問題如下：
$ qemu-system-i386 -m 64 win95_samba.qcow2 -vga cirrus -cdrom win95_osr2.iso -device ne2k_isa -netdev bridge,id=net0,br=br0 -net user,smb=/tmp
failed to create tun device: Operation not permitted
qemu-system-i386: -netdev bridge,id=net0,br=br0: bridge helper failed
解法如下：
$ sudo chmod u+s /usr/lib/qemu/qemu-bridge-helper