net: use eth_type_trans_careful() in __dev_forward_skb2()

A tc egress BPF program can push a tiny link-layer header with
bpf_skb_change_head() on an skb whose head is almost empty, then
redirect it to an Ethernet device ingress. __bpf_redirect_common()
only checks mac_header < network_header, and __dev_forward_skb2()
crashes in eth_type_trans():

  tc filter add dev lo egress pref 1 matchall action vlan pop_eth continue
  tc filter add dev lo egress pref 2 bpf da obj prog.o
  # prog: bpf_skb_change_head(skb, 1, 0);
  #       return bpf_redirect(veth1_ifindex, BPF_F_INGRESS);
  # AF_PACKET SOCK_RAW on lo: eth / eth / 8192 bytes of payload

  kernel BUG at include/linux/skbuff.h:2839!
  RIP: 0010:eth_type_trans (include/linux/etherdevice.h:635 net/ethernet/eth.c:163)
  Call Trace:
   __dev_forward_skb2 (net/core/dev.c:2507)
   dev_forward_skb_nomtu (net/core/dev.c:2546)
   __dev_queue_xmit (net/core/dev.c:4738 net/core/dev.c:4998)
   packet_sendmsg (net/packet/af_packet.c:3157 net/packet/af_packet.c:3189)

Use eth_type_trans_careful(), and drop the packet if the Ethernet
header can not be pulled. This also covers other dev_forward_skb()
users (veth, macvlan, ipvlan, netdevsim, l2tp_eth).

Update the empty_skb BPF selftest: at lwt_xmit, BPF_PROG_TEST_RUN
strips the Ethernet header, so the "veth ETH_HLEN+1" packet is a 1-byte
skb. Redirecting it to veth ingress used to read the Ethernet header
past skb->tail and deliver a 1-byte frame; it is now dropped with
NET_RX_DROP, as veth_xmit() already does at egress.

Fixes: 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure")
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Tested-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20261028122703.2093730-4-edumazet@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions