/*
 *  linux/net/sunrpc/gss_krb5_crypto.c
 *
 *  Copyright (c) 2000-2008 The Regents of the University of Michigan.
 *  All rights reserved.
 *
 *  Andy Adamson   <andros@umich.edu>
 *  Bruce Fields   <bfields@umich.edu>
 */

/*
 * Copyright (C) 1998 by the FundsXpress, INC.
 *
 * All rights reserved.
 *
 * Export of this software from the United States of America may require
 * a specific license from the United States Government.  It is the
 * responsibility of any person or organization contemplating export to
 * obtain such a license before exporting.
 *
 * WITHIN THAT CONSTRAINT, permission to use, copy, modify, and
 * distribute this software and its documentation for any purpose and
 * without fee is hereby granted, provided that the above copyright
 * notice appear in all copies and that both that copyright notice and
 * this permission notice appear in supporting documentation, and that
 * the name of FundsXpress. not be used in advertising or publicity pertaining
 * to distribution of the software without specific, written prior
 * permission.  FundsXpress makes no representations about the suitability of
 * this software for any purpose.  It is provided "as is" without express
 * or implied warranty.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
 * WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 */

#include <crypto/algapi.h>
#include <crypto/hash.h>
#include <crypto/skcipher.h>
#include <linux/err.h>
#include <linux/types.h>
#include <linux/mm.h>
#include <linux/scatterlist.h>
#include <linux/highmem.h>
#include <linux/pagemap.h>
#include <linux/random.h>
#include <linux/sunrpc/gss_krb5.h>
#include <linux/sunrpc/xdr.h>

#if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
# define RPCDBG_FACILITY        RPCDBG_AUTH
#endif

u32
krb5_encrypt(
        struct crypto_sync_skcipher *tfm,
        void * iv,
        void * in,
        void * out,
        int length)
{
        u32 ret = -EINVAL;
        struct scatterlist sg[1];
        u8 local_iv[GSS_KRB5_MAX_BLOCKSIZE] = {0};
        SYNC_SKCIPHER_REQUEST_ON_STACK(req, tfm);

        if (length % crypto_sync_skcipher_blocksize(tfm) != 0)
                goto out;

        if (crypto_sync_skcipher_ivsize(tfm) > GSS_KRB5_MAX_BLOCKSIZE) {
                dprintk("RPC:       gss_k5encrypt: tfm iv size too large %d\n",
                        crypto_sync_skcipher_ivsize(tfm));
                goto out;
        }

        if (iv)
                memcpy(local_iv, iv, crypto_sync_skcipher_ivsize(tfm));

        memcpy(out, in, length);
        sg_init_one(sg, out, length);

        skcipher_request_set_sync_tfm(req, tfm);
        skcipher_request_set_callback(req, 0, NULL, NULL);
        skcipher_request_set_crypt(req, sg, sg, length, local_iv);

        ret = crypto_skcipher_encrypt(req);
        skcipher_request_zero(req);
out:
        dprintk("RPC:       krb5_encrypt returns %d\n", ret);
        return ret;
}

u32
krb5_decrypt(
     struct crypto_sync_skcipher *tfm,
     void * iv,
     void * in,
     void * out,
     int length)
{
        u32 ret = -EINVAL;
        struct scatterlist sg[1];
        u8 local_iv[GSS_KRB5_MAX_BLOCKSIZE] = {0};
        SYNC_SKCIPHER_REQUEST_ON_STACK(req, tfm);

        if (length % crypto_sync_skcipher_blocksize(tfm) != 0)
                goto out;

        if (crypto_sync_skcipher_ivsize(tfm) > GSS_KRB5_MAX_BLOCKSIZE) {
                dprintk("RPC:       gss_k5decrypt: tfm iv size too large %d\n",
                        crypto_sync_skcipher_ivsize(tfm));
                goto out;
        }
        if (iv)
                memcpy(local_iv, iv, crypto_sync_skcipher_ivsize(tfm));

        memcpy(out, in, length);
        sg_init_one(sg, out, length);

        skcipher_request_set_sync_tfm(req, tfm);
        skcipher_request_set_callback(req, 0, NULL, NULL);
        skcipher_request_set_crypt(req, sg, sg, length, local_iv);

        ret = crypto_skcipher_decrypt(req);
        skcipher_request_zero(req);
out:
        dprintk("RPC:       gss_k5decrypt returns %d\n",ret);
        return ret;
}

static int
checksummer(struct scatterlist *sg, void *data)
{
        struct ahash_request *req = data;

        ahash_request_set_crypt(req, sg, NULL, sg->length);

        return crypto_ahash_update(req);
}

/*
 * checksum the plaintext data and hdrlen bytes of the token header
 * The checksum is performed over the first 8 bytes of the
 * gss token header and then over the data body
 */
u32
make_checksum(struct krb5_ctx *kctx, char *header, int hdrlen,
              struct xdr_buf *body, int body_offset, u8 *cksumkey,
              unsigned int usage, struct xdr_netobj *cksumout)
{
        struct crypto_ahash *tfm;
        struct ahash_request *req;
        struct scatterlist              sg[1];
        int err = -1;
        u8 *checksumdata;
        unsigned int checksumlen;

        if (cksumout->len < kctx->gk5e->cksumlength) {
                dprintk("%s: checksum buffer length, %u, too small for %s\n",
                        __func__, cksumout->len, kctx->gk5e->name);
                return GSS_S_FAILURE;
        }

        checksumdata = kmalloc(GSS_KRB5_MAX_CKSUM_LEN, GFP_NOFS);
        if (checksumdata == NULL)
                return GSS_S_FAILURE;

        tfm = crypto_alloc_ahash(kctx->gk5e->cksum_name, 0, CRYPTO_ALG_ASYNC);
        if (IS_ERR(tfm))
                goto out_free_cksum;

        req = ahash_request_alloc(tfm, GFP_NOFS);
        if (!req)
                goto out_free_ahash;

        ahash_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);

        checksumlen = crypto_ahash_digestsize(tfm);

        if (cksumkey != NULL) {
                err = crypto_ahash_setkey(tfm, cksumkey,
                                          kctx->gk5e->keylength);
                if (err)
                        goto out;
        }

        err = crypto_ahash_init(req);
        if (err)
                goto out;
        sg_init_one(sg, header, hdrlen);
        ahash_request_set_crypt(req, sg, NULL, hdrlen);
        err = crypto_ahash_update(req);
        if (err)
                goto out;
        err = xdr_process_buf(body, body_offset, body->len - body_offset,
                              checksummer, req);
        if (err)
                goto out;
        ahash_request_set_crypt(req, NULL, checksumdata, 0);
        err = crypto_ahash_final(req);
        if (err)
                goto out;

        switch (kctx->gk5e->ctype) {
        case CKSUMTYPE_RSA_MD5:
                err = kctx->gk5e->encrypt(kctx->seq, NULL, checksumdata,
                                          checksumdata, checksumlen);
                if (err)
                        goto out;
                memcpy(cksumout->data,
                       checksumdata + checksumlen - kctx->gk5e->cksumlength,
                       kctx->gk5e->cksumlength);
                break;
        case CKSUMTYPE_HMAC_SHA1_DES3:
                memcpy(cksumout->data, checksumdata, kctx->gk5e->cksumlength);
                break;
        default:
                BUG();
                break;
        }
        cksumout->len = kctx->gk5e->cksumlength;
out:
        ahash_request_free(req);
out_free_ahash:
        crypto_free_ahash(tfm);
out_free_cksum:
        kfree(checksumdata);
        return err ? GSS_S_FAILURE : 0;
}

/*
 * checksum the plaintext data and hdrlen bytes of the token header
 * Per rfc4121, sec. 4.2.4, the checksum is performed over the data
 * body then over the first 16 octets of the MIC token
 * Inclusion of the header data in the calculation of the
 * checksum is optional.
 */
u32
make_checksum_v2(struct krb5_ctx *kctx, char *header, int hdrlen,
                 struct xdr_buf *body, int body_offset, u8 *cksumkey,
                 unsigned int usage, struct xdr_netobj *cksumout)
{
        struct crypto_ahash *tfm;
        struct ahash_request *req;
        struct scatterlist sg[1];
        int err = -1;
        u8 *checksumdata;

        if (kctx->gk5e->keyed_cksum == 0) {
                dprintk("%s: expected keyed hash for %s\n",
                        __func__, kctx->gk5e->name);
                return GSS_S_FAILURE;
        }
        if (cksumkey == NULL) {
                dprintk("%s: no key supplied for %s\n",
                        __func__, kctx->gk5e->name);
                return GSS_S_FAILURE;
        }

        checksumdata = kmalloc(GSS_KRB5_MAX_CKSUM_LEN, GFP_NOFS);
        if (!checksumdata)
                return GSS_S_FAILURE;

        tfm = crypto_alloc_ahash(kctx->gk5e->cksum_name, 0, CRYPTO_ALG_ASYNC);
        if (IS_ERR(tfm))
                goto out_free_cksum;

        req = ahash_request_alloc(tfm, GFP_NOFS);
        if (!req)
                goto out_free_ahash;

        ahash_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);

        err = crypto_ahash_setkey(tfm, cksumkey, kctx->gk5e->keylength);
        if (err)
                goto out;

        err = crypto_ahash_init(req);
        if (err)
                goto out;
        err = xdr_process_buf(body, body_offset, body->len - body_offset,
                              checksummer, req);
        if (err)
                goto out;
        if (header != NULL) {
                sg_init_one(sg, header, hdrlen);
                ahash_request_set_crypt(req, sg, NULL, hdrlen);
                err = crypto_ahash_update(req);
                if (err)
                        goto out;
        }
        ahash_request_set_crypt(req, NULL, checksumdata, 0);
        err = crypto_ahash_final(req);
        if (err)
                goto out;

        cksumout->len = kctx->gk5e->cksumlength;

        switch (kctx->gk5e->ctype) {
        case CKSUMTYPE_HMAC_SHA1_96_AES128:
        case CKSUMTYPE_HMAC_SHA1_96_AES256:
                /* note that this truncates the hash */
                memcpy(cksumout->data, checksumdata, kctx->gk5e->cksumlength);
                break;
        default:
                BUG();
                break;
        }
out:
        ahash_request_free(req);
out_free_ahash:
        crypto_free_ahash(tfm);
out_free_cksum:
        kfree(checksumdata);
        return err ? GSS_S_FAILURE : 0;
}

struct encryptor_desc {
        u8 iv[GSS_KRB5_MAX_BLOCKSIZE];
        struct skcipher_request *req;
        int pos;
        struct xdr_buf *outbuf;
        struct page **pages;
        struct scatterlist infrags[4];
        struct scatterlist outfrags[4];
        int fragno;
        int fraglen;
};

static int
encryptor(struct scatterlist *sg, void *data)
{
        struct encryptor_desc *desc = data;
        struct xdr_buf *outbuf = desc->outbuf;
        struct crypto_sync_skcipher *tfm =
                crypto_sync_skcipher_reqtfm(desc->req);
        struct page *in_page;
        int thislen = desc->fraglen + sg->length;
        int fraglen, ret;
        int page_pos;

        /* Worst case is 4 fragments: head, end of page 1, start
         * of page 2, tail.  Anything more is a bug. */
        BUG_ON(desc->fragno > 3);

        page_pos = desc->pos - outbuf->head[0].iov_len;
        if (page_pos >= 0 && page_pos < outbuf->page_len) {
                /* pages are not in place: */
                int i = (page_pos + outbuf->page_base) >> PAGE_SHIFT;
                in_page = desc->pages[i];
        } else {
                in_page = sg_page(sg);
        }
        sg_set_page(&desc->infrags[desc->fragno], in_page, sg->length,
                    sg->offset);
        sg_set_page(&desc->outfrags[desc->fragno], sg_page(sg), sg->length,
                    sg->offset);
        desc->fragno++;
        desc->fraglen += sg->length;
        desc->pos += sg->length;

        fraglen = thislen & (crypto_sync_skcipher_blocksize(tfm) - 1);
        thislen -= fraglen;

        if (thislen == 0)
                return 0;

        sg_mark_end(&desc->infrags[desc->fragno - 1]);
        sg_mark_end(&desc->outfrags[desc->fragno - 1]);

        skcipher_request_set_crypt(desc->req, desc->infrags, desc->outfrags,
                                   thislen, desc->iv);

        ret = crypto_skcipher_encrypt(desc->req);
        if (ret)
                return ret;

        sg_init_table(desc->infrags, 4);
        sg_init_table(desc->outfrags, 4);

        if (fraglen) {
                sg_set_page(&desc->outfrags[0], sg_page(sg), fraglen,
                                sg->offset + sg->length - fraglen);
                desc->infrags[0] = desc->outfrags[0];
                sg_assign_page(&desc->infrags[0], in_page);
                desc->fragno = 1;
                desc->fraglen = fraglen;
        } else {
                desc->fragno = 0;
                desc->fraglen = 0;
        }
        return 0;
}

int
gss_encrypt_xdr_buf(struct crypto_sync_skcipher *tfm, struct xdr_buf *buf,
                    int offset, struct page **pages)
{
        int ret;
        struct encryptor_desc desc;
        SYNC_SKCIPHER_REQUEST_ON_STACK(req, tfm);

        BUG_ON((buf->len - offset) % crypto_sync_skcipher_blocksize(tfm) != 0);

        skcipher_request_set_sync_tfm(req, tfm);
        skcipher_request_set_callback(req, 0, NULL, NULL);

        memset(desc.iv, 0, sizeof(desc.iv));
        desc.req = req;
        desc.pos = offset;
        desc.outbuf = buf;
        desc.pages = pages;
        desc.fragno = 0;
        desc.fraglen = 0;

        sg_init_table(desc.infrags, 4);
        sg_init_table(desc.outfrags, 4);

        ret = xdr_process_buf(buf, offset, buf->len - offset, encryptor, &desc);
        skcipher_request_zero(req);
        return ret;
}

struct decryptor_desc {
        u8 iv[GSS_KRB5_MAX_BLOCKSIZE];
        struct skcipher_request *req;
        struct scatterlist frags[4];
        int fragno;
        int fraglen;
};

static int
decryptor(struct scatterlist *sg, void *data)
{
        struct decryptor_desc *desc = data;
        int thislen = desc->fraglen + sg->length;
        struct crypto_sync_skcipher *tfm =
                crypto_sync_skcipher_reqtfm(desc->req);
        int fraglen, ret;

        /* Worst case is 4 fragments: head, end of page 1, start
         * of page 2, tail.  Anything more is a bug. */
        BUG_ON(desc->fragno > 3);
        sg_set_page(&desc->frags[desc->fragno], sg_page(sg), sg->length,
                    sg->offset);
        desc->fragno++;
        desc->fraglen += sg->length;

        fraglen = thislen & (crypto_sync_skcipher_blocksize(tfm) - 1);
        thislen -= fraglen;

        if (thislen == 0)
                return 0;

        sg_mark_end(&desc->frags[desc->fragno - 1]);

        skcipher_request_set_crypt(desc->req, desc->frags, desc->frags,
                                   thislen, desc->iv);

        ret = crypto_skcipher_decrypt(desc->req);
        if (ret)
                return ret;

        sg_init_table(desc->frags, 4);

        if (fraglen) {
                sg_set_page(&desc->frags[0], sg_page(sg), fraglen,
                                sg->offset + sg->length - fraglen);
                desc->fragno = 1;
                desc->fraglen = fraglen;
        } else {
                desc->fragno = 0;
                desc->fraglen = 0;
        }
        return 0;
}

int
gss_decrypt_xdr_buf(struct crypto_sync_skcipher *tfm, struct xdr_buf *buf,
                    int offset)
{
        int ret;
        struct decryptor_desc desc;
        SYNC_SKCIPHER_REQUEST_ON_STACK(req, tfm);

        /* XXXJBF: */
        BUG_ON((buf->len - offset) % crypto_sync_skcipher_blocksize(tfm) != 0);

        skcipher_request_set_sync_tfm(req, tfm);
        skcipher_request_set_callback(req, 0, NULL, NULL);

        memset(desc.iv, 0, sizeof(desc.iv));
        desc.req = req;
        desc.fragno = 0;
        desc.fraglen = 0;

        sg_init_table(desc.frags, 4);

        ret = xdr_process_buf(buf, offset, buf->len - offset, decryptor, &desc);
        skcipher_request_zero(req);
        return ret;
}

/*
 * This function makes the assumption that it was ultimately called
 * from gss_wrap().
 *
 * The client auth_gss code moves any existing tail data into a
 * separate page before calling gss_wrap.
 * The server svcauth_gss code ensures that both the head and the
 * tail have slack space of RPC_MAX_AUTH_SIZE before calling gss_wrap.
 *
 * Even with that guarantee, this function may be called more than
 * once in the processing of gss_wrap().  The best we can do is
 * verify at compile-time (see GSS_KRB5_SLACK_CHECK) that the
 * largest expected shift will fit within RPC_MAX_AUTH_SIZE.
 * At run-time we can verify that a single invocation of this
 * function doesn't attempt to use more the RPC_MAX_AUTH_SIZE.
 */

int
xdr_extend_head(struct xdr_buf *buf, unsigned int base, unsigned int shiftlen)
{
        u8 *p;

        if (shiftlen == 0)
                return 0;

        BUILD_BUG_ON(GSS_KRB5_MAX_SLACK_NEEDED > RPC_MAX_AUTH_SIZE);
        BUG_ON(shiftlen > RPC_MAX_AUTH_SIZE);

        p = buf->head[0].iov_base + base;

        memmove(p + shiftlen, p, buf->head[0].iov_len - base);

        buf->head[0].iov_len += shiftlen;
        buf->len += shiftlen;

        return 0;
}

static u32
gss_krb5_cts_crypt(struct crypto_sync_skcipher *cipher, struct xdr_buf *buf,
                   u32 offset, u8 *iv, struct page **pages, int encrypt)
{
        u32 ret;
        struct scatterlist sg[1];
        SYNC_SKCIPHER_REQUEST_ON_STACK(req, cipher);
        u8 *data;
        struct page **save_pages;
        u32 len = buf->len - offset;

        if (len > GSS_KRB5_MAX_BLOCKSIZE * 2) {
                WARN_ON(0);
                return -ENOMEM;
        }
        data = kmalloc(GSS_KRB5_MAX_BLOCKSIZE * 2, GFP_NOFS);
        if (!data)
                return -ENOMEM;

        /*
         * For encryption, we want to read from the cleartext
         * page cache pages, and write the encrypted data to
         * the supplied xdr_buf pages.
         */
        save_pages = buf->pages;
        if (encrypt)
                buf->pages = pages;

        ret = read_bytes_from_xdr_buf(buf, offset, data, len);
        buf->pages = save_pages;
        if (ret)
                goto out;

        sg_init_one(sg, data, len);

        skcipher_request_set_sync_tfm(req, cipher);
        skcipher_request_set_callback(req, 0, NULL, NULL);
        skcipher_request_set_crypt(req, sg, sg, len, iv);

        if (encrypt)
                ret = crypto_skcipher_encrypt(req);
        else
                ret = crypto_skcipher_decrypt(req);

        skcipher_request_zero(req);

        if (ret)
                goto out;

        ret = write_bytes_to_xdr_buf(buf, offset, data, len);

out:
        kfree(data);
        return ret;
}

u32
gss_krb5_aes_encrypt(struct krb5_ctx *kctx, u32 offset,
                     struct xdr_buf *buf, struct page **pages)
{
        u32 err;
        struct xdr_netobj hmac;
        u8 *cksumkey;
        u8 *ecptr;
        struct crypto_sync_skcipher *cipher, *aux_cipher;
        int blocksize;
        struct page **save_pages;
        int nblocks, nbytes;
        struct encryptor_desc desc;
        u32 cbcbytes;
        unsigned int usage;

        if (kctx->initiate) {
                cipher = kctx->initiator_enc;
                aux_cipher = kctx->initiator_enc_aux;
                cksumkey = kctx->initiator_integ;
                usage = KG_USAGE_INITIATOR_SEAL;
        } else {
                cipher = kctx->acceptor_enc;
                aux_cipher = kctx->acceptor_enc_aux;
                cksumkey = kctx->acceptor_integ;
                usage = KG_USAGE_ACCEPTOR_SEAL;
        }
        blocksize = crypto_sync_skcipher_blocksize(cipher);

        /* hide the gss token header and insert the confounder */
        offset += GSS_KRB5_TOK_HDR_LEN;
        if (xdr_extend_head(buf, offset, kctx->gk5e->conflen))
                return GSS_S_FAILURE;
        gss_krb5_make_confounder(buf->head[0].iov_base + offset, kctx->gk5e->conflen);
        offset -= GSS_KRB5_TOK_HDR_LEN;

        if (buf->tail[0].iov_base != NULL) {
                ecptr = buf->tail[0].iov_base + buf->tail[0].iov_len;
        } else {
                buf->tail[0].iov_base = buf->head[0].iov_base
                                                        + buf->head[0].iov_len;
                buf->tail[0].iov_len = 0;
                ecptr = buf->tail[0].iov_base;
        }

        /* copy plaintext gss token header after filler (if any) */
        memcpy(ecptr, buf->head[0].iov_base + offset, GSS_KRB5_TOK_HDR_LEN);
        buf->tail[0].iov_len += GSS_KRB5_TOK_HDR_LEN;
        buf->len += GSS_KRB5_TOK_HDR_LEN;

        /* Do the HMAC */
        hmac.len = GSS_KRB5_MAX_CKSUM_LEN;
        hmac.data = buf->tail[0].iov_base + buf->tail[0].iov_len;

        /*
         * When we are called, pages points to the real page cache
         * data -- which we can't go and encrypt!  buf->pages points
         * to scratch pages which we are going to send off to the
         * client/server.  Swap in the plaintext pages to calculate
         * the hmac.
         */
        save_pages = buf->pages;
        buf->pages = pages;

        err = make_checksum_v2(kctx, NULL, 0, buf,
                               offset + GSS_KRB5_TOK_HDR_LEN,
                               cksumkey, usage, &hmac);
        buf->pages = save_pages;
        if (err)
                return GSS_S_FAILURE;

        nbytes = buf->len - offset - GSS_KRB5_TOK_HDR_LEN;
        nblocks = (nbytes + blocksize - 1) / blocksize;
        cbcbytes = 0;
        if (nblocks > 2)
                cbcbytes = (nblocks - 2) * blocksize;

        memset(desc.iv, 0, sizeof(desc.iv));

        if (cbcbytes) {
                SYNC_SKCIPHER_REQUEST_ON_STACK(req, aux_cipher);

                desc.pos = offset + GSS_KRB5_TOK_HDR_LEN;
                desc.fragno = 0;
                desc.fraglen = 0;
                desc.pages = pages;
                desc.outbuf = buf;
                desc.req = req;

                skcipher_request_set_sync_tfm(req, aux_cipher);
                skcipher_request_set_callback(req, 0, NULL, NULL);

                sg_init_table(desc.infrags, 4);
                sg_init_table(desc.outfrags, 4);

                err = xdr_process_buf(buf, offset + GSS_KRB5_TOK_HDR_LEN,
                                      cbcbytes, encryptor, &desc);
                skcipher_request_zero(req);
                if (err)
                        goto out_err;
        }

        /* Make sure IV carries forward from any CBC results. */
        err = gss_krb5_cts_crypt(cipher, buf,
                                 offset + GSS_KRB5_TOK_HDR_LEN + cbcbytes,
                                 desc.iv, pages, 1);
        if (err) {
                err = GSS_S_FAILURE;
                goto out_err;
        }

        /* Now update buf to account for HMAC */
        buf->tail[0].iov_len += kctx->gk5e->cksumlength;
        buf->len += kctx->gk5e->cksumlength;

out_err:
        if (err)
                err = GSS_S_FAILURE;
        return err;
}

u32
gss_krb5_aes_decrypt(struct krb5_ctx *kctx, u32 offset, u32 len,
                     struct xdr_buf *buf, u32 *headskip, u32 *tailskip)
{
        struct xdr_buf subbuf;
        u32 ret = 0;
        u8 *cksum_key;
        struct crypto_sync_skcipher *cipher, *aux_cipher;
        struct xdr_netobj our_hmac_obj;
        u8 our_hmac[GSS_KRB5_MAX_CKSUM_LEN];
        u8 pkt_hmac[GSS_KRB5_MAX_CKSUM_LEN];
        int nblocks, blocksize, cbcbytes;
        struct decryptor_desc desc;
        unsigned int usage;

        if (kctx->initiate) {
                cipher = kctx->acceptor_enc;
                aux_cipher = kctx->acceptor_enc_aux;
                cksum_key = kctx->acceptor_integ;
                usage = KG_USAGE_ACCEPTOR_SEAL;
        } else {
                cipher = kctx->initiator_enc;
                aux_cipher = kctx->initiator_enc_aux;
                cksum_key = kctx->initiator_integ;
                usage = KG_USAGE_INITIATOR_SEAL;
        }
        blocksize = crypto_sync_skcipher_blocksize(cipher);


        /* create a segment skipping the header and leaving out the checksum */
        xdr_buf_subsegment(buf, &subbuf, offset + GSS_KRB5_TOK_HDR_LEN,
                                    (len - offset - GSS_KRB5_TOK_HDR_LEN -
                                     kctx->gk5e->cksumlength));

        nblocks = (subbuf.len + blocksize - 1) / blocksize;

        cbcbytes = 0;
        if (nblocks > 2)
                cbcbytes = (nblocks - 2) * blocksize;

        memset(desc.iv, 0, sizeof(desc.iv));

        if (cbcbytes) {
                SYNC_SKCIPHER_REQUEST_ON_STACK(req, aux_cipher);

                desc.fragno = 0;
                desc.fraglen = 0;
                desc.req = req;

                skcipher_request_set_sync_tfm(req, aux_cipher);
                skcipher_request_set_callback(req, 0, NULL, NULL);

                sg_init_table(desc.frags, 4);

                ret = xdr_process_buf(&subbuf, 0, cbcbytes, decryptor, &desc);
                skcipher_request_zero(req);
                if (ret)
                        goto out_err;
        }

        /* Make sure IV carries forward from any CBC results. */
        ret = gss_krb5_cts_crypt(cipher, &subbuf, cbcbytes, desc.iv, NULL, 0);
        if (ret)
                goto out_err;


        /* Calculate our hmac over the plaintext data */
        our_hmac_obj.len = sizeof(our_hmac);
        our_hmac_obj.data = our_hmac;

        ret = make_checksum_v2(kctx, NULL, 0, &subbuf, 0,
                               cksum_key, usage, &our_hmac_obj);
        if (ret)
                goto out_err;

        /* Get the packet's hmac value */
        ret = read_bytes_from_xdr_buf(buf, len - kctx->gk5e->cksumlength,
                                      pkt_hmac, kctx->gk5e->cksumlength);
        if (ret)
                goto out_err;

        if (crypto_memneq(pkt_hmac, our_hmac, kctx->gk5e->cksumlength) != 0) {
                ret = GSS_S_BAD_SIG;
                goto out_err;
        }
        *headskip = kctx->gk5e->conflen;
        *tailskip = kctx->gk5e->cksumlength;
out_err:
        if (ret && ret != GSS_S_BAD_SIG)
                ret = GSS_S_FAILURE;
        return ret;
}