// SPDX-License-Identifier: GPL-2.0
/*  OpenVPN data channel offload
 *
 *  Copyright (C) 2020-2025 OpenVPN, Inc.
 *
 *  Author:     James Yonan <james@openvpn.net>
 *              Antonio Quartulli <antonio@openvpn.net>
 */

#include <crypto/aead.h>
#include <linux/skbuff.h>
#include <net/ip.h>
#include <net/ipv6.h>
#include <net/udp.h>

#include "ovpnpriv.h"
#include "main.h"
#include "io.h"
#include "pktid.h"
#include "crypto_aead.h"
#include "crypto.h"
#include "peer.h"
#include "proto.h"
#include "skb.h"

#define OVPN_AUTH_TAG_SIZE      16
#define OVPN_AAD_SIZE           (OVPN_OPCODE_SIZE + OVPN_NONCE_WIRE_SIZE)

#define ALG_NAME_AES            "gcm(aes)"
#define ALG_NAME_CHACHAPOLY     "rfc7539(chacha20,poly1305)"

static int ovpn_aead_encap_overhead(const struct ovpn_crypto_key_slot *ks)
{
        return  OVPN_OPCODE_SIZE +                      /* OP header size */
                sizeof(u32) +                           /* Packet ID */
                crypto_aead_authsize(ks->encrypt);      /* Auth Tag */
}

/**
 * ovpn_aead_crypto_tmp_size - compute the size of a temporary object containing
 *                             an AEAD request structure with extra space for SG
 *                             and IV.
 * @tfm: the AEAD cipher handle
 * @nfrags: the number of fragments in the skb
 *
 * This function calculates the size of a contiguous memory block that includes
 * the initialization vector (IV), the AEAD request, and an array of scatterlist
 * entries. For alignment considerations, the IV is placed first, followed by
 * the request, and then the scatterlist.
 * Additional alignment is applied according to the requirements of the
 * underlying structures.
 *
 * Return: the size of the temporary memory that needs to be allocated
 */
static unsigned int ovpn_aead_crypto_tmp_size(struct crypto_aead *tfm,
                                              const unsigned int nfrags)
{
        unsigned int len = OVPN_NONCE_SIZE;

        DEBUG_NET_WARN_ON_ONCE(crypto_aead_ivsize(tfm) != OVPN_NONCE_SIZE);

        /* min size for a buffer of ivsize, aligned to alignmask */
        len += crypto_aead_alignmask(tfm) & ~(crypto_tfm_ctx_alignment() - 1);
        /* round up to the next multiple of the crypto ctx alignment */
        len = ALIGN(len, crypto_tfm_ctx_alignment());

        /* reserve space for the AEAD request */
        len += sizeof(struct aead_request) + crypto_aead_reqsize(tfm);
        /* round up to the next multiple of the scatterlist alignment */
        len = ALIGN(len, __alignof__(struct scatterlist));

        /* add enough space for nfrags + 2 scatterlist entries */
        len += array_size(sizeof(struct scatterlist), nfrags + 2);
        return len;
}

/**
 * ovpn_aead_crypto_tmp_iv - retrieve the pointer to the IV within a temporary
 *                           buffer allocated using ovpn_aead_crypto_tmp_size
 * @aead: the AEAD cipher handle
 * @tmp: a pointer to the beginning of the temporary buffer
 *
 * This function retrieves a pointer to the initialization vector (IV) in the
 * temporary buffer. If the AEAD cipher specifies an IV size, the pointer is
 * adjusted using the AEAD's alignment mask to ensure proper alignment.
 *
 * Returns: a pointer to the IV within the temporary buffer
 */
static u8 *ovpn_aead_crypto_tmp_iv(struct crypto_aead *aead, void *tmp)
{
        return likely(crypto_aead_ivsize(aead)) ?
                      PTR_ALIGN((u8 *)tmp, crypto_aead_alignmask(aead) + 1) :
                      tmp;
}

/**
 * ovpn_aead_crypto_tmp_req - retrieve the pointer to the AEAD request structure
 *                            within a temporary buffer allocated using
 *                            ovpn_aead_crypto_tmp_size
 * @aead: the AEAD cipher handle
 * @iv: a pointer to the initialization vector in the temporary buffer
 *
 * This function computes the location of the AEAD request structure that
 * immediately follows the IV in the temporary buffer and it ensures the request
 * is aligned to the crypto transform context alignment.
 *
 * Returns: a pointer to the AEAD request structure
 */
static struct aead_request *ovpn_aead_crypto_tmp_req(struct crypto_aead *aead,
                                                     const u8 *iv)
{
        return (void *)PTR_ALIGN(iv + crypto_aead_ivsize(aead),
                                 crypto_tfm_ctx_alignment());
}

/**
 * ovpn_aead_crypto_req_sg - locate the scatterlist following the AEAD request
 *                           within a temporary buffer allocated using
 *                           ovpn_aead_crypto_tmp_size
 * @aead: the AEAD cipher handle
 * @req: a pointer to the AEAD request structure in the temporary buffer
 *
 * This function computes the starting address of the scatterlist that is
 * allocated immediately after the AEAD request structure. It aligns the pointer
 * based on the alignment requirements of the scatterlist structure.
 *
 * Returns: a pointer to the scatterlist
 */
static struct scatterlist *ovpn_aead_crypto_req_sg(struct crypto_aead *aead,
                                                   struct aead_request *req)
{
        return (void *)ALIGN((unsigned long)(req + 1) +
                             crypto_aead_reqsize(aead),
                             __alignof__(struct scatterlist));
}

int ovpn_aead_encrypt(struct ovpn_peer *peer, struct ovpn_crypto_key_slot *ks,
                      struct sk_buff *skb)
{
        const unsigned int tag_size = crypto_aead_authsize(ks->encrypt);
        struct aead_request *req;
        struct sk_buff *trailer;
        struct scatterlist *sg;
        int nfrags, ret;
        u32 pktid, op;
        void *tmp;
        u8 *iv;

        ovpn_skb_cb(skb)->peer = peer;
        ovpn_skb_cb(skb)->ks = ks;

        /* Sample AEAD header format:
         * 48000001 00000005 7e7046bd 444a7e28 cc6387b1 64a4d6c1 380275a...
         * [ OP32 ] [seq # ] [             auth tag            ] [ payload ... ]
         *          [4-byte
         *          IV head]
         */

        /* check that there's enough headroom in the skb for packet
         * encapsulation
         */
        if (unlikely(skb_cow_head(skb, OVPN_HEAD_ROOM)))
                return -ENOBUFS;

        /* get number of skb frags and ensure that packet data is writable */
        nfrags = skb_cow_data(skb, 0, &trailer);
        if (unlikely(nfrags < 0))
                return nfrags;

        if (unlikely(nfrags + 2 > (MAX_SKB_FRAGS + 2)))
                return -ENOSPC;

        /* allocate temporary memory for iv, sg and req */
        tmp = kmalloc(ovpn_aead_crypto_tmp_size(ks->encrypt, nfrags),
                      GFP_ATOMIC);
        if (unlikely(!tmp))
                return -ENOMEM;

        ovpn_skb_cb(skb)->crypto_tmp = tmp;

        iv = ovpn_aead_crypto_tmp_iv(ks->encrypt, tmp);
        req = ovpn_aead_crypto_tmp_req(ks->encrypt, iv);
        sg = ovpn_aead_crypto_req_sg(ks->encrypt, req);

        /* sg table:
         * 0: op, wire nonce (AD, len=OVPN_OP_SIZE_V2+OVPN_NONCE_WIRE_SIZE),
         * 1, 2, 3, ..., n: payload,
         * n+1: auth_tag (len=tag_size)
         */
        sg_init_table(sg, nfrags + 2);

        /* build scatterlist to encrypt packet payload */
        ret = skb_to_sgvec_nomark(skb, sg + 1, 0, skb->len);
        if (unlikely(ret < 0)) {
                netdev_err(peer->ovpn->dev,
                           "encrypt: cannot map skb to sg: %d\n", ret);
                return ret;
        }

        /* append auth_tag onto scatterlist */
        __skb_push(skb, tag_size);
        sg_set_buf(sg + ret + 1, skb->data, tag_size);

        /* obtain packet ID, which is used both as a first
         * 4 bytes of nonce and last 4 bytes of associated data.
         */
        ret = ovpn_pktid_xmit_next(&ks->pid_xmit, &pktid);
        if (unlikely(ret < 0))
                return ret;

        /* concat 4 bytes packet id and 8 bytes nonce tail into 12 bytes
         * nonce
         */
        ovpn_pktid_aead_write(pktid, ks->nonce_tail_xmit, iv);

        /* make space for packet id and push it to the front */
        __skb_push(skb, OVPN_NONCE_WIRE_SIZE);
        memcpy(skb->data, iv, OVPN_NONCE_WIRE_SIZE);

        /* add packet op as head of additional data */
        op = ovpn_opcode_compose(OVPN_DATA_V2, ks->key_id, peer->tx_id);
        __skb_push(skb, OVPN_OPCODE_SIZE);
        BUILD_BUG_ON(sizeof(op) != OVPN_OPCODE_SIZE);
        *((__force __be32 *)skb->data) = htonl(op);

        /* AEAD Additional data */
        sg_set_buf(sg, skb->data, OVPN_AAD_SIZE);

        /* setup async crypto operation */
        aead_request_set_tfm(req, ks->encrypt);
        aead_request_set_callback(req, 0, ovpn_encrypt_post, skb);
        aead_request_set_crypt(req, sg, sg,
                               skb->len - ovpn_aead_encap_overhead(ks), iv);
        aead_request_set_ad(req, OVPN_AAD_SIZE);

        /* encrypt it */
        return crypto_aead_encrypt(req);
}

int ovpn_aead_decrypt(struct ovpn_peer *peer, struct ovpn_crypto_key_slot *ks,
                      struct sk_buff *skb)
{
        const unsigned int tag_size = crypto_aead_authsize(ks->decrypt);
        int ret, payload_len, nfrags;
        unsigned int payload_offset;
        struct aead_request *req;
        struct sk_buff *trailer;
        struct scatterlist *sg;
        void *tmp;
        u8 *iv;

        payload_offset = OVPN_AAD_SIZE + tag_size;
        payload_len = skb->len - payload_offset;

        ovpn_skb_cb(skb)->payload_offset = payload_offset;
        ovpn_skb_cb(skb)->peer = peer;
        ovpn_skb_cb(skb)->ks = ks;

        /* sanity check on packet size, payload size must be >= 0 */
        if (unlikely(payload_len < 0))
                return -EINVAL;

        /* Prepare the skb data buffer to be accessed up until the auth tag.
         * This is required because this area is directly mapped into the sg
         * list.
         */
        if (unlikely(!pskb_may_pull(skb, payload_offset)))
                return -ENODATA;

        /* get number of skb frags and ensure that packet data is writable */
        nfrags = skb_cow_data(skb, 0, &trailer);
        if (unlikely(nfrags < 0))
                return nfrags;

        if (unlikely(nfrags + 2 > (MAX_SKB_FRAGS + 2)))
                return -ENOSPC;

        /* allocate temporary memory for iv, sg and req */
        tmp = kmalloc(ovpn_aead_crypto_tmp_size(ks->decrypt, nfrags),
                      GFP_ATOMIC);
        if (unlikely(!tmp))
                return -ENOMEM;

        ovpn_skb_cb(skb)->crypto_tmp = tmp;

        iv = ovpn_aead_crypto_tmp_iv(ks->decrypt, tmp);
        req = ovpn_aead_crypto_tmp_req(ks->decrypt, iv);
        sg = ovpn_aead_crypto_req_sg(ks->decrypt, req);

        /* sg table:
         * 0: op, wire nonce (AD, len=OVPN_OPCODE_SIZE+OVPN_NONCE_WIRE_SIZE),
         * 1, 2, 3, ..., n: payload,
         * n+1: auth_tag (len=tag_size)
         */
        sg_init_table(sg, nfrags + 2);

        /* packet op is head of additional data */
        sg_set_buf(sg, skb->data, OVPN_AAD_SIZE);

        /* build scatterlist to decrypt packet payload */
        ret = skb_to_sgvec_nomark(skb, sg + 1, payload_offset, payload_len);
        if (unlikely(ret < 0)) {
                netdev_err(peer->ovpn->dev,
                           "decrypt: cannot map skb to sg: %d\n", ret);
                return ret;
        }

        /* append auth_tag onto scatterlist */
        sg_set_buf(sg + ret + 1, skb->data + OVPN_AAD_SIZE, tag_size);

        /* copy nonce into IV buffer */
        memcpy(iv, skb->data + OVPN_OPCODE_SIZE, OVPN_NONCE_WIRE_SIZE);
        memcpy(iv + OVPN_NONCE_WIRE_SIZE, ks->nonce_tail_recv,
               OVPN_NONCE_TAIL_SIZE);

        /* setup async crypto operation */
        aead_request_set_tfm(req, ks->decrypt);
        aead_request_set_callback(req, 0, ovpn_decrypt_post, skb);
        aead_request_set_crypt(req, sg, sg, payload_len + tag_size, iv);

        aead_request_set_ad(req, OVPN_AAD_SIZE);

        /* decrypt it */
        return crypto_aead_decrypt(req);
}

/* Initialize a struct crypto_aead object */
static struct crypto_aead *ovpn_aead_init(const char *title,
                                          const char *alg_name,
                                          const unsigned char *key,
                                          unsigned int keylen)
{
        struct crypto_aead *aead;
        int ret;

        aead = crypto_alloc_aead(alg_name, 0, 0);
        if (IS_ERR(aead)) {
                ret = PTR_ERR(aead);
                pr_err("%s crypto_alloc_aead failed, err=%d\n", title, ret);
                aead = NULL;
                goto error;
        }

        ret = crypto_aead_setkey(aead, key, keylen);
        if (ret) {
                pr_err("%s crypto_aead_setkey size=%u failed, err=%d\n", title,
                       keylen, ret);
                goto error;
        }

        ret = crypto_aead_setauthsize(aead, OVPN_AUTH_TAG_SIZE);
        if (ret) {
                pr_err("%s crypto_aead_setauthsize failed, err=%d\n", title,
                       ret);
                goto error;
        }

        /* basic AEAD assumption
         * all current algorithms use OVPN_NONCE_SIZE.
         * ovpn_aead_crypto_tmp_size and ovpn_aead_encrypt/decrypt
         * expect this.
         */
        if (crypto_aead_ivsize(aead) != OVPN_NONCE_SIZE) {
                pr_err("%s IV size must be %d\n", title, OVPN_NONCE_SIZE);
                ret = -EINVAL;
                goto error;
        }

        pr_debug("********* Cipher %s (%s)\n", alg_name, title);
        pr_debug("*** IV size=%u\n", crypto_aead_ivsize(aead));
        pr_debug("*** req size=%u\n", crypto_aead_reqsize(aead));
        pr_debug("*** block size=%u\n", crypto_aead_blocksize(aead));
        pr_debug("*** auth size=%u\n", crypto_aead_authsize(aead));
        pr_debug("*** alignmask=0x%x\n", crypto_aead_alignmask(aead));

        return aead;

error:
        crypto_free_aead(aead);
        return ERR_PTR(ret);
}

void ovpn_aead_crypto_key_slot_destroy(struct ovpn_crypto_key_slot *ks)
{
        if (!ks)
                return;

        crypto_free_aead(ks->encrypt);
        crypto_free_aead(ks->decrypt);
        kfree(ks);
}

struct ovpn_crypto_key_slot *
ovpn_aead_crypto_key_slot_new(const struct ovpn_key_config *kc)
{
        struct ovpn_crypto_key_slot *ks = NULL;
        const char *alg_name;
        int ret;

        /* validate crypto alg */
        switch (kc->cipher_alg) {
        case OVPN_CIPHER_ALG_AES_GCM:
                alg_name = ALG_NAME_AES;
                break;
        case OVPN_CIPHER_ALG_CHACHA20_POLY1305:
                alg_name = ALG_NAME_CHACHAPOLY;
                break;
        default:
                return ERR_PTR(-EOPNOTSUPP);
        }

        if (kc->encrypt.nonce_tail_size != OVPN_NONCE_TAIL_SIZE ||
            kc->decrypt.nonce_tail_size != OVPN_NONCE_TAIL_SIZE)
                return ERR_PTR(-EINVAL);

        /* build the key slot */
        ks = kmalloc_obj(*ks);
        if (!ks)
                return ERR_PTR(-ENOMEM);

        ks->encrypt = NULL;
        ks->decrypt = NULL;
        kref_init(&ks->refcount);
        ks->key_id = kc->key_id;

        ks->encrypt = ovpn_aead_init("encrypt", alg_name,
                                     kc->encrypt.cipher_key,
                                     kc->encrypt.cipher_key_size);
        if (IS_ERR(ks->encrypt)) {
                ret = PTR_ERR(ks->encrypt);
                ks->encrypt = NULL;
                goto destroy_ks;
        }

        ks->decrypt = ovpn_aead_init("decrypt", alg_name,
                                     kc->decrypt.cipher_key,
                                     kc->decrypt.cipher_key_size);
        if (IS_ERR(ks->decrypt)) {
                ret = PTR_ERR(ks->decrypt);
                ks->decrypt = NULL;
                goto destroy_ks;
        }

        memcpy(ks->nonce_tail_xmit, kc->encrypt.nonce_tail,
               OVPN_NONCE_TAIL_SIZE);
        memcpy(ks->nonce_tail_recv, kc->decrypt.nonce_tail,
               OVPN_NONCE_TAIL_SIZE);

        /* init packet ID generation/validation */
        ovpn_pktid_xmit_init(&ks->pid_xmit);
        ovpn_pktid_recv_init(&ks->pid_recv);

        return ks;

destroy_ks:
        ovpn_aead_crypto_key_slot_destroy(ks);
        return ERR_PTR(ret);
}

enum ovpn_cipher_alg ovpn_aead_crypto_alg(struct ovpn_crypto_key_slot *ks)
{
        const char *alg_name;

        if (!ks->encrypt)
                return OVPN_CIPHER_ALG_NONE;

        alg_name = crypto_tfm_alg_name(crypto_aead_tfm(ks->encrypt));

        if (!strcmp(alg_name, ALG_NAME_AES))
                return OVPN_CIPHER_ALG_AES_GCM;
        else if (!strcmp(alg_name, ALG_NAME_CHACHAPOLY))
                return OVPN_CIPHER_ALG_CHACHA20_POLY1305;
        else
                return OVPN_CIPHER_ALG_NONE;
}