[PATCH 4.9 15/56] tun: properly test for IFF_UP

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@xxxxxxxxxx>

[ Upstream commit 4477138fa0ae4e1b699786ef0600863ea6e6c61c ]

Same reasons than the ones explained in commit 4179cb5a4c92
("vxlan: test dev->flags & IFF_UP before calling netif_rx()")

netif_rx_ni() or napi_gro_frags() must be called under a strict contract.

At device dismantle phase, core networking clears IFF_UP
and flush_all_backlogs() is called after rcu grace period
to make sure no incoming packet might be in a cpu backlog
and still referencing the device.

A similar protocol is used for gro layer.

Most drivers call netif_rx() from their interrupt handler,
and since the interrupts are disabled at device dismantle,
netif_rx() does not have to check dev->flags & IFF_UP

Virtual drivers do not have this guarantee, and must
therefore make the check themselves.

Fixes: 1bd4978a88ac ("tun: honor IFF_UP in tun_get_user()")
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/net/tun.c |   15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1194,9 +1194,6 @@ static ssize_t tun_get_user(struct tun_s
 	u32 rxhash;
 	ssize_t n;
 
-	if (!(tun->dev->flags & IFF_UP))
-		return -EIO;
-
 	if (!(tun->flags & IFF_NO_PI)) {
 		if (len < sizeof(pi))
 			return -EINVAL;
@@ -1273,9 +1270,11 @@ static ssize_t tun_get_user(struct tun_s
 		err = skb_copy_datagram_from_iter(skb, 0, from, len);
 
 	if (err) {
+		err = -EFAULT;
+drop:
 		this_cpu_inc(tun->pcpu_stats->rx_dropped);
 		kfree_skb(skb);
-		return -EFAULT;
+		return err;
 	}
 
 	err = virtio_net_hdr_to_skb(skb, &gso, tun_is_little_endian(tun));
@@ -1327,7 +1326,15 @@ static ssize_t tun_get_user(struct tun_s
 	skb_probe_transport_header(skb, 0);
 
 	rxhash = skb_get_hash(skb);
+
+	rcu_read_lock();
+	if (unlikely(!(tun->dev->flags & IFF_UP))) {
+		err = -EIO;
+		goto drop;
+	}
+
 	netif_rx_ni(skb);
+	rcu_read_unlock();
 
 	stats = get_cpu_ptr(tun->pcpu_stats);
 	u64_stats_update_begin(&stats->syncp);