[PATCH v2 2/3] pppoatm: fix race condition with destroying of vcc

[Krzysztof Mazur <krzysiek@podlesie.net>]

The pppoatm_send() calls vcc->send() and now also checks for
some vcc flags that indicate destroyed vcc without proper locking.

The vcc_sendmsg() uses lock_sock(sk). This lock is used by
vcc_release(), so vcc_destroy_socket() will not be called between
check and during ->send(). The vcc_release_async() sets ATM_VF_CLOSE,
but it should be safe to call ->send() after it, because
vcc->dev->ops->close() is not called.

The pppoatm_send() is called with BH disabled, so bh_lock_sock()
should be used instead of lock_sock().

Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Cc: David Woodhouse <dwmw2@infradead.org>
---
 net/atm/pppoatm.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/net/atm/pppoatm.c b/net/atm/pppoatm.c
index 0dcb5dc..e3b2d69 100644
--- a/net/atm/pppoatm.c
+++ b/net/atm/pppoatm.c
@@ -270,6 +270,7 @@ static int pppoatm_send(struct ppp_channel *chan, struct sk_buff *skb)
 {
 	struct pppoatm_vcc *pvcc = chan_to_pvcc(chan);
 	struct atm_vcc *vcc;
+	int ret;
 
 	ATM_SKB(skb)->vcc = pvcc->atmvcc;
 	pr_debug("(skb=0x%p, vcc=0x%p)\n", skb, pvcc->atmvcc);
@@ -304,17 +305,24 @@ static int pppoatm_send(struct ppp_channel *chan, struct sk_buff *skb)
 	}
 
 	vcc = ATM_SKB(skb)->vcc;
+	bh_lock_sock(sk_atm(vcc));
+	if (sock_owned_by_user(sk_atm(vcc)))
+		goto nospace_unlock_sock;
 	if (test_bit(ATM_VF_RELEASED, &vcc->flags)
 			|| test_bit(ATM_VF_CLOSE, &vcc->flags)
 			|| !test_bit(ATM_VF_READY, &vcc->flags))
-		goto nospace;
+		goto nospace_unlock_sock;
 
 	atomic_add(skb->truesize, &sk_atm(ATM_SKB(skb)->vcc)->sk_wmem_alloc);
 	ATM_SKB(skb)->atm_options = ATM_SKB(skb)->vcc->atm_options;
 	pr_debug("atm_skb(%p)->vcc(%p)->dev(%p)\n",
 		 skb, ATM_SKB(skb)->vcc, ATM_SKB(skb)->vcc->dev);
-	return ATM_SKB(skb)->vcc->send(ATM_SKB(skb)->vcc, skb)
+	ret = ATM_SKB(skb)->vcc->send(ATM_SKB(skb)->vcc, skb)
 	    ? DROP_PACKET : 1;
+	bh_unlock_sock(sk_atm(vcc));
+	return ret;
+nospace_unlock_sock:
+	bh_unlock_sock(sk_atm(vcc));
 nospace:
 	/*
 	 * We don't have space to send this SKB now, but we might have
-- 
1.8.0.2.g35080e9