[PATCH] Bluetooth: avoid page fault from sco_send_frame()

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

Since userfaultfd mechanism allows sleeping with kernel lock held,
avoiding page fault with kernel lock held where possible will make
the module more robust. This patch just brings memcpy_from_msg() calls
to out of sock lock.

This patch is an instant mitigation for CVE-2021-3640. To fully close
the race window for this use-after-free problem, we need more changes.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 net/bluetooth/sco.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c
index d9a4e88dacbb..e4b079b31ce9 100644
--- a/net/bluetooth/sco.c
+++ b/net/bluetooth/sco.c
@@ -273,7 +273,7 @@ static int sco_connect(struct sock *sk)
 	return err;
 }
 
-static int sco_send_frame(struct sock *sk, struct msghdr *msg, int len)
+static int sco_send_frame(struct sock *sk, const void *buf, int len, int flags)
 {
 	struct sco_conn *conn = sco_pi(sk)->conn;
 	struct sk_buff *skb;
@@ -285,14 +285,11 @@ static int sco_send_frame(struct sock *sk, struct msghdr *msg, int len)
 
 	BT_DBG("sk %p len %d", sk, len);
 
-	skb = bt_skb_send_alloc(sk, len, msg->msg_flags & MSG_DONTWAIT, &err);
+	skb = bt_skb_send_alloc(sk, len, flags & MSG_DONTWAIT, &err);
 	if (!skb)
 		return err;
 
-	if (memcpy_from_msg(skb_put(skb, len), msg, len)) {
-		kfree_skb(skb);
-		return -EFAULT;
-	}
+	memcpy(skb_put(skb, len), buf, len);
 
 	hci_send_sco(conn->hcon, skb);
 
@@ -714,6 +711,7 @@ static int sco_sock_sendmsg(struct socket *sock, struct msghdr *msg,
 			    size_t len)
 {
 	struct sock *sk = sock->sk;
+	void *buf;
 	int err;
 
 	BT_DBG("sock %p, sk %p", sock, sk);
@@ -725,14 +723,23 @@ static int sco_sock_sendmsg(struct socket *sock, struct msghdr *msg,
 	if (msg->msg_flags & MSG_OOB)
 		return -EOPNOTSUPP;
 
+	buf = kmalloc(len, GFP_KERNEL | __GFP_NOWARN);
+	if (!buf)
+		return -ENOMEM;
+	if (memcpy_from_msg(buf, msg, len)) {
+		kfree(buf);
+		return -EFAULT;
+	}
+
 	lock_sock(sk);
 
 	if (sk->sk_state == BT_CONNECTED)
-		err = sco_send_frame(sk, msg, len);
+		err = sco_send_frame(sk, buf, len, msg->msg_flags);
 	else
 		err = -ENOTCONN;
 
 	release_sock(sk);
+	kfree(buf);
 	return err;
 }
 
-- 
2.30.2