CVE ID:	CVE-2021-47555
Description:	In the Linux kernel, the following vulnerability has been resolved: net: vlan: fix underflow for the real_dev refcnt Inject error before dev_hold(real_dev) in register_vlan_dev(), and execute the following testcase: ip link add dev dummy1 type dummy ip link add name dummy1.100 link dummy1 type vlan id 100 ip link del dev dummy1 When the dummy netdevice is removed, we will get a WARNING as following: == ===================================================================== refcount_t: decrement hit 0; leaking memory. WARNING: CPU: 2 PID: 0 at lib/refcount.c:31 refcount_warn_saturate+0xbf/0x1e0 and an endless loop of: ============================================================= ========== unregister_netdevice: waiting for dummy1 to become free. Usage count = -1073741824 That is because dev_put(real_dev) in vlan_dev_free() be called without dev_hold(real_dev) in register_vlan_dev(). It makes the refcnt of real_dev underflow. Move the dev_hold(real_dev) to vlan_dev_init() which is the call-back of ndo_init(). That makes dev_hold() and dev_put() for vlan's real_dev symmetrical.
Test IDs:	None available
Cross References:	Common Vulnerability Exposure (CVE) ID: CVE-2021-47555 https://git.kernel.org/stable/c/01d9cc2dea3fde3bad6d27f464eff463496e2b00 https://git.kernel.org/stable/c/01d9cc2dea3fde3bad6d27f464eff463496e2b00 https://git.kernel.org/stable/c/5e44178864b38dd70b877985abd7d86fdb95f27d https://git.kernel.org/stable/c/5e44178864b38dd70b877985abd7d86fdb95f27d https://git.kernel.org/stable/c/6e800ee43218a56acc93676bbb3d93b74779e555 https://git.kernel.org/stable/c/6e800ee43218a56acc93676bbb3d93b74779e555 https://git.kernel.org/stable/c/f7fc72a508cf115c273a7a29350069def1041890 https://git.kernel.org/stable/c/f7fc72a508cf115c273a7a29350069def1041890