bpf: Reject negative head_room in __bpf_skb_change_head

Yinhao et al. recently reported:

  Our fuzzing tool was able to create a BPF program which triggered
  the below BUG condition inside pskb_expand_head.

  [   23.016047][T10006] kernel BUG at net/core/skbuff.c:2232!
  [...]
  [   23.017301][T10006] RIP: 0010:pskb_expand_head+0x1519/0x1530
  [...]
  [   23.021249][T10006] Call Trace:
  [   23.021387][T10006]  <TASK>
  [   23.021507][T10006]  ? __pfx_pskb_expand_head+0x10/0x10
  [   23.021725][T10006]  __bpf_skb_change_head+0x22a/0x520
  [   23.021939][T10006]  bpf_skb_change_head+0x34/0x1b0
  [   23.022143][T10006]  ___bpf_prog_run+0xf70/0xb670
  [   23.022342][T10006]  __bpf_prog_run32+0xed/0x140
  [...]

The problem is that in __bpf_skb_change_head() we need to reject a
negative head_room as otherwise this propagates all the way to the
pskb_expand_head() from skb_cow(). For example, if the BPF test infra
passes a skb with gso_skb:1 to the BPF helper with a negative head_room
of -22, then this gets passed into skb_cow(). __skb_cow() in this
example calculates a delta of -86 which gets aligned to -64, and then
triggers BUG_ON(nhead < 0). Thus, reject malformed negative input.

Fixes: 3a0af8fd ("bpf: BPF for lightweight tunnel infrastructure")
Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Link: https://patch.msgid.link/20251023125532.182262-1-daniel@iogearbox.net