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

[Daniel Borkmann]

[ +David ]

On 11/19/20 8:04 AM, xiakaixu1987@xxxxxxxxx wrote:
> From: Kaixu Xia <kaixuxia@xxxxxxxxxxx>
>
> 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 <kaixuxia@xxxxxxxxxxx>
> ---
>   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 <daniel@xxxxxxxxxxxxx>
---
 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