Bluetooth: Provide L2CAP ops callback for memcpy_fromiovec

The highly optimized TX path for L2CAP channels and its fragmentation
within the HCI ACL packets requires to copy data from user provided
IO vectors and also kernel provided memory buffers.

This patch allows channel clients to provide a memcpy_fromiovec callback
to keep this optimized behavior, but adapt it to kernel vs user memory
for the TX path. For all kernel internal L2CAP channels, a default
implementation is provided that can be referenced.

In case of A2MP, this fixes a long-standing issue with wrongly accessing
kernel memory as user memory.

This patch originally by Marcel Holtmann.
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/net/bluetooth/l2cap.h

@@ -602,6 +602,10 @@ struct l2cap_ops {
 	struct sk_buff		*(*alloc_skb) (struct l2cap_chan *chan,
 					       unsigned long hdr_len,
 					       unsigned long len, int nb);
+	int			(*memcpy_fromiovec) (struct l2cap_chan *chan,
+						     unsigned char *kdata,
+						     struct iovec *iov,
+						     int len);
 };
 
 struct l2cap_conn {
@@ -857,6 +861,31 @@ static inline long l2cap_chan_no_get_sndtimeo(struct l2cap_chan *chan)
 	return 0;
 }
 
+static inline int l2cap_chan_no_memcpy_fromiovec(struct l2cap_chan *chan,
+						 unsigned char *kdata,
+						 struct iovec *iov,
+						 int len)
+{
+	/* Following is safe since for compiler definitions of kvec and
+	 * iovec are identical, yielding the same in-core layout and alignment
+	 */
+	struct kvec *vec = (struct kvec *)iov;
+
+	while (len > 0) {
+		if (vec->iov_len) {
+			int copy = min_t(unsigned int, len, vec->iov_len);
+			memcpy(kdata, vec->iov_base, copy);
+			len -= copy;
+			kdata += copy;
+			vec->iov_base += copy;
+			vec->iov_len -= copy;
+		}
+		vec++;
+	}
+
+	return 0;
+}
+
 extern bool disable_ertm;
 
 int l2cap_init_sockets(void);

net/bluetooth/a2mp.c

@@ -720,6 +720,7 @@ static const struct l2cap_ops a2mp_chan_ops = {
 	.resume = l2cap_chan_no_resume,
 	.set_shutdown = l2cap_chan_no_set_shutdown,
 	.get_sndtimeo = l2cap_chan_no_get_sndtimeo,
+	.memcpy_fromiovec = l2cap_chan_no_memcpy_fromiovec,
 };
 
 static struct l2cap_chan *a2mp_chan_open(struct l2cap_conn *conn, bool locked)

net/bluetooth/l2cap_core.c

@@ -2118,7 +2118,8 @@ static inline int l2cap_skbuff_fromiovec(struct l2cap_chan *chan,
 	struct sk_buff **frag;
 	int sent = 0;
 
-	if (memcpy_fromiovec(skb_put(skb, count), msg->msg_iov, count))
+	if (chan->ops->memcpy_fromiovec(chan, skb_put(skb, count),
+					msg->msg_iov, count))
 		return -EFAULT;
 
 	sent += count;
@@ -2138,7 +2139,8 @@ static inline int l2cap_skbuff_fromiovec(struct l2cap_chan *chan,
 
 		*frag = tmp;
 
-		if (memcpy_fromiovec(skb_put(*frag, count), msg->msg_iov, count))
+		if (chan->ops->memcpy_fromiovec(chan, skb_put(*frag, count),
+						msg->msg_iov, count))
 			return -EFAULT;
 
 		sent += count;

net/bluetooth/l2cap_sock.c

@@ -1313,6 +1313,13 @@ static struct sk_buff *l2cap_sock_alloc_skb_cb(struct l2cap_chan *chan,
 	return skb;
 }
 
+static int l2cap_sock_memcpy_fromiovec_cb(struct l2cap_chan *chan,
+					  unsigned char *kdata,
+					  struct iovec *iov, int len)
+{
+	return memcpy_fromiovec(kdata, iov, len);
+}
+
 static void l2cap_sock_ready_cb(struct l2cap_chan *chan)
 {
 	struct sock *sk = chan->data;
@@ -1392,6 +1399,7 @@ static const struct l2cap_ops l2cap_chan_ops = {
 	.set_shutdown		= l2cap_sock_set_shutdown_cb,
 	.get_sndtimeo		= l2cap_sock_get_sndtimeo_cb,
 	.alloc_skb		= l2cap_sock_alloc_skb_cb,
+	.memcpy_fromiovec	= l2cap_sock_memcpy_fromiovec_cb,
 };
 
 static void l2cap_sock_destruct(struct sock *sk)