Eric Dumazet wrote:

Corey Minyard found a race added in commit 
271b72c7fa82c2c7a795bc16896149933110672d
(udp: RCU handling for Unicast packets.)

"If the socket is moved from one list to another list in-between the 
time  the hash is calculated and the next field is accessed, and the 
socket  has moved to the end of the new list, the traversal will not 
complete  properly on the list it should have, since the socket will 
be on the end  of the new list and there's not a way to tell it's on a 
new list and  restart the list traversal.  I think that this can be 
solved by  pre-fetching the "next" field (with proper barriers) before 
checking the  hash."

This patch corrects this problem, introducing a new 
sk_for_each_rcu_safenext()
macro.

You also need the appropriate smp_wmb() in udp_lib_get_port() after 
sk_hash is set, I think, so the next field is guaranteed to be changed 
after the hash value is changed.


-corey