[PATCH bpf-next] bpf: Fix dev pointer dereference from sk_skb

[Joe Stringer]

Dan Carpenter reports:

The patch 6acc9b432e67: "bpf: Add helper to retrieve socket in BPF"
from Oct 2, 2018, leads to the following Smatch complaint:

    net/core/filter.c:4893 bpf_sk_lookup()
    error: we previously assumed 'skb->dev' could be null (see line 4885)

Fix this issue by checking skb->dev before using it.

Signed-off-by: Joe Stringer <j...@wand.net.nz>
---
 net/core/filter.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 4bbc6567fcb8..b844761b5d4c 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4821,9 +4821,12 @@ static const struct bpf_func_proto 
bpf_lwt_seg6_adjust_srh_proto = {
 static struct sock *sk_lookup(struct net *net, struct bpf_sock_tuple *tuple,
                              struct sk_buff *skb, u8 family, u8 proto)
 {
-       int dif = skb->dev->ifindex;
        bool refcounted = false;
        struct sock *sk = NULL;
+       int dif = 0;
+
+       if (skb->dev)
+               dif = skb->dev->ifindex;
 
        if (family == AF_INET) {
                __be32 src4 = tuple->ipv4.saddr;
-- 
2.17.1