net: use skb_header_pointer() for TCPv4 GSO frag_off check

Syzbot reported a KMSAN uninit-value warning in gso_features_check()
called from netif_skb_features() [1].

gso_features_check() reads iph->frag_off to decide whether to clear
mangleid_features. Accessing the IPv4 header via ip_hdr()/inner_ip_hdr()
can rely on skb header offsets that are not always safe for direct
dereference on packets injected from PF_PACKET paths.

Use skb_header_pointer() for the TCPv4 frag_off check so the header read
is robust whether data is already linear or needs copying.

[1] https://syzkaller.appspot.com/bug?extid=1543a7d954d9c6d00407

Link: https://lore.kernel.org/netdev/[email protected]/
Fixes: cbc53e0 ("GSO: Add GSO type for fixed IPv4 ID")
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=1543a7d954d9c6d00407
Tested-by: [email protected]
Signed-off-by: Guoyu Su <[email protected]>
Reviewed-by: Willem de Bruijn <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: CN-Scars

net/core/dev.c

@@ -3821,10 +3821,15 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb,
 	 * segmentation-offloads.rst).
 	 */
 	if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4) {
-		struct iphdr *iph = skb->encapsulation ?
-				    inner_ip_hdr(skb) : ip_hdr(skb);
+		const struct iphdr *iph;
+		struct iphdr _iph;
+		int nhoff = skb->encapsulation ?
+			    skb_inner_network_offset(skb) :
+			    skb_network_offset(skb);
 
-		if (!(iph->frag_off & htons(IP_DF)))
+		iph = skb_header_pointer(skb, nhoff, sizeof(_iph), &_iph);
+
+		if (!iph || !(iph->frag_off & htons(IP_DF)))
 			features &= ~dev->mangleid_features;
 	}