net/sunrpc/auth_gss/gss_krb5_wrap.c - kernel/msm-4.9 - Gitiles

 #include <linux/types.h>
 #include <linux/slab.h>
 #include <linux/jiffies.h>
 #include <linux/sunrpc/gss_krb5.h>
 #include <linux/random.h>
 #include <linux/pagemap.h>
 #include <asm/scatterlist.h>
 #include <linux/crypto.h>

 #ifdef RPC_DEBUG
 # define RPCDBG_FACILITY	RPCDBG_AUTH
 #endif

 static inline int
 gss_krb5_padding(int blocksize, int length)
 {
 	/* Most of the code is block-size independent but currently we
 	 * use only 8: */
 	BUG_ON(blocksize != 8);
 	return 8 - (length & 7);
 }

 static inline void
 gss_krb5_add_padding(struct xdr_buf *buf, int offset, int blocksize)
 {
 	int padding = gss_krb5_padding(blocksize, buf->len - offset);
 	char *p;
 	struct kvec *iov;

 	if (buf->page_len || buf->tail[0].iov_len)
 		iov = &buf->tail[0];
 	else
 		iov = &buf->head[0];
 	p = iov->iov_base + iov->iov_len;
 	iov->iov_len += padding;
 	buf->len += padding;
 	memset(p, padding, padding);
 }

 static inline int
 gss_krb5_remove_padding(struct xdr_buf *buf, int blocksize)
 {
 	u8 *ptr;
 	u8 pad;
 	int len = buf->len;

 	if (len <= buf->head[0].iov_len) {
 		pad = *(u8 *)(buf->head[0].iov_base + len - 1);
 		if (pad > buf->head[0].iov_len)
 			return -EINVAL;
 		buf->head[0].iov_len -= pad;
 		goto out;
 	} else
 		len -= buf->head[0].iov_len;
 	if (len <= buf->page_len) {
 		int last = (buf->page_base + len - 1)
 					>>PAGE_CACHE_SHIFT;
 		int offset = (buf->page_base + len - 1)
 					& (PAGE_CACHE_SIZE - 1);
 		ptr = kmap_atomic(buf->pages[last], KM_SKB_SUNRPC_DATA);
 		pad = *(ptr + offset);
 		kunmap_atomic(ptr, KM_SKB_SUNRPC_DATA);
 		goto out;
 	} else
 		len -= buf->page_len;
 	BUG_ON(len > buf->tail[0].iov_len);
 	pad = *(u8 *)(buf->tail[0].iov_base + len - 1);
 out:
 	/* XXX: NOTE: we do not adjust the page lengths--they represent
 	 * a range of data in the real filesystem page cache, and we need
 	 * to know that range so the xdr code can properly place read data.
 	 * However adjusting the head length, as we do above, is harmless.
 	 * In the case of a request that fits into a single page, the server
 	 * also uses length and head length together to determine the original
 	 * start of the request to copy the request for deferal; so it's
 	 * easier on the server if we adjust head and tail length in tandem.
 	 * It's not really a problem that we don't fool with the page and
 	 * tail lengths, though--at worst badly formed xdr might lead the
 	 * server to attempt to parse the padding.
 	 * XXX: Document all these weird requirements for gss mechanism
 	 * wrap/unwrap functions. */
 	if (pad > blocksize)
 		return -EINVAL;
 	if (buf->len > pad)
 		buf->len -= pad;
 	else
 		return -EINVAL;
 	return 0;
 }

 static inline void
 make_confounder(char *p, int blocksize)
 {
 	static u64 i = 0;
 	u64 *q = (u64 *)p;

 	/* rfc1964 claims this should be "random".  But all that's really
 	 * necessary is that it be unique.  And not even that is necessary in
 	 * our case since our "gssapi" implementation exists only to support
 	 * rpcsec_gss, so we know that the only buffers we will ever encrypt
 	 * already begin with a unique sequence number.  Just to hedge my bets
 	 * I'll make a half-hearted attempt at something unique, but ensuring
 	 * uniqueness would mean worrying about atomicity and rollover, and I
 	 * don't care enough. */

 	BUG_ON(blocksize != 8);
 	*q = i++;
 }

 /* Assumptions: the head and tail of inbuf are ours to play with.
  * The pages, however, may be real pages in the page cache and we replace
  * them with scratch pages from **pages before writing to them. */
 /* XXX: obviously the above should be documentation of wrap interface,
  * and shouldn't be in this kerberos-specific file. */

 /* XXX factor out common code with seal/unseal. */

 u32
 gss_wrap_kerberos(struct gss_ctx *ctx, int offset,
 		struct xdr_buf *buf, struct page **pages)
 {
 	struct krb5_ctx		*kctx = ctx->internal_ctx_id;
 	s32			checksum_type;
 	char			cksumdata[16];
 	struct xdr_netobj	md5cksum = {.len = 0, .data = cksumdata};
 	int			blocksize = 0, plainlen;
 	unsigned char		*ptr, *krb5_hdr, *msg_start;
 	s32			now;
 	int			headlen;
 	struct page		**tmp_pages;
 	u32			seq_send;

 	dprintk("RPC:     gss_wrap_kerberos\n");

 	now = get_seconds();

 	switch (kctx->signalg) {
 		case SGN_ALG_DES_MAC_MD5:
 			checksum_type = CKSUMTYPE_RSA_MD5;
 			break;
 		default:
 			dprintk("RPC:      gss_krb5_seal: kctx->signalg %d not"
 				" supported\n", kctx->signalg);
 			goto out_err;
 	}
 	if (kctx->sealalg != SEAL_ALG_NONE && kctx->sealalg != SEAL_ALG_DES) {
 		dprintk("RPC:      gss_krb5_seal: kctx->sealalg %d not supported\n",
 			kctx->sealalg);
 		goto out_err;
 	}

 	blocksize = crypto_tfm_alg_blocksize(kctx->enc);
 	gss_krb5_add_padding(buf, offset, blocksize);
 	BUG_ON((buf->len - offset) % blocksize);
 	plainlen = blocksize + buf->len - offset;

 	headlen = g_token_size(&kctx->mech_used, 22 + plainlen) -
 						(buf->len - offset);

 	ptr = buf->head[0].iov_base + offset;
 	/* shift data to make room for header. */
 	/* XXX Would be cleverer to encrypt while copying. */
 	/* XXX bounds checking, slack, etc. */
 	memmove(ptr + headlen, ptr, buf->head[0].iov_len - offset);
 	buf->head[0].iov_len += headlen;
 	buf->len += headlen;
 	BUG_ON((buf->len - offset - headlen) % blocksize);

 	g_make_token_header(&kctx->mech_used, 22 + plainlen, &ptr);


 	*ptr++ = (unsigned char) ((KG_TOK_WRAP_MSG>>8)&0xff);
 	*ptr++ = (unsigned char) (KG_TOK_WRAP_MSG&0xff);

 	/* ptr now at byte 2 of header described in rfc 1964, section 1.2.1: */
 	krb5_hdr = ptr - 2;
 	msg_start = krb5_hdr + 24;
 	/* XXXJBF: */ BUG_ON(buf->head[0].iov_base + offset + headlen != msg_start + blocksize);

 	*(u16 *)(krb5_hdr + 2) = htons(kctx->signalg);
 	memset(krb5_hdr + 4, 0xff, 4);
 	*(u16 *)(krb5_hdr + 4) = htons(kctx->sealalg);

 	make_confounder(msg_start, blocksize);

 	/* XXXJBF: UGH!: */
 	tmp_pages = buf->pages;
 	buf->pages = pages;
 	if (make_checksum(checksum_type, krb5_hdr, 8, buf,
 				offset + headlen - blocksize, &md5cksum))
 		goto out_err;
 	buf->pages = tmp_pages;

 	switch (kctx->signalg) {
 	case SGN_ALG_DES_MAC_MD5:
 		if (krb5_encrypt(kctx->seq, NULL, md5cksum.data,
 				  md5cksum.data, md5cksum.len))
 			goto out_err;
 		memcpy(krb5_hdr + 16,
 		       md5cksum.data + md5cksum.len - KRB5_CKSUM_LENGTH,
 		       KRB5_CKSUM_LENGTH);

 		dprintk("RPC:      make_seal_token: cksum data: \n");
 		print_hexl((u32 *) (krb5_hdr + 16), KRB5_CKSUM_LENGTH, 0);
 		break;
 	default:
 		BUG();
 	}

 	spin_lock(&krb5_seq_lock);
 	seq_send = kctx->seq_send++;
 	spin_unlock(&krb5_seq_lock);

 	/* XXX would probably be more efficient to compute checksum
 	 * and encrypt at the same time: */
 	if ((krb5_make_seq_num(kctx->seq, kctx->initiate ? 0 : 0xff,
 			       seq_send, krb5_hdr + 16, krb5_hdr + 8)))
 		goto out_err;

 	if (gss_encrypt_xdr_buf(kctx->enc, buf, offset + headlen - blocksize,
 									pages))
 		goto out_err;

 	return ((kctx->endtime < now) ? GSS_S_CONTEXT_EXPIRED : GSS_S_COMPLETE);
 out_err:
 	return GSS_S_FAILURE;
 }

 u32
 gss_unwrap_kerberos(struct gss_ctx *ctx, int offset, struct xdr_buf *buf)
 {
 	struct krb5_ctx		*kctx = ctx->internal_ctx_id;
 	int			signalg;
 	int			sealalg;
 	s32			checksum_type;
 	char			cksumdata[16];
 	struct xdr_netobj	md5cksum = {.len = 0, .data = cksumdata};
 	s32			now;
 	int			direction;
 	s32			seqnum;
 	unsigned char		*ptr;
 	int			bodysize;
 	u32			ret = GSS_S_DEFECTIVE_TOKEN;
 	void			*data_start, *orig_start;
 	int			data_len;
 	int			blocksize;

 	dprintk("RPC:      gss_unwrap_kerberos\n");

 	ptr = (u8 *)buf->head[0].iov_base + offset;
 	if (g_verify_token_header(&kctx->mech_used, &bodysize, &ptr,
 					buf->len - offset))
 		goto out;

 	if ((*ptr++ != ((KG_TOK_WRAP_MSG>>8)&0xff)) ||
 	    (*ptr++ !=  (KG_TOK_WRAP_MSG    &0xff))   )
 		goto out;

 	/* XXX sanity-check bodysize?? */

 	/* get the sign and seal algorithms */

 	signalg = ptr[0] + (ptr[1] << 8);
 	sealalg = ptr[2] + (ptr[3] << 8);

 	/* Sanity checks */

 	if ((ptr[4] != 0xff) || (ptr[5] != 0xff))
 		goto out;

 	if (sealalg == 0xffff)
 		goto out;

 	/* in the current spec, there is only one valid seal algorithm per
 	   key type, so a simple comparison is ok */

 	if (sealalg != kctx->sealalg)
 		goto out;

 	/* there are several mappings of seal algorithms to sign algorithms,
 	   but few enough that we can try them all. */

 	if ((kctx->sealalg == SEAL_ALG_NONE && signalg > 1) ||
 	    (kctx->sealalg == SEAL_ALG_1 && signalg != SGN_ALG_3) ||
 	    (kctx->sealalg == SEAL_ALG_DES3KD &&
 	     signalg != SGN_ALG_HMAC_SHA1_DES3_KD))
 		goto out;

 	if (gss_decrypt_xdr_buf(kctx->enc, buf,
 			ptr + 22 - (unsigned char *)buf->head[0].iov_base))
 		goto out;

 	/* compute the checksum of the message */

 	/* initialize the the cksum */
 	switch (signalg) {
 	case SGN_ALG_DES_MAC_MD5:
 		checksum_type = CKSUMTYPE_RSA_MD5;
 		break;
 	default:
 		ret = GSS_S_DEFECTIVE_TOKEN;
 		goto out;
 	}

 	switch (signalg) {
 	case SGN_ALG_DES_MAC_MD5:
 		ret = make_checksum(checksum_type, ptr - 2, 8, buf,
 			 ptr + 22 - (unsigned char *)buf->head[0].iov_base, &md5cksum);
 		if (ret)
 			goto out;

 		ret = krb5_encrypt(kctx->seq, NULL, md5cksum.data,
 				   md5cksum.data, md5cksum.len);
 		if (ret)
 			goto out;

 		if (memcmp(md5cksum.data + 8, ptr + 14, 8)) {
 			ret = GSS_S_BAD_SIG;
 			goto out;
 		}
 		break;
 	default:
 		ret = GSS_S_DEFECTIVE_TOKEN;
 		goto out;
 	}

 	/* it got through unscathed.  Make sure the context is unexpired */

 	now = get_seconds();

 	ret = GSS_S_CONTEXT_EXPIRED;
 	if (now > kctx->endtime)
 		goto out;

 	/* do sequencing checks */

 	ret = GSS_S_BAD_SIG;
 	if ((ret = krb5_get_seq_num(kctx->seq, ptr + 14, ptr + 6, &direction,
 				    &seqnum)))
 		goto out;

 	if ((kctx->initiate && direction != 0xff) ||
 	    (!kctx->initiate && direction != 0))
 		goto out;

 	/* Copy the data back to the right position.  XXX: Would probably be
 	 * better to copy and encrypt at the same time. */

 	blocksize = crypto_tfm_alg_blocksize(kctx->enc);
 	data_start = ptr + 22 + blocksize;
 	orig_start = buf->head[0].iov_base + offset;
 	data_len = (buf->head[0].iov_base + buf->head[0].iov_len) - data_start;
 	memmove(orig_start, data_start, data_len);
 	buf->head[0].iov_len -= (data_start - orig_start);
 	buf->len -= (data_start - orig_start);

 	ret = GSS_S_DEFECTIVE_TOKEN;
 	if (gss_krb5_remove_padding(buf, blocksize))
 		goto out;

 	ret = GSS_S_COMPLETE;
 out:
 	return ret;
 }
	#include <linux/types.h>
	#include <linux/slab.h>
	#include <linux/jiffies.h>
	#include <linux/sunrpc/gss_krb5.h>
	#include <linux/random.h>
	#include <linux/pagemap.h>
	#include <asm/scatterlist.h>
	#include <linux/crypto.h>

	#ifdef RPC_DEBUG
	# define RPCDBG_FACILITY RPCDBG_AUTH
	#endif

	static inline int
	gss_krb5_padding(int blocksize, int length)
	{
	/* Most of the code is block-size independent but currently we
	* use only 8: */
	BUG_ON(blocksize != 8);
	return 8 - (length & 7);
	}

	static inline void
	gss_krb5_add_padding(struct xdr_buf *buf, int offset, int blocksize)
	{
	int padding = gss_krb5_padding(blocksize, buf->len - offset);
	char *p;
	struct kvec *iov;

	if (buf->page_len \|\| buf->tail[0].iov_len)
	iov = &buf->tail[0];
	else
	iov = &buf->head[0];
	p = iov->iov_base + iov->iov_len;
	iov->iov_len += padding;
	buf->len += padding;
	memset(p, padding, padding);
	}

	static inline int
	gss_krb5_remove_padding(struct xdr_buf *buf, int blocksize)
	{
	u8 *ptr;
	u8 pad;
	int len = buf->len;

	if (len <= buf->head[0].iov_len) {
	pad = (u8 )(buf->head[0].iov_base + len - 1);
	if (pad > buf->head[0].iov_len)
	return -EINVAL;
	buf->head[0].iov_len -= pad;
	goto out;
	} else
	len -= buf->head[0].iov_len;
	if (len <= buf->page_len) {
	int last = (buf->page_base + len - 1)
	>>PAGE_CACHE_SHIFT;
	int offset = (buf->page_base + len - 1)
	& (PAGE_CACHE_SIZE - 1);
	ptr = kmap_atomic(buf->pages[last], KM_SKB_SUNRPC_DATA);
	pad = *(ptr + offset);
	kunmap_atomic(ptr, KM_SKB_SUNRPC_DATA);
	goto out;
	} else
	len -= buf->page_len;
	BUG_ON(len > buf->tail[0].iov_len);
	pad = (u8 )(buf->tail[0].iov_base + len - 1);
	out:
	/* XXX: NOTE: we do not adjust the page lengths--they represent
	* a range of data in the real filesystem page cache, and we need
	* to know that range so the xdr code can properly place read data.
	* However adjusting the head length, as we do above, is harmless.
	* In the case of a request that fits into a single page, the server
	* also uses length and head length together to determine the original
	* start of the request to copy the request for deferal; so it's
	* easier on the server if we adjust head and tail length in tandem.
	* It's not really a problem that we don't fool with the page and
	* tail lengths, though--at worst badly formed xdr might lead the
	* server to attempt to parse the padding.
	* XXX: Document all these weird requirements for gss mechanism
	* wrap/unwrap functions. */
	if (pad > blocksize)
	return -EINVAL;
	if (buf->len > pad)
	buf->len -= pad;
	else
	return -EINVAL;
	return 0;
	}

	static inline void
	make_confounder(char *p, int blocksize)
	{
	static u64 i = 0;
	u64 q = (u64 )p;

	/* rfc1964 claims this should be "random". But all that's really
	* necessary is that it be unique. And not even that is necessary in
	* our case since our "gssapi" implementation exists only to support
	* rpcsec_gss, so we know that the only buffers we will ever encrypt
	* already begin with a unique sequence number. Just to hedge my bets
	* I'll make a half-hearted attempt at something unique, but ensuring
	* uniqueness would mean worrying about atomicity and rollover, and I
	* don't care enough. */

	BUG_ON(blocksize != 8);
	*q = i++;
	}

	/* Assumptions: the head and tail of inbuf are ours to play with.
	* The pages, however, may be real pages in the page cache and we replace
	* them with scratch pages from *pages before writing to them. /
	/* XXX: obviously the above should be documentation of wrap interface,
	* and shouldn't be in this kerberos-specific file. */

	/* XXX factor out common code with seal/unseal. */

	u32
	gss_wrap_kerberos(struct gss_ctx *ctx, int offset,
	struct xdr_buf buf, struct page *pages)
	{
	struct krb5_ctx *kctx = ctx->internal_ctx_id;
	s32 checksum_type;
	char cksumdata[16];
	struct xdr_netobj md5cksum = {.len = 0, .data = cksumdata};
	int blocksize = 0, plainlen;
	unsigned char ptr, krb5_hdr, *msg_start;
	s32 now;
	int headlen;
	struct page **tmp_pages;
	u32 seq_send;

	dprintk("RPC: gss_wrap_kerberos\n");

	now = get_seconds();

	switch (kctx->signalg) {
	case SGN_ALG_DES_MAC_MD5:
	checksum_type = CKSUMTYPE_RSA_MD5;
	break;
	default:
	dprintk("RPC: gss_krb5_seal: kctx->signalg %d not"
	" supported\n", kctx->signalg);
	goto out_err;
	}
	if (kctx->sealalg != SEAL_ALG_NONE && kctx->sealalg != SEAL_ALG_DES) {
	dprintk("RPC: gss_krb5_seal: kctx->sealalg %d not supported\n",
	kctx->sealalg);
	goto out_err;
	}

	blocksize = crypto_tfm_alg_blocksize(kctx->enc);
	gss_krb5_add_padding(buf, offset, blocksize);
	BUG_ON((buf->len - offset) % blocksize);
	plainlen = blocksize + buf->len - offset;

	headlen = g_token_size(&kctx->mech_used, 22 + plainlen) -
	(buf->len - offset);

	ptr = buf->head[0].iov_base + offset;
	/* shift data to make room for header. */
	/* XXX Would be cleverer to encrypt while copying. */
	/* XXX bounds checking, slack, etc. */
	memmove(ptr + headlen, ptr, buf->head[0].iov_len - offset);
	buf->head[0].iov_len += headlen;
	buf->len += headlen;
	BUG_ON((buf->len - offset - headlen) % blocksize);

	g_make_token_header(&kctx->mech_used, 22 + plainlen, &ptr);


	*ptr++ = (unsigned char) ((KG_TOK_WRAP_MSG>>8)&0xff);
	*ptr++ = (unsigned char) (KG_TOK_WRAP_MSG&0xff);

	/* ptr now at byte 2 of header described in rfc 1964, section 1.2.1: */
	krb5_hdr = ptr - 2;
	msg_start = krb5_hdr + 24;
	/* XXXJBF: */ BUG_ON(buf->head[0].iov_base + offset + headlen != msg_start + blocksize);

	(u16 )(krb5_hdr + 2) = htons(kctx->signalg);
	memset(krb5_hdr + 4, 0xff, 4);
	(u16 )(krb5_hdr + 4) = htons(kctx->sealalg);

	make_confounder(msg_start, blocksize);

	/* XXXJBF: UGH!: */
	tmp_pages = buf->pages;
	buf->pages = pages;
	if (make_checksum(checksum_type, krb5_hdr, 8, buf,
	offset + headlen - blocksize, &md5cksum))
	goto out_err;
	buf->pages = tmp_pages;

	switch (kctx->signalg) {
	case SGN_ALG_DES_MAC_MD5:
	if (krb5_encrypt(kctx->seq, NULL, md5cksum.data,
	md5cksum.data, md5cksum.len))
	goto out_err;
	memcpy(krb5_hdr + 16,
	md5cksum.data + md5cksum.len - KRB5_CKSUM_LENGTH,
	KRB5_CKSUM_LENGTH);

	dprintk("RPC: make_seal_token: cksum data: \n");
	print_hexl((u32 *) (krb5_hdr + 16), KRB5_CKSUM_LENGTH, 0);
	break;
	default:
	BUG();
	}

	spin_lock(&krb5_seq_lock);
	seq_send = kctx->seq_send++;
	spin_unlock(&krb5_seq_lock);

	/* XXX would probably be more efficient to compute checksum
	* and encrypt at the same time: */
	if ((krb5_make_seq_num(kctx->seq, kctx->initiate ? 0 : 0xff,
	seq_send, krb5_hdr + 16, krb5_hdr + 8)))
	goto out_err;

	if (gss_encrypt_xdr_buf(kctx->enc, buf, offset + headlen - blocksize,
	pages))
	goto out_err;

	return ((kctx->endtime < now) ? GSS_S_CONTEXT_EXPIRED : GSS_S_COMPLETE);
	out_err:
	return GSS_S_FAILURE;
	}

	u32
	gss_unwrap_kerberos(struct gss_ctx ctx, int offset, struct xdr_buf buf)
	{
	struct krb5_ctx *kctx = ctx->internal_ctx_id;
	int signalg;
	int sealalg;
	s32 checksum_type;
	char cksumdata[16];
	struct xdr_netobj md5cksum = {.len = 0, .data = cksumdata};
	s32 now;
	int direction;
	s32 seqnum;
	unsigned char *ptr;
	int bodysize;
	u32 ret = GSS_S_DEFECTIVE_TOKEN;
	void data_start, orig_start;
	int data_len;
	int blocksize;

	dprintk("RPC: gss_unwrap_kerberos\n");

	ptr = (u8 *)buf->head[0].iov_base + offset;
	if (g_verify_token_header(&kctx->mech_used, &bodysize, &ptr,
	buf->len - offset))
	goto out;

	if ((*ptr++ != ((KG_TOK_WRAP_MSG>>8)&0xff)) \|\|
	(*ptr++ != (KG_TOK_WRAP_MSG &0xff)) )
	goto out;

	/* XXX sanity-check bodysize?? */

	/* get the sign and seal algorithms */

	signalg = ptr[0] + (ptr[1] << 8);
	sealalg = ptr[2] + (ptr[3] << 8);

	/* Sanity checks */

	if ((ptr[4] != 0xff) \|\| (ptr[5] != 0xff))
	goto out;

	if (sealalg == 0xffff)
	goto out;

	/* in the current spec, there is only one valid seal algorithm per
	key type, so a simple comparison is ok */

	if (sealalg != kctx->sealalg)
	goto out;

	/* there are several mappings of seal algorithms to sign algorithms,
	but few enough that we can try them all. */

	if ((kctx->sealalg == SEAL_ALG_NONE && signalg > 1) \|\|
	(kctx->sealalg == SEAL_ALG_1 && signalg != SGN_ALG_3) \|\|
	(kctx->sealalg == SEAL_ALG_DES3KD &&
	signalg != SGN_ALG_HMAC_SHA1_DES3_KD))
	goto out;

	if (gss_decrypt_xdr_buf(kctx->enc, buf,
	ptr + 22 - (unsigned char *)buf->head[0].iov_base))
	goto out;

	/* compute the checksum of the message */

	/* initialize the the cksum */
	switch (signalg) {
	case SGN_ALG_DES_MAC_MD5:
	checksum_type = CKSUMTYPE_RSA_MD5;
	break;
	default:
	ret = GSS_S_DEFECTIVE_TOKEN;
	goto out;
	}

	switch (signalg) {
	case SGN_ALG_DES_MAC_MD5:
	ret = make_checksum(checksum_type, ptr - 2, 8, buf,
	ptr + 22 - (unsigned char *)buf->head[0].iov_base, &md5cksum);
	if (ret)
	goto out;

	ret = krb5_encrypt(kctx->seq, NULL, md5cksum.data,
	md5cksum.data, md5cksum.len);
	if (ret)
	goto out;

	if (memcmp(md5cksum.data + 8, ptr + 14, 8)) {
	ret = GSS_S_BAD_SIG;
	goto out;
	}
	break;
	default:
	ret = GSS_S_DEFECTIVE_TOKEN;
	goto out;
	}

	/* it got through unscathed. Make sure the context is unexpired */

	now = get_seconds();

	ret = GSS_S_CONTEXT_EXPIRED;
	if (now > kctx->endtime)
	goto out;

	/* do sequencing checks */

	ret = GSS_S_BAD_SIG;
	if ((ret = krb5_get_seq_num(kctx->seq, ptr + 14, ptr + 6, &direction,
	&seqnum)))
	goto out;

	if ((kctx->initiate && direction != 0xff) \|\|
	(!kctx->initiate && direction != 0))
	goto out;

	/* Copy the data back to the right position. XXX: Would probably be
	* better to copy and encrypt at the same time. */

	blocksize = crypto_tfm_alg_blocksize(kctx->enc);
	data_start = ptr + 22 + blocksize;
	orig_start = buf->head[0].iov_base + offset;
	data_len = (buf->head[0].iov_base + buf->head[0].iov_len) - data_start;
	memmove(orig_start, data_start, data_len);
	buf->head[0].iov_len -= (data_start - orig_start);
	buf->len -= (data_start - orig_start);

	ret = GSS_S_DEFECTIVE_TOKEN;
	if (gss_krb5_remove_padding(buf, blocksize))
	goto out;

	ret = GSS_S_COMPLETE;
	out:
	return ret;
	}