netdev - [PATCH bpf] bpf: lwtunnel: fix reroute supplying invalid dst

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Date:   Wed,  9 Oct 2019 10:31:24 +0200
From:   Jiri Benc <jbenc@...hat.com>
To:     bpf@...r.kernel.org, netdev@...r.kernel.org
Cc:     Peter Oskolkov <posk@...gle.com>
Subject: [PATCH bpf] bpf: lwtunnel: fix reroute supplying invalid dst

The dst in bpf_input() has lwtstate field set. As it is of the
LWTUNNEL_ENCAP_BPF type, lwtstate->data is struct bpf_lwt. When the bpf
program returns BPF_LWT_REROUTE, ip_route_input_noref is directly called on
this skb. This causes invalid memory access, as ip_route_input_slow calls
skb_tunnel_info(skb) that expects the dst->lwstate->data to be
struct ip_tunnel_info. This results to struct bpf_lwt being accessed as
struct ip_tunnel_info.

Drop the dst before calling the IP route input functions (both for IPv4 and
IPv6).

Reported by KASAN.

Fixes: 3bd0b15281af ("bpf: add handling of BPF_LWT_REROUTE to lwt_bpf.c")
Cc: Peter Oskolkov <posk@...gle.com>
Signed-off-by: Jiri Benc <jbenc@...hat.com>
---
 net/core/lwt_bpf.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index f93785e5833c..74cfb8b5ab33 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -88,11 +88,16 @@ static int bpf_lwt_input_reroute(struct sk_buff *skb)
 	int err = -EINVAL;

 	if (skb->protocol == htons(ETH_P_IP)) {
+		struct net_device *dev = skb_dst(skb)->dev;
 		struct iphdr *iph = ip_hdr(skb);

+		dev_hold(dev);
+		skb_dst_drop(skb);
 		err = ip_route_input_noref(skb, iph->daddr, iph->saddr,
-					   iph->tos, skb_dst(skb)->dev);
+					   iph->tos, dev);
+		dev_put(dev);
 	} else if (skb->protocol == htons(ETH_P_IPV6)) {
+		skb_dst_drop(skb);
 		err = ipv6_stub->ipv6_route_input(skb);
 	} else {
 		err = -EAFNOSUPPORT;
-- 
2.18.1