[PATCH net 2/3] rxrpc: Fix DATA decrypt vs splice() by copying data to buffer in recvmsg

[David Howells <dhowells@redhat.com>]

This improves the fix for CVE-2026-43500.

Fix the pagecache corruption from in-place decryption of a DATA packet
transmitted locally by splice() by getting rid of the packet sharing in the
I/O thread and unconditionally extracting the packet content into a bounce
buffer in which the buffer is decrypted.  recvmsg() (or the kernel
equivalent) then copies the data from the bounce buffer to the destination
buffer.  The sk_buff then remains unmodified.

This has an additional advantage in that the packet is then arranged in the
buffer with the correct alignment required for the crypto algorithms to
process directly.  The performance of the crypto does seem to be a little
faster and, surprisingly, the unencrypted performance doesn't seem to
change much - possibly due to removing complexity from the I/O thread.

Yet another advantage is that the I/O thread doesn't have to copy packets
which would slow down packet distribution, ACK generation, etc..

The buffer belongs to the call and is allocated initially at 2K,
sufficiently large to hold a whole jumbo subpacket, but the buffer will be
increased in size if needed.  There is one downside here, and that's if a
MSG_PEEK of more than one byte occurs, it may move on to the next packet,
replacing the content of the buffer.  In such a case, it has to go back and
re-decrypt the current packet.

Note that rx_pkt_offset may legitimately see 0 as a valid offset now, so
switch to using USHRT_MAX to indicate an invalid offset.

Note also that I would generally prefer to replace the buffers of the
current sk_buff with a new kmalloc'd buffer of the right size, ditching the
old data and frags as this makes the handling of MSG_PEEK easier and
removes the double-decryption issue, but this looks like quite a
complicated thing to achieve.  skb_morph() looks half way to what I want,
but I don't want to have to allocate a new sk_buff.

Fixes: d0d5c0cd1e71 ("rxrpc: Use skb_unshare() rather than skb_cow_data()")
Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Closes: https://lore.kernel.org/r/afKV2zGR6rrelPC7@v4bel/
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: Jiayuan Chen <jiayuan.chen@linux.dev>
cc: netdev@vger.kernel.org
cc: linux-afs@lists.infradead.org
cc: stable@vger.kernel.org
---
 net/rxrpc/ar-internal.h |  7 +++-
 net/rxrpc/call_event.c  | 22 +----------
 net/rxrpc/call_object.c |  2 +
 net/rxrpc/insecure.c    |  3 --
 net/rxrpc/recvmsg.c     | 72 +++++++++++++++++++++++++++-------
 net/rxrpc/rxgk.c        | 49 +++++++++++------------
 net/rxrpc/rxgk_common.h | 79 +++++++++++++++++++++++++++++++++++++
 net/rxrpc/rxkad.c       | 86 +++++++++++++++--------------------------
 8 files changed, 200 insertions(+), 120 deletions(-)

diff --git a/net/rxrpc/ar-internal.h b/net/rxrpc/ar-internal.h
index 27c2aa2dd023..783367eea798 100644
--- a/net/rxrpc/ar-internal.h
+++ b/net/rxrpc/ar-internal.h
@@ -213,8 +213,6 @@ struct rxrpc_skb_priv {
 		struct {
 			u16		offset;		/* Offset of data */
 			u16		len;		/* Length of data */
-			u8		flags;
-#define RXRPC_RX_VERIFIED	0x01
 		};
 		struct {
 			rxrpc_seq_t	first_ack;	/* First packet in acks table */
@@ -774,6 +772,11 @@ struct rxrpc_call {
 	struct sk_buff_head	recvmsg_queue;	/* Queue of packets ready for recvmsg() */
 	struct sk_buff_head	rx_queue;	/* Queue of packets for this call to receive */
 	struct sk_buff_head	rx_oos_queue;	/* Queue of out of sequence packets */
+	void			*rx_dec_buffer;	/* Decryption buffer */
+	unsigned short		rx_dec_bsize;	/* rx_dec_buffer size */
+	unsigned short		rx_dec_offset;	/* Decrypted packet data offset */
+	unsigned short		rx_dec_len;	/* Decrypted packet data len */
+	rxrpc_seq_t		rx_dec_seq;	/* Packet in decryption buffer */
 
 	rxrpc_seq_t		rx_highest_seq;	/* Higest sequence number received */
 	rxrpc_seq_t		rx_consumed;	/* Highest packet consumed */
diff --git a/net/rxrpc/call_event.c b/net/rxrpc/call_event.c
index 2b19b252225e..fec59d9338b9 100644
--- a/net/rxrpc/call_event.c
+++ b/net/rxrpc/call_event.c
@@ -332,27 +332,7 @@ bool rxrpc_input_call_event(struct rxrpc_call *call)
 
 			saw_ack |= sp->hdr.type == RXRPC_PACKET_TYPE_ACK;
 
-			if (sp->hdr.type == RXRPC_PACKET_TYPE_DATA &&
-			    sp->hdr.securityIndex != 0 &&
-			    (skb_cloned(skb) ||
-			     skb_has_frag_list(skb) ||
-			     skb_has_shared_frag(skb))) {
-				/* Unshare the packet so that it can be
-				 * modified by in-place decryption.
-				 */
-				struct sk_buff *nskb = skb_copy(skb, GFP_ATOMIC);
-
-				if (nskb) {
-					rxrpc_new_skb(nskb, rxrpc_skb_new_unshared);
-					rxrpc_input_call_packet(call, nskb);
-					rxrpc_free_skb(nskb, rxrpc_skb_put_call_rx);
-				} else {
-					/* OOM - Drop the packet. */
-					rxrpc_see_skb(skb, rxrpc_skb_see_unshare_nomem);
-				}
-			} else {
-				rxrpc_input_call_packet(call, skb);
-			}
+			rxrpc_input_call_packet(call, skb);
 			rxrpc_free_skb(skb, rxrpc_skb_put_call_rx);
 			did_receive = true;
 		}
diff --git a/net/rxrpc/call_object.c b/net/rxrpc/call_object.c
index f035f486c139..fcb9d38bb521 100644
--- a/net/rxrpc/call_object.c
+++ b/net/rxrpc/call_object.c
@@ -152,6 +152,7 @@ struct rxrpc_call *rxrpc_alloc_call(struct rxrpc_sock *rx, gfp_t gfp,
 	spin_lock_init(&call->notify_lock);
 	refcount_set(&call->ref, 1);
 	call->debug_id		= debug_id;
+	call->rx_pkt_offset	= USHRT_MAX;
 	call->tx_total_len	= -1;
 	call->tx_jumbo_max	= 1;
 	call->next_rx_timo	= 20 * HZ;
@@ -553,6 +554,7 @@ static void rxrpc_cleanup_rx_buffers(struct rxrpc_call *call)
 	rxrpc_purge_queue(&call->recvmsg_queue);
 	rxrpc_purge_queue(&call->rx_queue);
 	rxrpc_purge_queue(&call->rx_oos_queue);
+	kfree(call->rx_dec_buffer);
 }
 
 /*
diff --git a/net/rxrpc/insecure.c b/net/rxrpc/insecure.c
index 0a260df45d25..7a26c6097d03 100644
--- a/net/rxrpc/insecure.c
+++ b/net/rxrpc/insecure.c
@@ -32,9 +32,6 @@ static int none_secure_packet(struct rxrpc_call *call, struct rxrpc_txbuf *txb)
 
 static int none_verify_packet(struct rxrpc_call *call, struct sk_buff *skb)
 {
-	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
-
-	sp->flags |= RXRPC_RX_VERIFIED;
 	return 0;
 }
 
diff --git a/net/rxrpc/recvmsg.c b/net/rxrpc/recvmsg.c
index e1f7513a46db..865e368381d5 100644
--- a/net/rxrpc/recvmsg.c
+++ b/net/rxrpc/recvmsg.c
@@ -147,15 +147,55 @@ static void rxrpc_rotate_rx_window(struct rxrpc_call *call)
 }
 
 /*
- * Decrypt and verify a DATA packet.
+ * Decrypt and verify a DATA packet.  The content of the packet is pulled out
+ * into a flat buffer rather than decrypting in place in the skbuff.  This also
+ * has the advantage of aligning the buffer correctly for the crypto routines.
+ *
+ * We keep track of the sequence number of the packet currently decrypted into
+ * the buffer in ->rx_dec_seq.  Unfortunately, this means that a MSG_PEEK of
+ * more than one byte may cause a later packet to be decrypted into the buffer,
+ * requiring the original to be re-decrypted when recvmsg() is called again.
  */
 static int rxrpc_verify_data(struct rxrpc_call *call, struct sk_buff *skb)
 {
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
+	int ret;
 
-	if (sp->flags & RXRPC_RX_VERIFIED)
+	if (call->rx_dec_seq == sp->hdr.seq && call->rx_dec_buffer)
 		return 0;
-	return call->security->verify_packet(call, skb);
+
+	if (call->rx_dec_bsize < sp->len) {
+		/* Make sure we can hold a 1412-byte jumbo subpacket and make
+		 * sure that the buffer size is aligned to a crypto blocksize.
+		 */
+		size_t size = umin(round_up(sp->len, 32), 2048);
+		void *buffer = krealloc(call->rx_dec_buffer, size, GFP_NOFS);
+
+		if (!buffer)
+			return -ENOMEM;
+		call->rx_dec_buffer = buffer;
+		call->rx_dec_bsize = size;
+	}
+
+	ret = -EFAULT;
+	if (skb_copy_bits(skb, sp->offset, call->rx_dec_buffer, sp->len) < 0)
+		goto err;
+
+	call->rx_dec_offset = 0;
+	call->rx_dec_len = sp->len;
+	call->rx_dec_seq = sp->hdr.seq;
+	ret = call->security->verify_packet(call, skb);
+	if (ret < 0)
+		goto err;
+	return 0;
+
+err:
+	kfree(call->rx_dec_buffer);
+	call->rx_dec_buffer = NULL;
+	call->rx_dec_bsize = 0;
+	call->rx_dec_offset = 0;
+	call->rx_dec_len = 0;
+	return ret;
 }
 
 /*
@@ -283,16 +323,19 @@ static int rxrpc_recvmsg_data(struct socket *sock, struct rxrpc_call *call,
 		if (msg)
 			sock_recv_timestamp(msg, sock->sk, skb);
 
-		if (rx_pkt_offset == 0) {
+		if (rx_pkt_offset == USHRT_MAX) {
 			ret2 = rxrpc_verify_data(call, skb);
 			trace_rxrpc_recvdata(call, rxrpc_recvmsg_next, seq,
-					     sp->offset, sp->len, ret2);
+					     call->rx_dec_offset,
+					     call->rx_dec_len, ret2);
 			if (ret2 < 0) {
 				ret = ret2;
 				goto out;
 			}
-			rx_pkt_offset = sp->offset;
-			rx_pkt_len = sp->len;
+			sp = rxrpc_skb(skb);
+			seq = sp->hdr.seq;
+			rx_pkt_offset = call->rx_dec_offset;
+			rx_pkt_len = call->rx_dec_len;
 		} else {
 			trace_rxrpc_recvdata(call, rxrpc_recvmsg_cont, seq,
 					     rx_pkt_offset, rx_pkt_len, 0);
@@ -304,10 +347,10 @@ static int rxrpc_recvmsg_data(struct socket *sock, struct rxrpc_call *call,
 		if (copy > remain)
 			copy = remain;
 		if (copy > 0) {
-			ret2 = skb_copy_datagram_iter(skb, rx_pkt_offset, iter,
-						      copy);
-			if (ret2 < 0) {
-				ret = ret2;
+			ret2 = copy_to_iter(call->rx_dec_buffer + rx_pkt_offset,
+					    copy, iter);
+			if (ret2 != copy) {
+				ret = -EFAULT;
 				goto out;
 			}
 
@@ -328,13 +371,14 @@ static int rxrpc_recvmsg_data(struct socket *sock, struct rxrpc_call *call,
 		/* The whole packet has been transferred. */
 		if (sp->hdr.flags & RXRPC_LAST_PACKET)
 			ret = 1;
-		rx_pkt_offset = 0;
+		rx_pkt_offset = USHRT_MAX;
 		rx_pkt_len = 0;
+		if (unlikely(flags & MSG_PEEK))
+			break;
 
 		skb = skb_peek_next(skb, &call->recvmsg_queue);
 
-		if (!(flags & MSG_PEEK))
-			rxrpc_rotate_rx_window(call);
+		rxrpc_rotate_rx_window(call);
 
 		if (!rx->app_ops &&
 		    !skb_queue_empty_lockless(&rx->recvmsg_oobq)) {
diff --git a/net/rxrpc/rxgk.c b/net/rxrpc/rxgk.c
index 0d5e654da918..88e651dd0e90 100644
--- a/net/rxrpc/rxgk.c
+++ b/net/rxrpc/rxgk.c
@@ -473,8 +473,9 @@ static int rxgk_verify_packet_integrity(struct rxrpc_call *call,
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
 	struct rxgk_header *hdr;
 	struct krb5_buffer metadata;
-	unsigned int offset = sp->offset, len = sp->len;
+	unsigned int offset = 0, len = call->rx_dec_len;
 	size_t data_offset = 0, data_len = len;
+	void *data = call->rx_dec_buffer;
 	u32 ac = 0;
 	int ret = -ENOMEM;
 
@@ -496,16 +497,16 @@ static int rxgk_verify_packet_integrity(struct rxrpc_call *call,
 
 	metadata.len = sizeof(*hdr);
 	metadata.data = hdr;
-	ret = rxgk_verify_mic_skb(gk->krb5, gk->rx_Kc, &metadata,
-				  skb, &offset, &len, &ac);
+	ret = rxgk_verify_mic(gk->krb5, gk->rx_Kc, &metadata,
+			      data, &offset, &len, &ac);
 	kfree(hdr);
 	if (ret < 0) {
 		if (ret != -ENOMEM)
 			rxrpc_abort_eproto(call, skb, ac,
 					   rxgk_abort_1_verify_mic_eproto);
 	} else {
-		sp->offset = offset;
-		sp->len = len;
+		call->rx_dec_offset = offset;
+		call->rx_dec_len = len;
 	}
 
 put_gk:
@@ -522,49 +523,45 @@ static int rxgk_verify_packet_encrypted(struct rxrpc_call *call,
 					struct sk_buff *skb)
 {
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
-	struct rxgk_header hdr;
-	unsigned int offset = sp->offset, len = sp->len;
+	struct rxgk_header *hdr;
+	unsigned int offset = 0, len = call->rx_dec_len;
+	void *data = call->rx_dec_buffer;
 	int ret;
 	u32 ac = 0;
 
 	_enter("");
 
-	ret = rxgk_decrypt_skb(gk->krb5, gk->rx_enc, skb, &offset, &len, &ac);
+	ret = rxgk_decrypt(gk->krb5, gk->rx_enc, data, &offset, &len, &ac);
 	if (ret < 0) {
 		if (ret != -ENOMEM)
 			rxrpc_abort_eproto(call, skb, ac, rxgk_abort_2_decrypt_eproto);
 		goto error;
 	}
 
-	if (len < sizeof(hdr)) {
+	if (len < sizeof(*hdr)) {
 		ret = rxrpc_abort_eproto(call, skb, RXGK_PACKETSHORT,
 					 rxgk_abort_2_short_header);
 		goto error;
 	}
 
 	/* Extract the header from the skb */
-	ret = skb_copy_bits(skb, offset, &hdr, sizeof(hdr));
-	if (ret < 0) {
-		ret = rxrpc_abort_eproto(call, skb, RXGK_PACKETSHORT,
-					 rxgk_abort_2_short_encdata);
-		goto error;
-	}
-	offset += sizeof(hdr);
-	len -= sizeof(hdr);
-
-	if (ntohl(hdr.epoch)		!= call->conn->proto.epoch ||
-	    ntohl(hdr.cid)		!= call->cid ||
-	    ntohl(hdr.call_number)	!= call->call_id ||
-	    ntohl(hdr.seq)		!= sp->hdr.seq ||
-	    ntohl(hdr.sec_index)	!= call->security_ix ||
-	    ntohl(hdr.data_len)		> len) {
+	hdr = data + offset;
+	offset += sizeof(*hdr);
+	len -= sizeof(*hdr);
+
+	if (ntohl(hdr->epoch)		!= call->conn->proto.epoch ||
+	    ntohl(hdr->cid)		!= call->cid ||
+	    ntohl(hdr->call_number)	!= call->call_id ||
+	    ntohl(hdr->seq)		!= sp->hdr.seq ||
+	    ntohl(hdr->sec_index)	!= call->security_ix ||
+	    ntohl(hdr->data_len)	> len) {
 		ret = rxrpc_abort_eproto(call, skb, RXGK_SEALEDINCON,
 					 rxgk_abort_2_short_data);
 		goto error;
 	}
 
-	sp->offset = offset;
-	sp->len = ntohl(hdr.data_len);
+	call->rx_dec_offset = offset;
+	call->rx_dec_len = ntohl(hdr->data_len);
 	ret = 0;
 error:
 	rxgk_put(gk);
diff --git a/net/rxrpc/rxgk_common.h b/net/rxrpc/rxgk_common.h
index 1e257d7ab8ec..dc8b0f106104 100644
--- a/net/rxrpc/rxgk_common.h
+++ b/net/rxrpc/rxgk_common.h
@@ -105,6 +105,45 @@ int rxgk_decrypt_skb(const struct krb5_enctype *krb5,
 	return ret;
 }
 
+/*
+ * Apply decryption and checksumming functions a flat data buffer.  The offset
+ * and length are updated to reflect the actual content of the encrypted
+ * region.
+ */
+static inline int rxgk_decrypt(const struct krb5_enctype *krb5,
+			       struct crypto_aead *aead,
+			       void *data,
+			       unsigned int *_offset, unsigned int *_len,
+			       int *_error_code)
+{
+	struct scatterlist sg[1];
+	size_t offset = 0, len = *_len;
+	int ret;
+
+	sg_init_one(sg, data, len);
+
+	ret = crypto_krb5_decrypt(krb5, aead, sg, 1, &offset, &len);
+	switch (ret) {
+	case 0:
+		*_offset += offset;
+		*_len = len;
+		break;
+	case -EBADMSG: /* Checksum mismatch. */
+	case -EPROTO:
+		*_error_code = RXGK_SEALEDINCON;
+		break;
+	case -EMSGSIZE:
+		*_error_code = RXGK_PACKETSHORT;
+		break;
+	case -ENOPKG: /* Would prefer RXGK_BADETYPE, but not available for YFS. */
+	default:
+		*_error_code = RXGK_INCONSISTENCY;
+		break;
+	}
+
+	return ret;
+}
+
 /*
  * Check the MIC on a region of an skbuff.  The offset and length are updated
  * to reflect the actual content of the secure region.
@@ -148,3 +187,43 @@ int rxgk_verify_mic_skb(const struct krb5_enctype *krb5,
 
 	return ret;
 }
+
+/*
+ * Check the MIC on a flat buffer.  The offset and length are updated to
+ * reflect the actual content of the secure region.
+ */
+static inline
+int rxgk_verify_mic(const struct krb5_enctype *krb5,
+		    struct crypto_shash *shash,
+		    const struct krb5_buffer *metadata,
+		    void *data,
+		    unsigned int *_offset, unsigned int *_len,
+		    u32 *_error_code)
+{
+	struct scatterlist sg[1];
+	size_t offset = 0, len = *_len;
+	int ret;
+
+	sg_init_one(sg, data, len);
+
+	ret = crypto_krb5_verify_mic(krb5, shash, metadata, sg, 1, &offset, &len);
+	switch (ret) {
+	case 0:
+		*_offset += offset;
+		*_len = len;
+		break;
+	case -EBADMSG: /* Checksum mismatch */
+	case -EPROTO:
+		*_error_code = RXGK_SEALEDINCON;
+		break;
+	case -EMSGSIZE:
+		*_error_code = RXGK_PACKETSHORT;
+		break;
+	case -ENOPKG: /* Would prefer RXGK_BADETYPE, but not available for YFS. */
+	default:
+		*_error_code = RXGK_INCONSISTENCY;
+		break;
+	}
+
+	return ret;
+}
diff --git a/net/rxrpc/rxkad.c b/net/rxrpc/rxkad.c
index cba7935977f0..075936337836 100644
--- a/net/rxrpc/rxkad.c
+++ b/net/rxrpc/rxkad.c
@@ -430,27 +430,25 @@ static int rxkad_verify_packet_1(struct rxrpc_call *call, struct sk_buff *skb,
 				 rxrpc_seq_t seq,
 				 struct skcipher_request *req)
 {
-	struct rxkad_level1_hdr sechdr;
+	struct rxkad_level1_hdr *sechdr;
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
 	struct rxrpc_crypt iv;
-	struct scatterlist sg[16];
-	u32 data_size, buf;
+	struct scatterlist sg[1];
+	void *data = call->rx_dec_buffer;
+	u32 len = sp->len, data_size, buf;
 	u16 check;
 	int ret;
 
 	_enter("");
 
-	if (sp->len < 8)
+	if (len < 8)
 		return rxrpc_abort_eproto(call, skb, RXKADSEALEDINCON,
 					  rxkad_abort_1_short_header);
 
 	/* Decrypt the skbuff in-place.  TODO: We really want to decrypt
 	 * directly into the target buffer.
 	 */
-	sg_init_table(sg, ARRAY_SIZE(sg));
-	ret = skb_to_sgvec(skb, sg, sp->offset, 8);
-	if (unlikely(ret < 0))
-		return ret;
+	sg_init_one(sg, data, len);
 
 	/* start the decryption afresh */
 	memset(&iv, 0, sizeof(iv));
@@ -464,13 +462,11 @@ static int rxkad_verify_packet_1(struct rxrpc_call *call, struct sk_buff *skb,
 		return ret;
 
 	/* Extract the decrypted packet length */
-	if (skb_copy_bits(skb, sp->offset, &sechdr, sizeof(sechdr)) < 0)
-		return rxrpc_abort_eproto(call, skb, RXKADDATALEN,
-					  rxkad_abort_1_short_encdata);
-	sp->offset += sizeof(sechdr);
-	sp->len    -= sizeof(sechdr);
+	sechdr = data;
+	call->rx_dec_offset = sizeof(*sechdr);
+	len -= sizeof(*sechdr);
 
-	buf = ntohl(sechdr.data_size);
+	buf = ntohl(sechdr->data_size);
 	data_size = buf & 0xffff;
 
 	check = buf >> 16;
@@ -479,10 +475,10 @@ static int rxkad_verify_packet_1(struct rxrpc_call *call, struct sk_buff *skb,
 	if (check != 0)
 		return rxrpc_abort_eproto(call, skb, RXKADSEALEDINCON,
 					  rxkad_abort_1_short_check);
-	if (data_size > sp->len)
+	if (data_size > len)
 		return rxrpc_abort_eproto(call, skb, RXKADDATALEN,
 					  rxkad_abort_1_short_data);
-	sp->len = data_size;
+	call->rx_dec_len = data_size;
 
 	_leave(" = 0 [dlen=%x]", data_size);
 	return 0;
@@ -496,43 +492,28 @@ static int rxkad_verify_packet_2(struct rxrpc_call *call, struct sk_buff *skb,
 				 struct skcipher_request *req)
 {
 	const struct rxrpc_key_token *token;
-	struct rxkad_level2_hdr sechdr;
+	struct rxkad_level2_hdr *sechdr;
 	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
 	struct rxrpc_crypt iv;
-	struct scatterlist _sg[4], *sg;
-	u32 data_size, buf;
+	struct scatterlist sg[1];
+	void *data = call->rx_dec_buffer;
+	u32 len = sp->len, data_size, buf;
 	u16 check;
-	int nsg, ret;
+	int ret;
 
-	_enter(",{%d}", sp->len);
+	_enter(",{%d}", len);
 
-	if (sp->len < 8)
+	if (len < 8)
 		return rxrpc_abort_eproto(call, skb, RXKADSEALEDINCON,
 					  rxkad_abort_2_short_header);
 
 	/* Don't let the crypto algo see a misaligned length. */
-	sp->len = round_down(sp->len, 8);
+	len = round_down(len, 8);
 
-	/* Decrypt the skbuff in-place.  TODO: We really want to decrypt
-	 * directly into the target buffer.
+	/* Decrypt in place in the call's decryption buffer.  TODO: We really
+	 * want to decrypt directly into the target buffer.
 	 */
-	sg = _sg;
-	nsg = skb_shinfo(skb)->nr_frags + 1;
-	if (nsg <= 4) {
-		nsg = 4;
-	} else {
-		sg = kmalloc_objs(*sg, nsg, GFP_NOIO);
-		if (!sg)
-			return -ENOMEM;
-	}
-
-	sg_init_table(sg, nsg);
-	ret = skb_to_sgvec(skb, sg, sp->offset, sp->len);
-	if (unlikely(ret < 0)) {
-		if (sg != _sg)
-			kfree(sg);
-		return ret;
-	}
+	sg_init_one(sg, data, len);
 
 	/* decrypt from the session key */
 	token = call->conn->key->payload.data[0];
@@ -540,11 +521,9 @@ static int rxkad_verify_packet_2(struct rxrpc_call *call, struct sk_buff *skb,
 
 	skcipher_request_set_sync_tfm(req, call->conn->rxkad.cipher);
 	skcipher_request_set_callback(req, 0, NULL, NULL);
-	skcipher_request_set_crypt(req, sg, sg, sp->len, iv.x);
+	skcipher_request_set_crypt(req, sg, sg, len, iv.x);
 	ret = crypto_skcipher_decrypt(req);
 	skcipher_request_zero(req);
-	if (sg != _sg)
-		kfree(sg);
 	if (ret < 0) {
 		if (ret == -ENOMEM)
 			return ret;
@@ -553,13 +532,11 @@ static int rxkad_verify_packet_2(struct rxrpc_call *call, struct sk_buff *skb,
 	}
 
 	/* Extract the decrypted packet length */
-	if (skb_copy_bits(skb, sp->offset, &sechdr, sizeof(sechdr)) < 0)
-		return rxrpc_abort_eproto(call, skb, RXKADDATALEN,
-					  rxkad_abort_2_short_len);
-	sp->offset += sizeof(sechdr);
-	sp->len    -= sizeof(sechdr);
+	sechdr = data;
+	call->rx_dec_offset = sizeof(*sechdr);
+	len -= sizeof(*sechdr);
 
-	buf = ntohl(sechdr.data_size);
+	buf = ntohl(sechdr->data_size);
 	data_size = buf & 0xffff;
 
 	check = buf >> 16;
@@ -569,17 +546,18 @@ static int rxkad_verify_packet_2(struct rxrpc_call *call, struct sk_buff *skb,
 		return rxrpc_abort_eproto(call, skb, RXKADSEALEDINCON,
 					  rxkad_abort_2_short_check);
 
-	if (data_size > sp->len)
+	if (data_size > len)
 		return rxrpc_abort_eproto(call, skb, RXKADDATALEN,
 					  rxkad_abort_2_short_data);
 
-	sp->len = data_size;
+	call->rx_dec_len = data_size;
 	_leave(" = 0 [dlen=%x]", data_size);
 	return 0;
 }
 
 /*
- * Verify the security on a received packet and the subpackets therein.
+ * Verify the security on a received (sub)packet.  If the packet needs
+ * modifying (e.g. decrypting), it must be copied.
  */
 static int rxkad_verify_packet(struct rxrpc_call *call, struct sk_buff *skb)
 {