto;
	/* request and response are in unencrypted memory */
	struct snp_guest_msg *request, *response;

	/*
	 * Avoid information leakage by double-buffering shared messages
	 * in fields that are in regular encrypted memory.
	 */
	struct snp_guest_msg secret_request, secret_response;

	struct snp_secrets_page *secrets;
	struct snp_req_data input;
	union {
		struct snp_report_req report;
		struct snp_derived_key_req derived_key;
		struct snp_ext_report_req ext_report;
	} req;
	u32 *os_area_msg_seqno;
	u8 *vmpck;
};

/*
 * The VMPCK ID represents the key used by the SNP guest to communicate with the
 * SEV firmware in the AMD Secure Processor (ASP, aka PSP). By default, the key
 * used will be the key associated with the VMPL at which the guest is running.
 * Should the default key be wiped (see snp_disable_vmpck()), this parameter
 * allows for using one of the remaining VMPCKs.
 */
static int vmpck_id = -1;
module_param(vmpck_id, int, 0444);
MODULE_PARM_DESC(vmpck_id, "The VMPCK ID to use when communicating with the PSP.");

/* Mutex to serialize the shared buffer access and command handling. */
static DEFINE_MUTEX(snp_cmd_mutex);

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);

	return true;
}

/*
 * If an error is received from the host or AMD Secure Processor (ASP) there
 * are two options. Either retry the exact same encrypted request or discontinue
 * using the VMPCK.
 *
 * This is because in the current encryption scheme GHCB v2 uses AES-GCM to
 * encrypt the requests. The IV for this scheme is the sequence number. GCM
 * cannot tolerate IV reuse.
 *
 * The ASP FW v1.51 only increments the sequence numbers on a successful
 * guest<->ASP back and forth and only accepts messages at its exact sequence
 * number.
 *
 * So if the sequence number were to be reused the encryption scheme is
 * vulnerable. If the sequence number were incremented for a fresh IV the ASP
 * will reject the request.
 */
static void snp_disable_vmpck(struct snp_guest_dev *snp_dev)
{
	dev_alert(snp_dev->dev, "Disabling VMPCK%d communication key to prevent IV reuse.\n",
		  vmpck_id);
	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) {
		dev_err(snp_dev->dev, "request message sequence counter overflow\n");
		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