[PATCH net v4] net: use skb_header_pointer() only for DODGY TCPv4 GSO skbs

[Guoyu Su]

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

The current direct skb->len check is not sufficient for SKB_GSO_DODGY
packets. In the AF_PACKET/PACKET_VNET_HDR path, packet_snd() can build
a DODGY GSO skb whose total length is large enough, while the IPv4
header is not fully available as initialized linear data for a direct
iph->frag_off access.

Use skb_header_pointer() to fetch the IPv4 header only for DODGY packets
and clear mangleid_features conservatively if the header cannot be
accessed or the DF bit is not set.

Keep the existing direct ip_hdr()/inner_ip_hdr() access for non-DODGY
packets so that buggy internal callers are still exposed.

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

Fixes: cbc53e08a793 ("GSO: Add GSO type for fixed IPv4 ID")
Reported-by: syzbot+1543a7d954d9c6d00407@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=1543a7d954d9c6d00407
Tested-by: syzbot+1543a7d954d9c6d00407@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Guoyu Su <yss2813483011xxl@xxxxxxxxx>
---
v4:
 - Move struct iphdr _iph to the outer scope so a pointer returned by
   skb_header_pointer() remains valid when iph->frag_off is checked.

v3: https://lore.kernel.org/netdev/20260312104351.185370-1-yss2813483011xxl@xxxxxxxxx/
v2: https://lore.kernel.org/netdev/20260308083319.1255118-1-yss2813483011xxl@xxxxxxxxx/
v1: https://lore.kernel.org/netdev/20260307162905.3697050-1-yss2813483011xxl@xxxxxxxxx/

 net/core/dev.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 14a83f2035b9..d47bf7b1fa99 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3805,10 +3805,20 @@ 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);
+		int nhoff = skb->encapsulation ?
+			    skb_inner_network_offset(skb) :
+			    skb_network_offset(skb);
+		const struct iphdr *iph;
+		struct iphdr _iph;
+
+		if (unlikely(skb_shinfo(skb)->gso_type & SKB_GSO_DODGY)) {
+			iph = nhoff < 0 ? NULL :
+			      skb_header_pointer(skb, nhoff, sizeof(_iph), &_iph);
+		} else {
+			iph = skb->encapsulation ? inner_ip_hdr(skb) : ip_hdr(skb);
+		}
 
-		if (!(iph->frag_off & htons(IP_DF)))
+		if (!iph || !(iph->frag_off & htons(IP_DF)))
 			features &= ~dev->mangleid_features;
 	}
 
-- 
2.34.1