net: test for not too small csum_start in virtio_net_hdr_to_skb()

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

net: test for not too small csum_start in virtio_net_hdr_to_skb()

syzbot was able to trigger this warning [1], after injecting a
malicious packet through af_packet, setting skb->csum_start and thus
the transport header to an incorrect value.

We can at least make sure the transport header is after
the end of the network header (with a estimated minimal size).

[1]
[ 67.873027] skb len=4096 headroom=16 headlen=14 tailroom=0
mac=(-1,-1) mac_len=0 net=(16,-6) trans=10
shinfo(txflags=0 nr_frags=1 gso(size=0 type=0 segs=0))
csum(0xa start=10 offset=0 ip_summed=3 complete_sw=0 valid=0 level=0)
hash(0x0 sw=0 l4=0) proto=0x0800 pkttype=0 iif=0
priority=0x0 mark=0x0 alloc_cpu=10 vlan_all=0x0
encapsulation=0 inner(proto=0x0000, mac=0, net=0, trans=0)
[ 67.877172] dev name=veth0_vlan feat=0x000061164fdd09e9
[ 67.877764] sk family=17 type=3 proto=0
[ 67.878279] skb linear: 00000000: 00 00 10 00 00 00 00 00 0f 00 00 00 08 00
[ 67.879128] skb frag: 00000000: 0e 00 07 00 00 00 28 00 08 80 1c 00 04 00 00 02
[ 67.879877] skb frag: 00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.880647] skb frag: 00000020: 00 00 02 00 00 00 08 00 1b 00 00 00 00 00 00 00
[ 67.881156] skb frag: 00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.881753] skb frag: 00000040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.882173] skb frag: 00000050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.882790] skb frag: 00000060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.883171] skb frag: 00000070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.883733] skb frag: 00000080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.884206] skb frag: 00000090: 00 00 00 00 00 00 00 00 00 00 69 70 76 6c 61 6e
[ 67.884704] skb frag: 000000a0: 31 00 00 00 00 00 00 00 00 00 2b 00 00 00 00 00
[ 67.885139] skb frag: 000000b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.885677] skb frag: 000000c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.886042] skb frag: 000000d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.886408] skb frag: 000000e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.887020] skb frag: 000000f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 67.887384] skb frag: 00000100: 00 00
[ 67.887878] ------------[ cut here ]------------
[ 67.887908] offset (-6) >= skb_headlen() (14)
[ 67.888445] WARNING: CPU: 10 PID: 2088 at net/core/dev.c:3332 skb_checksum_help (net/core/dev.c:3332 (discriminator 2))
[ 67.889353] Modules linked in: macsec macvtap macvlan hsr wireguard curve25519_x86_64 libcurve25519_generic libchacha20poly1305 chacha_x86_64 libchacha poly1305_x86_64 dummy bridge sr_mod cdrom evdev pcspkr i2c_piix4 9pnet_virtio 9p 9pnet netfs
[ 67.890111] CPU: 10 UID: 0 PID: 2088 Comm: b363492833 Not tainted 6.11.0-virtme #1011
[ 67.890183] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[ 67.890309] RIP: 0010:skb_checksum_help (net/core/dev.c:3332 (discriminator 2))
[ 67.891043] Call Trace:
[ 67.891173] <TASK>
[ 67.891274] ? __warn (kernel/panic.c:741)
[ 67.891320] ? skb_checksum_help (net/core/dev.c:3332 (discriminator 2))
[ 67.891333] ? report_bug (lib/bug.c:180 lib/bug.c:219)
[ 67.891348] ? handle_bug (arch/x86/kernel/traps.c:239)
[ 67.891363] ? exc_invalid_op (arch/x86/kernel/traps.c:260 (discriminator 1))
[ 67.891372] ? asm_exc_invalid_op (./arch/x86/include/asm/idtentry.h:621)
[ 67.891388] ? skb_checksum_help (net/core/dev.c:3332 (discriminator 2))
[ 67.891399] ? skb_checksum_help (net/core/dev.c:3332 (discriminator 2))
[ 67.891416] ip_do_fragment (net/ipv4/ip_output.c:777 (discriminator 1))
[ 67.891448] ? __ip_local_out (./include/linux/skbuff.h:1146 ./include/net/l3mdev.h:196 ./include/net/l3mdev.h:213 ne
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