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/willemdebruijn.kernel.1a9f35039caab@gmail.com/
Fixes: cbc53e08a793 ("GSO: Add GSO type for fixed IPv4 ID")
Reported-by: syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1543a7d954d9c6d00407
Tested-by: syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com
Signed-off-by: Guoyu Su <yss2813483011xxl@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260327153507.39742-1-yss2813483011xxl@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/core/dev.c b/net/core/dev.c
index fc555706241436d790b7910451ec10de310ccce1..831129f2a69b5005e99b8309107f2391badf11db 100644 (file)
--- a/net/core/dev.c
+++ b/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;
        }