netfilter: nf_reject_ipv6: fix potential crash in nf_send_reset6()

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_reject_ipv6: fix potential crash in nf_send_reset6()

I got a syzbot report without a repro [1] crashing in nf_send_reset6()

I think the issue is that dev->hard_header_len is zero, and we attempt
later to push an Ethernet header.

Use LL_MAX_HEADER, as other functions in net/ipv6/netfilter/nf_reject_ipv6.c.

[1]

skbuff: skb_under_panic: text:ffffffff89b1d008 len:74 put:14 head:ffff88803123aa00 data:ffff88803123a9f2 tail:0x3c end:0x140 dev:syz_tun
kernel BUG at net/core/skbuff.c:206 !
Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 0 UID: 0 PID: 7373 Comm: syz.1.568 Not tainted 6.12.0-rc2-syzkaller-00631-g6d858708d465 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
RIP: 0010:skb_panic net/core/skbuff.c:206 [inline]
RIP: 0010:skb_under_panic+0x14b/0x150 net/core/skbuff.c:216
Code: 0d 8d 48 c7 c6 60 a6 29 8e 48 8b 54 24 08 8b 0c 24 44 8b 44 24 04 4d 89 e9 50 41 54 41 57 41 56 e8 ba 30 38 02 48 83 c4 20 90 <0f> 0b 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3
RSP: 0018:ffffc900045269b0 EFLAGS: 00010282
RAX: 0000000000000088 RBX: dffffc0000000000 RCX: cd66dacdc5d8e800
RDX: 0000000000000000 RSI: 0000000000000200 RDI: 0000000000000000
RBP: ffff88802d39a3d0 R08: ffffffff8174afec R09: 1ffff920008a4ccc
R10: dffffc0000000000 R11: fffff520008a4ccd R12: 0000000000000140
R13: ffff88803123aa00 R14: ffff88803123a9f2 R15: 000000000000003c
FS: 00007fdbee5ff6c0(0000) GS:ffff8880b8600000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000005d322000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
skb_push+0xe5/0x100 net/core/skbuff.c:2636
eth_header+0x38/0x1f0 net/ethernet/eth.c:83
dev_hard_header include/linux/netdevice.h:3208 [inline]
nf_send_reset6+0xce6/0x1270 net/ipv6/netfilter/nf_reject_ipv6.c:358
nft_reject_inet_eval+0x3b9/0x690 net/netfilter/nft_reject_inet.c:48
expr_call_ops_eval net/netfilter/nf_tables_core.c:240 [inline]
nft_do_chain+0x4ad/0x1da0 net/netfilter/nf_tables_core.c:288
nft_do_chain_inet+0x418/0x6b0 net/netfilter/nft_chain_filter.c:161
nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline]
nf_hook_slow+0xc3/0x220 net/netfilter/core.c:626
nf_hook include/linux/netfilter.h:269 [inline]
NF_HOOK include/linux/netfilter.h:312 [inline]
br_nf_pre_routing_ipv6+0x63e/0x770 net/bridge/br_netfilter_ipv6.c:184
nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline]
nf_hook_bridge_pre net/bridge/br_input.c:277 [inline]
br_handle_frame+0x9fd/0x1530 net/bridge/br_input.c:424
__netif_receive_skb_core+0x13e8/0x4570 net/core/dev.c:5562
__netif_receive_skb_one_core net/core/dev.c:5666 [inline]
__netif_receive_skb+0x12f/0x650 net/core/dev.c:5781
netif_receive_skb_internal net/core/dev.c:5867 [inline]
netif_receive_skb+0x1e8/0x890 net/core/dev.c:5926
tun_rx_batched+0x1b7/0x8f0 drivers/net/tun.c:1550
tun_get_user+0x3056/0x47e0 drivers/net/tun.c:2007
tun_chr_write_iter+0x10d/0x1f0 drivers/net/tun.c:2053
new_sync_write fs/read_write.c:590 [inline]
vfs_write+0xa6d/0xc90 fs/read_write.c:683
ksys_write+0x183/0x2b0 fs/read_write.c:736
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fdbeeb7d1ff
Code: 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 c9 8d 02 00 48 8b 54 24 18 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 31 44 89 c7 48 89 44 24 08 e8 1c 8e 02 00 48
RSP: 002b:00007fdbee5ff000 EFLAGS: 00000293 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fdbeed36058 RCX: 00007fdbeeb7d1ff
RDX: 000000000000008e RSI: 0000000020000040 RDI: 00000000000000c8
RBP: 00007fdbeebf12be R08: 0000000
---truncated---