[PATCH bpf] xsk: fix registration of Rx-only sockets

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

Having Rx-only AF_XDP sockets can potentially lead to a crash in the
system by a NULL pointer dereference in xsk_umem_consume_tx(). This
function iterates through a list of all sockets tied to a umem and
checks if there are any packets to send on the Tx ring. Rx-only
sockets do not have a Tx ring, so this will cause a NULL pointer
dereference. This will happen if you have registered one or more
Rx-only sockets to a umem and the driver is checking the Tx ring even
on Rx, or if the XDP_SHARED_UMEM mode is used and there is a mix of
Rx-only and other sockets tied to the same umem.

Fixed by only putting sockets with a Tx component on the list that
xsk_umem_consume_tx() iterates over.

Fixes: ac98d8aab61b ("xsk: wire upp Tx zero-copy functions")
Reported-by: Kal Cutter Conley <kal.conley@dectris.com>
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
---
 net/xdp/xdp_umem.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
index 16d5f35..3049af2 100644
--- a/net/xdp/xdp_umem.c
+++ b/net/xdp/xdp_umem.c
@@ -27,6 +27,9 @@ void xdp_add_sk_umem(struct xdp_umem *umem, struct xdp_sock *xs)
 {
 	unsigned long flags;
 
+	if (!xs->tx)
+		return;
+
 	spin_lock_irqsave(&umem->xsk_list_lock, flags);
 	list_add_rcu(&xs->list, &umem->xsk_list);
 	spin_unlock_irqrestore(&umem->xsk_list_lock, flags);
@@ -36,6 +39,9 @@ void xdp_del_sk_umem(struct xdp_umem *umem, struct xdp_sock *xs)
 {
 	unsigned long flags;
 
+	if (!xs->tx)
+		return;
+
 	spin_lock_irqsave(&umem->xsk_list_lock, flags);
 	list_del_rcu(&xs->list);
 	spin_unlock_irqrestore(&umem->xsk_list_lock, flags);
-- 
2.7.4