net:emac/emac-mac: Fix a use after free in emac_mac_tx_buf_send

stable inclusion
from stable-v4.19.191
commit 16d8c44be52e3650917736d45f5904384a9da834
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I94JI5
CVE: CVE-2021-47013

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=16d8c44be52e3650917736d45f5904384a9da834



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[ Upstream commit 6d72e7c7 ]

In emac_mac_tx_buf_send, it calls emac_tx_fill_tpd(..,skb,..).
If some error happens in emac_tx_fill_tpd(), the skb will be freed via
dev_kfree_skb(skb) in error branch of emac_tx_fill_tpd().
But the freed skb is still used via skb->len by netdev_sent_queue(,skb->len).

As i observed that emac_tx_fill_tpd() haven't modified the value of skb->len,
thus my patch assigns skb->len to 'len' before the possible free and
use 'len' instead of skb->len later.

Fixes: b9b17deb ("net: emac: emac gigabit ethernet controller driver")
Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>