[PATCH bpf 4/4] xsk: fix potential race in SKB TX completion code

[Magnus Karlsson <magnus.karlsson@intel.com>]

There was a potential race in the TX completion code for
the SKB case when the TX napi thread and the error path
of the sendmsg code could both call the SKB destructor
at the same time. Fixed by introducing a spin_lock in the
destructor.

Fixes: 35fcde7f8deb ("xsk: support for Tx")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
---
 include/net/xdp_sock.h | 4 ++++
 net/xdp/xsk.c          | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
index 9fe472f2ac95..7161856bcf9c 100644
--- a/include/net/xdp_sock.h
+++ b/include/net/xdp_sock.h
@@ -60,6 +60,10 @@ struct xdp_sock {
 	bool zc;
 	/* Protects multiple processes in the control path */
 	struct mutex mutex;
+	/* Mutual exclusion of NAPI TX thread and sendmsg error paths
+	 * in the SKB destructor callback.
+	 */
+	spinlock_t tx_completion_lock;
 	u64 rx_dropped;
 };
 
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index d482f727f4c2..650c4da8dc5a 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -199,8 +199,11 @@ static void xsk_destruct_skb(struct sk_buff *skb)
 {
 	u64 addr = (u64)(long)skb_shinfo(skb)->destructor_arg;
 	struct xdp_sock *xs = xdp_sk(skb->sk);
+	unsigned long flags;
 
+	spin_lock_irqsave(&xs->tx_completion_lock, flags);
 	WARN_ON_ONCE(xskq_produce_addr(xs->umem->cq, addr));
+	spin_unlock_irqrestore(&xs->tx_completion_lock, flags);
 
 	sock_wfree(skb);
 }
@@ -754,6 +757,7 @@ static int xsk_create(struct net *net, struct socket *sock, int protocol,
 
 	xs = xdp_sk(sk);
 	mutex_init(&xs->mutex);
+	spin_lock_init(&xs->tx_completion_lock);
 
 	local_bh_disable();
 	sock_prot_inuse_add(net, &xsk_proto, 1);
-- 
2.7.4