RPCSEC_GSS: Add support for privacy to krb5 rpcsec_gss mechanism.

 Add support for privacy to the krb5 rpcsec_gss mechanism.

 Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

7 files changed, 552 insertions, 7 deletions

diff --git a/include/linux/sunrpc/gss_krb5.h b/include/linux/sunrpc/gss_krb5.h
index cb35833..7f93c2d 100644
--- a/include/linux/sunrpc/gss_krb5.h
+++ b/include/linux/sunrpc/gss_krb5.h
@@ -116,7 +116,7 @@ enum seal_alg {
 
 s32
 make_checksum(s32 cksumtype, char *header, int hdrlen, struct xdr_buf *body,
-		   struct xdr_netobj *cksum);
+		   int body_offset, struct xdr_netobj *cksum);
 
 u32
 krb5_make_token(struct krb5_ctx *context_handle, int qop_req,
@@ -129,6 +129,15 @@ krb5_read_token(struct krb5_ctx *context_handle,
 	  struct xdr_buf *message_buffer, int *qop_state);
 
 u32
+gss_wrap_kerberos(struct gss_ctx *ctx_id, u32 qop, int offset,
+		struct xdr_buf *outbuf, struct page **pages);
+
+u32
+gss_unwrap_kerberos(struct gss_ctx *ctx_id, u32 *qop, int offset,
+		struct xdr_buf *buf);
+
+
+u32
 krb5_encrypt(struct crypto_tfm * key,
 	     void *iv, void *in, void *out, int length);
 
@@ -136,6 +145,13 @@ u32
 krb5_decrypt(struct crypto_tfm * key,
 	     void *iv, void *in, void *out, int length); 
 
+int
+gss_encrypt_xdr_buf(struct crypto_tfm *tfm, struct xdr_buf *outbuf, int offset,
+		struct page **pages);
+
+int
+gss_decrypt_xdr_buf(struct crypto_tfm *tfm, struct xdr_buf *inbuf, int offset);
+
 s32
 krb5_make_seq_num(struct crypto_tfm * key,
 		int direction,
diff --git a/net/sunrpc/auth_gss/Makefile b/net/sunrpc/auth_gss/Makefile
index fe1b874..f3431a7 100644
--- a/net/sunrpc/auth_gss/Makefile
+++ b/net/sunrpc/auth_gss/Makefile
@@ -10,7 +10,7 @@ auth_rpcgss-objs := auth_gss.o gss_generic_token.o \
 obj-$(CONFIG_RPCSEC_GSS_KRB5) += rpcsec_gss_krb5.o
 
 rpcsec_gss_krb5-objs := gss_krb5_mech.o gss_krb5_seal.o gss_krb5_unseal.o \
-	gss_krb5_seqnum.o
+	gss_krb5_seqnum.o gss_krb5_wrap.o
 
 obj-$(CONFIG_RPCSEC_GSS_SPKM3) += rpcsec_gss_spkm3.o
 
diff --git a/net/sunrpc/auth_gss/gss_krb5_crypto.c b/net/sunrpc/auth_gss/gss_krb5_crypto.c
index 2baf93f..3f3d543 100644
--- a/net/sunrpc/auth_gss/gss_krb5_crypto.c
+++ b/net/sunrpc/auth_gss/gss_krb5_crypto.c
@@ -218,7 +218,7 @@ checksummer(struct scatterlist *sg, void *data)
 /* checksum the plaintext data and hdrlen bytes of the token header */
 s32
 make_checksum(s32 cksumtype, char *header, int hdrlen, struct xdr_buf *body,
-		   struct xdr_netobj *cksum)
+		   int body_offset, struct xdr_netobj *cksum)
 {
 	char                            *cksumname;
 	struct crypto_tfm               *tfm = NULL; /* XXX add to ctx? */
@@ -243,7 +243,8 @@ make_checksum(s32 cksumtype, char *header, int hdrlen, struct xdr_buf *body,
 	crypto_digest_init(tfm);
 	buf_to_sg(sg, header, hdrlen);
 	crypto_digest_update(tfm, sg, 1);
-	process_xdr_buf(body, 0, body->len, checksummer, tfm);
+	process_xdr_buf(body, body_offset, body->len - body_offset,
+			checksummer, tfm);
 	crypto_digest_final(tfm, cksum->data);
 	code = 0;
 out:
@@ -252,3 +253,154 @@ out:
 }
 
 EXPORT_SYMBOL(make_checksum);
+
+struct encryptor_desc {
+	u8 iv[8]; /* XXX hard-coded blocksize */
+	struct crypto_tfm *tfm;
+	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 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);
+	desc->infrags[desc->fragno] = *sg;
+	desc->outfrags[desc->fragno] = *sg;
+
+	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_CACHE_SHIFT;
+		in_page = desc->pages[i];
+	} else {
+		in_page = sg->page;
+	}
+	desc->infrags[desc->fragno].page = in_page;
+	desc->fragno++;
+	desc->fraglen += sg->length;
+	desc->pos += sg->length;
+
+	fraglen = thislen & 7; /* XXX hardcoded blocksize */
+	thislen -= fraglen;
+
+	if (thislen == 0)
+		return 0;
+
+	ret = crypto_cipher_encrypt_iv(desc->tfm, desc->outfrags, desc->infrags,
+					thislen, desc->iv);
+	if (ret)
+		return ret;
+	if (fraglen) {
+		desc->outfrags[0].page = sg->page;
+		desc->outfrags[0].offset = sg->offset + sg->length - fraglen;
+		desc->outfrags[0].length = fraglen;
+		desc->infrags[0] = desc->outfrags[0];
+		desc->infrags[0].page = in_page;
+		desc->fragno = 1;
+		desc->fraglen = fraglen;
+	} else {
+		desc->fragno = 0;
+		desc->fraglen = 0;
+	}
+	return 0;
+}
+
+int
+gss_encrypt_xdr_buf(struct crypto_tfm *tfm, struct xdr_buf *buf, int offset,
+		struct page **pages)
+{
+	int ret;
+	struct encryptor_desc desc;
+
+	BUG_ON((buf->len - offset) % crypto_tfm_alg_blocksize(tfm) != 0);
+
+	memset(desc.iv, 0, sizeof(desc.iv));
+	desc.tfm = tfm;
+	desc.pos = offset;
+	desc.outbuf = buf;
+	desc.pages = pages;
+	desc.fragno = 0;
+	desc.fraglen = 0;
+
+	ret = process_xdr_buf(buf, offset, buf->len - offset, encryptor, &desc);
+	return ret;
+}
+
+EXPORT_SYMBOL(gss_encrypt_xdr_buf);
+
+struct decryptor_desc {
+	u8 iv[8]; /* XXX hard-coded blocksize */
+	struct crypto_tfm *tfm;
+	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;
+	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);
+	desc->frags[desc->fragno] = *sg;
+	desc->fragno++;
+	desc->fraglen += sg->length;
+
+	fraglen = thislen & 7; /* XXX hardcoded blocksize */
+	thislen -= fraglen;
+
+	if (thislen == 0)
+		return 0;
+
+	ret = crypto_cipher_decrypt_iv(desc->tfm, desc->frags, desc->frags,
+					thislen, desc->iv);
+	if (ret)
+		return ret;
+	if (fraglen) {
+		desc->frags[0].page = sg->page;
+		desc->frags[0].offset = sg->offset + sg->length - fraglen;
+		desc->frags[0].length = fraglen;
+		desc->fragno = 1;
+		desc->fraglen = fraglen;
+	} else {
+		desc->fragno = 0;
+		desc->fraglen = 0;
+	}
+	return 0;
+}
+
+int
+gss_decrypt_xdr_buf(struct crypto_tfm *tfm, struct xdr_buf *buf, int offset)
+{
+	struct decryptor_desc desc;
+
+	/* XXXJBF: */
+	BUG_ON((buf->len - offset) % crypto_tfm_alg_blocksize(tfm) != 0);
+
+	memset(desc.iv, 0, sizeof(desc.iv));
+	desc.tfm = tfm;
+	desc.fragno = 0;
+	desc.fraglen = 0;
+	return process_xdr_buf(buf, offset, buf->len - offset, decryptor, &desc);
+}
+
+EXPORT_SYMBOL(gss_decrypt_xdr_buf);
diff --git a/net/sunrpc/auth_gss/gss_krb5_mech.c b/net/sunrpc/auth_gss/gss_krb5_mech.c
index 8b9066f..37a9ad9 100644
--- a/net/sunrpc/auth_gss/gss_krb5_mech.c
+++ b/net/sunrpc/auth_gss/gss_krb5_mech.c
@@ -226,6 +226,8 @@ static struct gss_api_ops gss_kerberos_ops = {
 	.gss_import_sec_context	= gss_import_sec_context_kerberos,
 	.gss_get_mic		= gss_get_mic_kerberos,
 	.gss_verify_mic		= gss_verify_mic_kerberos,
+	.gss_wrap		= gss_wrap_kerberos,
+	.gss_unwrap		= gss_unwrap_kerberos,
 	.gss_delete_sec_context	= gss_delete_sec_context_kerberos,
 };
 
@@ -240,6 +242,11 @@ static struct pf_desc gss_kerberos_pfs[] = {
 		.service = RPC_GSS_SVC_INTEGRITY,
 		.name = "krb5i",
 	},
+	[2] = {
+		.pseudoflavor = RPC_AUTH_GSS_KRB5P,
+		.service = RPC_GSS_SVC_PRIVACY,
+		.name = "krb5p",
+	},
 };
 
 static struct gss_api_mech gss_kerberos_mech = {
diff --git a/net/sunrpc/auth_gss/gss_krb5_seal.c b/net/sunrpc/auth_gss/gss_krb5_seal.c
index 2511834..fb852d9 100644
--- a/net/sunrpc/auth_gss/gss_krb5_seal.c
+++ b/net/sunrpc/auth_gss/gss_krb5_seal.c
@@ -116,8 +116,8 @@ krb5_make_token(struct krb5_ctx *ctx, int qop_req,
 	*(u16 *)(krb5_hdr + 2) = htons(ctx->signalg);
 	memset(krb5_hdr + 4, 0xff, 4);
 
-	if (make_checksum(checksum_type, krb5_hdr, 8, text, &md5cksum))
-		goto out_err;
+	if (make_checksum(checksum_type, krb5_hdr, 8, text, 0, &md5cksum))
+			goto out_err;
 
 	switch (ctx->signalg) {
 	case SGN_ALG_DES_MAC_MD5:
diff --git a/net/sunrpc/auth_gss/gss_krb5_unseal.c b/net/sunrpc/auth_gss/gss_krb5_unseal.c
index 19eba3d..c3d6d1b 100644
--- a/net/sunrpc/auth_gss/gss_krb5_unseal.c
+++ b/net/sunrpc/auth_gss/gss_krb5_unseal.c
@@ -136,7 +136,7 @@ krb5_read_token(struct krb5_ctx *ctx,
 	switch (signalg) {
 	case SGN_ALG_DES_MAC_MD5:
 		ret = make_checksum(checksum_type, ptr - 2, 8,
-					 message_buffer, &md5cksum);
+					 message_buffer, 0, &md5cksum);
 		if (ret)
 			goto out;
 
diff --git a/net/sunrpc/auth_gss/gss_krb5_wrap.c b/net/sunrpc/auth_gss/gss_krb5_wrap.c
new file mode 100644
index 0000000..ddcde6e
--- /dev/null
+++ b/net/sunrpc/auth_gss/gss_krb5_wrap.c
@@ -0,0 +1,370 @@
+#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, u32 qop, int offset,
+		struct xdr_buf *buf, struct page **pages)
+{
+	struct krb5_ctx		*kctx = ctx->internal_ctx_id;
+	s32			checksum_type;
+	struct xdr_netobj	md5cksum = {.len = 0, .data = NULL};
+	int			blocksize = 0, plainlen;
+	unsigned char		*ptr, *krb5_hdr, *msg_start;
+	s32			now;
+	int			headlen;
+	struct page		**tmp_pages;
+
+	dprintk("RPC:     gss_wrap_kerberos\n");
+
+	now = get_seconds();
+
+	if (qop != 0)
+		goto out_err;
+
+	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();
+	}
+
+	kfree(md5cksum.data);
+
+	/* 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,
+			       kctx->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;
+
+	kctx->seq_send++;
+
+	return ((kctx->endtime < now) ? GSS_S_CONTEXT_EXPIRED : GSS_S_COMPLETE);
+out_err:
+	if (md5cksum.data) kfree(md5cksum.data);
+	return GSS_S_FAILURE;
+}
+
+u32
+gss_unwrap_kerberos(struct gss_ctx *ctx, u32 *qop, int offset,
+			struct xdr_buf *buf)
+{
+	struct krb5_ctx		*kctx = ctx->internal_ctx_id;
+	int			signalg;
+	int			sealalg;
+	s32			checksum_type;
+	struct xdr_netobj	md5cksum = {.len = 0, .data = NULL};
+	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 */
+
+	if (qop)
+		*qop = GSS_C_QOP_DEFAULT;
+
+	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:
+	if (md5cksum.data) kfree(md5cksum.data);
+	return ret;
+}