CVE-2021-47236: Missing Release of Memory after Effective Lifetime

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

net: cdc_eem: fix tx fixup skb leak

when usbnet transmit a skb, eem fixup it in eem_tx_fixup(), if skb_copy_expand() failed, it return NULL, usbnet_start_xmit() will have no chance to free original skb.

fix it by free orginal skb in eem_tx_fixup() first, then check skb clone status, if failed, return NULL to usbnet.