net: skbuff: add overflow debug check to pull/push helpers

syzbot managed to trigger following splat:
BUG: KASAN: use-after-free in __skb_flow_dissect+0x4a3b/0x5e50
Read of size 1 at addr ffff888208a4000e by task a.out/2313
[..]
  __skb_flow_dissect+0x4a3b/0x5e50
  __skb_get_hash+0xb4/0x400
  ip_tunnel_xmit+0x77e/0x26f0
  ipip_tunnel_xmit+0x298/0x410
  ..

Analysis shows that the skb has a valid ->head, but bogus ->data
pointer.

skb->data gets its bogus value via the neigh layer, which does:

1556    __skb_pull(skb, skb_network_offset(skb));

... and the skb was already dodgy at this point:

skb_network_offset(skb) returns a negative value due to an
earlier overflow of skb->network_header (u16).  __skb_pull thus
"adjusts" skb->data by a huge offset, pointing outside skb->head
area.

Allow debug builds to splat when we try to pull/push more than
INT_MAX bytes.

After this, the syzkaller reproducer yields a more precise splat
before the flow dissector attempts to read off skb->data memory:

WARNING: CPU: 5 PID: 2313 at include/linux/skbuff.h:2653 neigh_connected_output+0x28e/0x400
  ip_finish_output2+0xb25/0xed0
  iptunnel_xmit+0x4ff/0x870
  ipgre_xmit+0x78e/0xbb0

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240216113700.23013-1-fw@strlen.de


Signed-off-by: Paolo Abeni <pabeni@redhat.com>