[PATCH net] net/netfilter: bpf: avoid leakage of skb

[D. Wythe]

From: "D. Wythe" <alibuda@xxxxxxxxxxxxxxxxx>

A malicious eBPF program can interrupt the subsequent processing of
a skb by returning an exceptional retval, and no one will be responsible
for releasing the very skb.

Moreover, normal programs can also have the demand to return NF_STOLEN,
usually, the hook needs to take responsibility for releasing this skb
itself, but currently, there is no such helper function to achieve that.
Ignoring NF_STOLEN will also lead to skb leakage.

Fixes: fd9c663b9ad6 ("bpf: minimal support for programs hooked into netfilter framework")
Signed-off-by: D. Wythe <alibuda@xxxxxxxxxxxxxxxxx>
---
 net/netfilter/nf_bpf_link.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
index e502ec0..03c47d6 100644
--- a/net/netfilter/nf_bpf_link.c
+++ b/net/netfilter/nf_bpf_link.c
@@ -12,12 +12,29 @@ static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb,
 				    const struct nf_hook_state *s)
 {
 	const struct bpf_prog *prog = bpf_prog;
+	unsigned int verdict;
 	struct bpf_nf_ctx ctx = {
 		.state = s,
 		.skb = skb,
 	};
 
-	return bpf_prog_run(prog, &ctx);
+	verdict = bpf_prog_run(prog, &ctx);
+	switch (verdict) {
+	case NF_STOLEN:
+		consume_skb(skb);
+		fallthrough;
+	case NF_ACCEPT:
+	case NF_DROP:
+	case NF_QUEUE:
+		/* restrict the retval of the ebpf programs */
+		break;
+	default:
+		/* force it to be dropped */
+		verdict = NF_DROP_ERR(-EINVAL);
+		break;
+	}
+
+	return verdict;
 }
 
 struct bpf_nf_link {
-- 
1.8.3.1