net: ensure mac header is set in virtio_net_hdr_to_skb()

Commit 924a9bc3 ("net: check if protocol extracted by virtio_net_hdr_set_proto is correct")
added a call to dev_parse_header_protocol() but mac_header is not yet set.

This means that eth_hdr() reads complete garbage, and syzbot complained about it [1]

This patch resets mac_header earlier, to get more coverage about this change.

Audit of virtio_net_hdr_to_skb() callers shows that this change should be safe.

[1]

BUG: KASAN: use-after-free in eth_header_parse_protocol+0xdc/0xe0 net/ethernet/eth.c:282
Read of size 2 at addr ffff888017a6200b by task syz-executor313/8409

CPU: 1 PID: 8409 Comm: syz-executor313 Not tainted 5.12.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:79 [inline]
 dump_stack+0x141/0x1d7 lib/dump_stack.c:120
 print_address_description.constprop.0.cold+0x5b/0x2f8 mm/kasan/report.c:232
 __kasan_report mm/kasan/report.c:399 [inline]
 kasan_report.cold+0x7c/0xd8 mm/kasan/report.c:416
 eth_header_parse_protocol+0xdc/0xe0 net/ethernet/eth.c:282
 dev_parse_header_protocol include/linux/netdevice.h:3177 [inline]
 virtio_net_hdr_to_skb.constprop.0+0x99d/0xcd0 include/linux/virtio_net.h:83
 packet_snd net/packet/af_packet.c:2994 [inline]
 packet_sendmsg+0x2325/0x52b0 net/packet/af_packet.c:3031
 sock_sendmsg_nosec net/socket.c:654 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:674
 sock_no_sendpage+0xf3/0x130 net/core/sock.c:2860
 kernel_sendpage.part.0+0x1ab/0x350 net/socket.c:3631
 kernel_sendpage net/socket.c:3628 [inline]
 sock_sendpage+0xe5/0x140 net/socket.c:947
 pipe_to_sendpage+0x2ad/0x380 fs/splice.c:364
 splice_from_pipe_feed fs/splice.c:418 [inline]
 __splice_from_pipe+0x43e/0x8a0 fs/splice.c:562
 splice_from_pipe fs/splice.c:597 [inline]
 generic_splice_sendpage+0xd4/0x140 fs/splice.c:746
 do_splice_from fs/splice.c:767 [inline]
 do_splice+0xb7e/0x1940 fs/splice.c:1079
 __do_splice+0x134/0x250 fs/splice.c:1144
 __do_sys_splice fs/splice.c:1350 [inline]
 __se_sys_splice fs/splice.c:1332 [inline]
 __x64_sys_splice+0x198/0x250 fs/splice.c:1332
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46

Fixes: 924a9bc3 ("net: check if protocol extracted by virtio_net_hdr_set_proto is correct")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Balazs Nemeth <bnemeth@redhat.com>
Cc: Willem de Bruijn <willemb@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/virtio_net.h

@@ -62,6 +62,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
 			return -EINVAL;
 	}
 
+	skb_reset_mac_header(skb);
+
 	if (hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
 		u16 start = __virtio16_to_cpu(little_endian, hdr->csum_start);
 		u16 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);