[PATCH net v3] atm: Fix Use-After-Free in do_vcc_ioctl

[Hyunwoo Kim <v4bel@theori.io>]

Because do_vcc_ioctl() accesses sk->sk_receive_queue
without holding a sk->sk_receive_queue.lock, it can
cause a race with vcc_recvmsg().
A use-after-free for skb occurs with the following flow.
```
do_vcc_ioctl() -> skb_peek()
vcc_recvmsg() -> skb_recv_datagram() -> skb_free_datagram()
```
Add sk->sk_receive_queue.lock to do_vcc_ioctl() to fix this issue.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Hyunwoo Kim <v4bel@theori.io>
---
v1 -> v2: Change the code style
v2 -> v3: Change to 'int amount'
---
 net/atm/ioctl.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/atm/ioctl.c b/net/atm/ioctl.c
index 838ebf0cabbf..f81f8d56f5c0 100644
--- a/net/atm/ioctl.c
+++ b/net/atm/ioctl.c
@@ -73,14 +73,17 @@ static int do_vcc_ioctl(struct socket *sock, unsigned int cmd,
 	case SIOCINQ:
 	{
 		struct sk_buff *skb;
+		int amount;
 
 		if (sock->state != SS_CONNECTED) {
 			error = -EINVAL;
 			goto done;
 		}
+		spin_lock_irq(&sk->sk_receive_queue.lock);
 		skb = skb_peek(&sk->sk_receive_queue);
-		error = put_user(skb ? skb->len : 0,
-				 (int __user *)argp) ? -EFAULT : 0;
+		amount = skb ? skb->len : 0;
+		spin_unlock_irq(&sk->sk_receive_queue.lock);
+		error = put_user(amount, (int __user *)argp) ? -EFAULT : 0;
 		goto done;
 	}
 	case ATM_SETSC:
-- 
2.25.1