Security critical race condition in epoll code

[John Gardiner Myers <jgmyers@netscape.com>]

There appears to be a race condition in the epoll patch which permits 
user space to scribble in the kernel's free memory.

First a user space program creates an epoll fd and adds a socket to it 
using sys_epoll_ctl(...EP_CTL_ADD...)

Then the program creates two threads, A and B.  Simultaneously, A calls 
sys_epoll_ctl(...EP_CTL_MOD...) and B calls 
sys_epoll_ctl(...EP_CTL_DEL...) on the socket that was previously added.

Thread A runs up through the point where ep_find() returns the (struct 
epitem *) for the socket.

Thread B then runs and ep_remove() frees the (struct epitem *).

Thread A then runs some more and stores the value of events into the now 
freed block of memory pointed to by dpi.