bpf, testing: Introduce 'gso_linear_no_head_frag' skb_segment test

Following reports of skb_segment() hitting a BUG_ON when working on
GROed skbs which have their gso_size mangled (e.g. after a
bpf_skb_change_proto call), add a reproducer test that mimics the
input skbs that lead to the mentioned BUG_ON as in [1] and validates the
fix submitted in [2].

[1] https://lists.openwall.net/netdev/2019/08/26/110
[2] commit 3dcbdb13 ("net: gso: Fix skb_segment splat when splitting gso_size mangled skb having linear-headed frag_list")
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191025134223.2761-3-shmulik.ladkani@gmail.com

lib/test_bpf.c

@@ -6859,6 +6859,60 @@ static __init struct sk_buff *build_test_skb(void)
 	return NULL;
 }
 
+static __init struct sk_buff *build_test_skb_linear_no_head_frag(void)
+{
+	unsigned int alloc_size = 2000;
+	unsigned int headroom = 102, doffset = 72, data_size = 1308;
+	struct sk_buff *skb[2];
+	int i;
+
+	/* skbs linked in a frag_list, both with linear data, with head_frag=0
+	 * (data allocated by kmalloc), both have tcp data of 1308 bytes
+	 * (total payload is 2616 bytes).
+	 * Data offset is 72 bytes (40 ipv6 hdr, 32 tcp hdr). Some headroom.
+	 */
+	for (i = 0; i < 2; i++) {
+		skb[i] = alloc_skb(alloc_size, GFP_KERNEL);
+		if (!skb[i]) {
+			if (i == 0)
+				goto err_skb0;
+			else
+				goto err_skb1;
+		}
+
+		skb[i]->protocol = htons(ETH_P_IPV6);
+		skb_reserve(skb[i], headroom);
+		skb_put(skb[i], doffset + data_size);
+		skb_reset_network_header(skb[i]);
+		if (i == 0)
+			skb_reset_mac_header(skb[i]);
+		else
+			skb_set_mac_header(skb[i], -ETH_HLEN);
+		__skb_pull(skb[i], doffset);
+	}
+
+	/* setup shinfo.
+	 * mimic bpf_skb_proto_4_to_6, which resets gso_segs and assigns a
+	 * reduced gso_size.
+	 */
+	skb_shinfo(skb[0])->gso_size = 1288;
+	skb_shinfo(skb[0])->gso_type = SKB_GSO_TCPV6 | SKB_GSO_DODGY;
+	skb_shinfo(skb[0])->gso_segs = 0;
+	skb_shinfo(skb[0])->frag_list = skb[1];
+
+	/* adjust skb[0]'s len */
+	skb[0]->len += skb[1]->len;
+	skb[0]->data_len += skb[1]->len;
+	skb[0]->truesize += skb[1]->truesize;
+
+	return skb[0];
+
+err_skb1:
+	kfree_skb(skb[0]);
+err_skb0:
+	return NULL;
+}
+
 struct skb_segment_test {
 	const char *descr;
 	struct sk_buff *(*build_skb)(void);
@@ -6871,6 +6925,15 @@ static struct skb_segment_test skb_segment_tests[] __initconst = {
 		.build_skb = build_test_skb,
 		.features = NETIF_F_SG | NETIF_F_GSO_PARTIAL | NETIF_F_IP_CSUM |
 			    NETIF_F_IPV6_CSUM | NETIF_F_RXCSUM
+	},
+	{
+		.descr = "gso_linear_no_head_frag",
+		.build_skb = build_test_skb_linear_no_head_frag,
+		.features = NETIF_F_SG | NETIF_F_FRAGLIST |
+			    NETIF_F_HW_VLAN_CTAG_TX | NETIF_F_GSO |
+			    NETIF_F_LLTX_BIT | NETIF_F_GRO |
+			    NETIF_F_IPV6_CSUM | NETIF_F_RXCSUM |
+			    NETIF_F_HW_VLAN_STAG_TX_BIT
 	}
 };