Re: [PATCH] bpf: Check the return value of dev_get_by_index_rcu()

[Daniel Borkmann]

[ +David ]

On 11/19/20 8:04 AM, xiakaixu1...@gmail.com wrote:

From: Kaixu Xia <kaixu...@tencent.com>

The return value of dev_get_by_index_rcu() can be NULL, so here it
is need to check the return value and return error code if it is NULL.

Signed-off-by: Kaixu Xia <kaixu...@tencent.com>
---
  net/core/filter.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 2ca5eecebacf..1263fe07170a 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5573,6 +5573,8 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
                struct net_device *dev;

dev = dev_get_by_index_rcu(net, params->ifindex);

+               if (unlikely(!dev))
+                       return -EINVAL;
                if (!is_skb_forwardable(dev, skb))
                        rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;

The above logic is quite ugly anyway given we fetched the dev pointer already 
earlier
in bpf_ipv{4,6}_fib_lookup() and now need to redo it again ... so yeah there 
could be
a tiny race in here. We wanted do bring this logic closer to what XDP does 
anyway,
something like below, for example. David, thoughts? Thx

Subject: [PATCH] diff mtu check

Signed-off-by: Daniel Borkmann <dan...@iogearbox.net>
---
 net/core/filter.c | 22 +++++-----------------
 1 file changed, 5 insertions(+), 17 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 2ca5eecebacf..3bab0a97fa38 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5547,9 +5547,6 @@ static const struct bpf_func_proto 
bpf_xdp_fib_lookup_proto = {
 BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
           struct bpf_fib_lookup *, params, int, plen, u32, flags)
 {
-       struct net *net = dev_net(skb->dev);
-       int rc = -EAFNOSUPPORT;
-
        if (plen < sizeof(*params))
                return -EINVAL;

@@ -5559,25 +5556,16 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
        switch (params->family) {
 #if IS_ENABLED(CONFIG_INET)
        case AF_INET:
-               rc = bpf_ipv4_fib_lookup(net, params, flags, false);
-               break;
+               return bpf_ipv4_fib_lookup(dev_net(skb->dev), params, flags,
+                                          !skb_is_gso(skb));
 #endif
 #if IS_ENABLED(CONFIG_IPV6)
        case AF_INET6:
-               rc = bpf_ipv6_fib_lookup(net, params, flags, false);
-               break;
+               return bpf_ipv6_fib_lookup(dev_net(skb->dev), params, flags,
+                                          !skb_is_gso(skb));
 #endif
        }
-
-       if (!rc) {
-               struct net_device *dev;
-
-               dev = dev_get_by_index_rcu(net, params->ifindex);
-               if (!is_skb_forwardable(dev, skb))
-                       rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
-       }
-
-       return rc;
+       return -EAFNOSUPPORT;
 }

 static const struct bpf_func_proto bpf_skb_fib_lookup_proto = {
--
2.21.0