[PATCH v5 bpf 1/4] lwt: fix return values of BPF ops

[Yan Zhai <yan@cloudflare.com>]

BPF encap ops can return different types of positive values, such like
NET_RX_DROP, NET_XMIT_CN, NETDEV_TX_BUSY, and so on, from function
skb_do_redirect and bpf_lwt_xmit_reroute. At the xmit hook, such return
values would be treated implicitly as LWTUNNEL_XMIT_CONTINUE in
ip(6)_finish_output2. When this happens, skbs that have been freed would
continue to the neighbor subsystem, causing use-after-free bug and
kernel crashes.

To fix the incorrect behavior, skb_do_redirect return values can be
simply discarded, the same as tc-egress behavior. On the other hand,
bpf_lwt_xmit_reroute returns useful errors to local senders, e.g. PMTU
information. Thus convert its return values to avoid the conflict with
LWTUNNEL_XMIT_CONTINUE.

Fixes: 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure")
Suggested-by: Martin KaFai Lau <martin.lau@linux.dev>
Suggested-by: Stanislav Fomichev <sdf@google.com>
Reported-by: Jordan Griege <jgriege@cloudflare.com>
Signed-off-by: Yan Zhai <yan@cloudflare.com>
---
* v5: discards skb_do_redirect return instead; convert
      bpf_lwt_xmit_reroute return;
* v4: minor commit message changes
* v3: converts skb_do_redirect statuses from both ingress and egress
* v2: code style amend
---
 net/core/lwt_bpf.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index 8b6b5e72b217..4a0797f0a154 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -60,9 +60,8 @@ static int run_lwt_bpf(struct sk_buff *skb, struct bpf_lwt_prog *lwt,
 			ret = BPF_OK;
 		} else {
 			skb_reset_mac_header(skb);
-			ret = skb_do_redirect(skb);
-			if (ret == 0)
-				ret = BPF_REDIRECT;
+			skb_do_redirect(skb);
+			ret = BPF_REDIRECT;
 		}
 		break;
 
@@ -255,7 +254,7 @@ static int bpf_lwt_xmit_reroute(struct sk_buff *skb)
 
 	err = dst_output(dev_net(skb_dst(skb)->dev), skb->sk, skb);
 	if (unlikely(err))
-		return err;
+		return net_xmit_errno(err);
 
 	/* ip[6]_finish_output2 understand LWTUNNEL_XMIT_DONE */
 	return LWTUNNEL_XMIT_DONE;
-- 
2.30.2