Merge branch 'net-af_packet-be-careful-when-expanding-mac-header-size'

Eric Dumazet says:

====================
net: af_packet: be careful when expanding mac header size

A recent regression in af_packet needed a preliminary debug patch,
which will presumably be useful for next bugs hunting.

The af_packet fix is to make sure MAC headers are contained in
skb linear part, as GSO stack requests.

v2: CONFIG_DEBUG_NET depends on CONFIG_NET to avoid compile
   errors found by kernel bots.
====================

Link: https://lore.kernel.org/r/20220602161859.2546399-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/skbuff.h

@@ -2696,7 +2696,14 @@ void *skb_pull(struct sk_buff *skb, unsigned int len);
 static inline void *__skb_pull(struct sk_buff *skb, unsigned int len)
 {
 	skb->len -= len;
-	BUG_ON(skb->len < skb->data_len);
+	if (unlikely(skb->len < skb->data_len)) {
+#if defined(CONFIG_DEBUG_NET)
+		skb->len += len;
+		pr_err("__skb_pull(len=%u)\n", len);
+		skb_dump(KERN_ERR, skb, false);
+#endif
+		BUG();
+	}
 	return skb->data += len;
 }
 

net/Kconfig.debug

@@ -20,7 +20,7 @@ config NET_NS_REFCNT_TRACKER
 
 config DEBUG_NET
 	bool "Add generic networking debug"
-	depends on DEBUG_KERNEL
+	depends on DEBUG_KERNEL && NET
 	help
 	  Enable extra sanity checks in networking.
 	  This is mostly used by fuzzers, but is safe to select.

net/packet/af_packet.c

@@ -1935,8 +1935,10 @@ static void packet_parse_headers(struct sk_buff *skb, struct socket *sock)
 	/* Move network header to the right position for VLAN tagged packets */
 	if (likely(skb->dev->type == ARPHRD_ETHER) &&
 	    eth_type_vlan(skb->protocol) &&
-	    __vlan_get_protocol(skb, skb->protocol, &depth) != 0)
-		skb_set_network_header(skb, depth);
+	    __vlan_get_protocol(skb, skb->protocol, &depth) != 0) {
+		if (pskb_may_pull(skb, depth))
+			skb_set_network_header(skb, depth);
+	}
 
 	skb_probe_transport_header(skb);
 }