[PATCH v4 bpf 1/2] bpf: fix skb_do_redirect return values

[Yan Zhai <yan@cloudflare.com>]

skb_do_redirect returns various of values: error code (negative),
0 (success), and some positive status code, e.g. NET_XMIT_CN,
NET_RX_DROP. Commit 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel
infrastructure") didn't check the return code correctly, so positive
values are propagated back along call chain:

  ip_finish_output2
    -> bpf_xmit
      -> run_lwt_bpf
        -> skb_do_redirect

Inside ip_finish_output2, redirected skb will continue to neighbor
subsystem as if LWTUNNEL_XMIT_CONTINUE is returned, despite that this
skb could have been freed. The bug can trigger use-after-free warning
and crashes kernel afterwards:

https://gist.github.com/zhaiyan920/8fbac245b261fe316a7ef04c9b1eba48

Convert positive statuses from skb_do_redirect eliminates this issue.

Fixes: 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure")
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Suggested-by: Markus Elfring <Markus.Elfring@web.de>
Suggested-by: Stanislav Fomichev <sdf@google.com>
Reported-by: Jordan Griege <jgriege@cloudflare.com>
Signed-off-by: Yan Zhai <yan@cloudflare.com>
---
 include/linux/netdevice.h | 2 ++
 net/core/filter.c         | 9 +++++++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index b828c7a75be2..520d808eec15 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -87,6 +87,8 @@ void netdev_sw_irq_coalesce_default_on(struct net_device *dev);
 #define NET_RX_SUCCESS		0	/* keep 'em coming, baby */
 #define NET_RX_DROP		1	/* packet dropped */
 
+#define net_rx_errno(e)		((e) == NET_RX_DROP ? -ENOBUFS : (e))
+
 #define MAX_NEST_DEV 8
 
 /*
diff --git a/net/core/filter.c b/net/core/filter.c
index 06ba0e56e369..564104543737 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2095,7 +2095,9 @@ static const struct bpf_func_proto bpf_csum_level_proto = {
 
 static inline int __bpf_rx_skb(struct net_device *dev, struct sk_buff *skb)
 {
-	return dev_forward_skb_nomtu(dev, skb);
+	int ret = dev_forward_skb_nomtu(dev, skb);
+
+	return net_rx_errno(ret);
 }
 
 static inline int __bpf_rx_skb_no_mac(struct net_device *dev,
@@ -2108,7 +2110,7 @@ static inline int __bpf_rx_skb_no_mac(struct net_device *dev,
 		ret = netif_rx(skb);
 	}
 
-	return ret;
+	return net_rx_errno(ret);
 }
 
 static inline int __bpf_tx_skb(struct net_device *dev, struct sk_buff *skb)
@@ -2129,6 +2131,9 @@ static inline int __bpf_tx_skb(struct net_device *dev, struct sk_buff *skb)
 	ret = dev_queue_xmit(skb);
 	dev_xmit_recursion_dec();
 
+	if (unlikely(ret > 0))
+		ret = net_xmit_errno(ret);
+
 	return ret;
 }
 
-- 
2.30.2