Enable promisc mode of PF, set VF link state to enable, and
run iperf of the VF, then do self test of the PF. The self test
will fail with a low frequency, and may cause a use-after-free
problem.

[   87.142126] selftest:000004a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   87.159722] ==================================================================
[   87.174187] BUG: KASAN: use-after-free in hex_dump_to_buffer+0x140/0x608
[   87.187600] Read of size 1 at addr ffff003b22828000 by task ethtool/1186
[   87.201012]
[   87.203978] CPU: 7 PID: 1186 Comm: ethtool Not tainted 5.5.0-rc4-gfd51c473-dirty #4
[   87.219306] Hardware name: Huawei TaiShan 2280 V2/BC82AMDA, BIOS TA BIOS 2280-A CS V2.B160.01 01/15/2020
[   87.238292] Call trace:
[   87.243173]  dump_backtrace+0x0/0x280
[   87.250491]  show_stack+0x24/0x30
[   87.257114]  dump_stack+0xe8/0x140
[   87.263911]  print_address_description.isra.8+0x70/0x380
[   87.274538]  __kasan_report+0x12c/0x230
[   87.282203]  kasan_report+0xc/0x18
[   87.288999]  __asan_load1+0x60/0x68
[   87.295969]  hex_dump_to_buffer+0x140/0x608
[   87.304332]  print_hex_dump+0x140/0x1e0
[   87.312000]  hns3_lb_check_skb_data+0x168/0x170
[   87.321060]  hns3_clean_rx_ring+0xa94/0xfe0
[   87.329422]  hns3_self_test+0x708/0x8c0

The length of packet sent by the selftest process is only
128 + 14 bytes, and the min buffer size of a BD is 256 bytes,
and the receive process will make sure the packet sent by
the selftest process is in the linear part, so only check
the linear part in hns3_lb_check_skb_data().

So fix this use-after-free by using skb_headlen() to dump
skb->data instead of skb->len.

Fixes: c39c4d98 ("net: hns3: Add mac loopback selftest support in hns3 driver")
Signed-off-by: Yonglong Liu <liuyonglong@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>