From: Nathan Chancellor <redacted>
Date: Tue, 28 Apr 2020 10:54:56 -0700

netvsc_start_xmit is used as a callback function for the ndo_start_xmit
function pointer. ndo_start_xmit's return type is netdev_tx_t but
netvsc_start_xmit's return type is int.

This causes a failure with Control Flow Integrity (CFI), which requires
function pointer prototypes and callback function definitions to match
exactly. When CFI is in enforcing, the kernel panics. When booting a
CFI kernel with WSL 2, the VM is immediately terminated because of this.

The splat when CONFIG_CFI_PERMISSIVE is used:

 ...

Avoid this by using the right return type for netvsc_start_xmit.

Fixes: fceaf24a943d8 ("Staging: hv: add the Hyper-V virtual network driver")
Link: https://github.com/ClangBuiltLinux/linux/issues/1009
Signed-off-by: Nathan Chancellor <redacted>

Applied.