ipv6: reject malicious packets in ipv6_gso_segment()

syzbot was able to craft a packet with very long IPv6 extension headers
leading to an overflow of skb->transport_header.

This 16bit field has a limited range.

Add skb_reset_transport_header_careful() helper and use it
from ipv6_gso_segment()

WARNING: CPU: 0 PID: 5871 at ./include/linux/skbuff.h:3032 skb_reset_transport_header include/linux/skbuff.h:3032 [inline]
WARNING: CPU: 0 PID: 5871 at ./include/linux/skbuff.h:3032 ipv6_gso_segment+0x15e2/0x21e0 net/ipv6/ip6_offload.c:151
Modules linked in:
CPU: 0 UID: 0 PID: 5871 Comm: syz-executor211 Not tainted 6.16.0-rc6-syzkaller-g7abc678e3084 #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
 RIP: 0010:skb_reset_transport_header include/linux/skbuff.h:3032 [inline]
 RIP: 0010:ipv6_gso_segment+0x15e2/0x21e0 net/ipv6/ip6_offload.c:151
Call Trace:
 <TASK>
  skb_mac_gso_segment+0x31c/0x640 net/core/gso.c:53
  nsh_gso_segment+0x54a/0xe10 net/nsh/nsh.c:110
  skb_mac_gso_segment+0x31c/0x640 net/core/gso.c:53
  __skb_gso_segment+0x342/0x510 net/core/gso.c:124
  skb_gso_segment include/net/gso.h:83 [inline]
  validate_xmit_skb+0x857/0x11b0 net/core/dev.c:3950
  validate_xmit_skb_list+0x84/0x120 net/core/dev.c:4000
  sch_direct_xmit+0xd3/0x4b0 net/sched/sch_generic.c:329
  __dev_xmit_skb net/core/dev.c:4102 [inline]
  __dev_queue_xmit+0x17b6/0x3a70 net/core/dev.c:4679

Fixes: d1da932 ("ipv6: Separate ipv6 offload support")
Reported-by: syzbot+af43e647fd835acc02df@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/688a1a05.050a0220.5d226.0008.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Dawid Osuchowski <dawid.osuchowski@linux.intel.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250730131738.3385939-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

include/linux/skbuff.h

@@ -3033,6 +3033,29 @@ static inline void skb_reset_transport_header(struct sk_buff *skb)
 	skb->transport_header = offset;
 }
 
+/**
+ * skb_reset_transport_header_careful - conditionally reset transport header
+ * @skb: buffer to alter
+ *
+ * Hardened version of skb_reset_transport_header().
+ *
+ * Returns: true if the operation was a success.
+ */
+static inline bool __must_check
+skb_reset_transport_header_careful(struct sk_buff *skb)
+{
+	long offset = skb->data - skb->head;
+
+	if (unlikely(offset != (typeof(skb->transport_header))offset))
+		return false;
+
+	if (unlikely(offset == (typeof(skb->transport_header))~0U))
+		return false;
+
+	skb->transport_header = offset;
+	return true;
+}
+
 static inline void skb_set_transport_header(struct sk_buff *skb,
 					    const int offset)
 {

net/ipv6/ip6_offload.c

@@ -148,7 +148,9 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
 
 	ops = rcu_dereference(inet6_offloads[proto]);
 	if (likely(ops && ops->callbacks.gso_segment)) {
-		skb_reset_transport_header(skb);
+		if (!skb_reset_transport_header_careful(skb))
+			goto out;
+
 		segs = ops->callbacks.gso_segment(skb, features);
 		if (!segs)
 			skb->network_header = skb_mac_header(skb) + nhoff - skb->head;