[PATCH net v6] net: use skb_header_pointer() for TCPv4 GSO frag_off

[Guoyu Su]

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.

This also removes the SKB_GSO_DODGY special casing: skb_header_pointer()
already fast-paths linear data, so a separate direct-access path is not
needed.

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

Link: https://lore.kernel.org/netdev/willemdebruijn.kernel.1a9f35039caab@xxxxxxxxx/
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>
---
v6:
 - Use skb_header_pointer() for both DODGY and non-DODGY TCPv4 GSO
   packets in gso_features_check().
 - Drop the SKB_GSO_DODGY special-casing for IPv4 header access.

v5: https://lore.kernel.org/netdev/20260320141459.9691-1-yss2813483011xxl@xxxxxxxxx/
v4: https://lore.kernel.org/netdev/20260319005421.14908-1-yss2813483011xxl@xxxxxxxxx/
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 | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 14a83f2035b9..8a15ca67cfed 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3805,10 +3805,16 @@ 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;
 
-		if (!(iph->frag_off & htons(IP_DF)))
+		int nhoff = skb->encapsulation ?
+			    skb_inner_network_offset(skb) :
+			    skb_network_offset(skb);
+
+		iph = skb_header_pointer(skb, nhoff, sizeof(_iph), &_iph);
+
+		if (!iph || !(iph->frag_off & htons(IP_DF)))
 			features &= ~dev->mangleid_features;
 	}
 
-- 
2.34.1