In the Linux kernel, the following vulnerability has been resolved: atm: clip: Fix potential null-ptr-deref in toatmarpd(). atmarpd is protected by RTNL since commit f3a0592b37b8 ("[ATM]: clip causes unregister hang"). However, it is not enough because toatmarpd() is called without RTNL, especially clipneighsolicit() / neighops->solicit() is unsleepable. Also, there is no RTNL dependency around atmarpd. Let's use a private mutex and RCU to protect access to atmarpd in toatmarpd().