In the Linux kernel, the following vulnerability has been resolved:net: fix __dst_negative_advice() race__dst_negative_advice() does not enforce proper RCU rules whensk->dst_cache must be cleared, leading to possible UAF.RCU rules are that we must first clear sk->sk_dst_cache,then call dst_release(old_dst).Note that sk_dst_reset(sk) is implementing this protocol correctly,while __dst_negative_advice() uses the wrong order.Given that ip6_negative_advice() has special logicagainst RTF_CACHE, this means each of the three ->negative_advice()existing methods must perform the sk_dst_reset() themselves.Note the check against NULL dst is centralized in__dst_negative_advice(), there is no need to duplicateit in various callbacks.Many thanks to Clement Lecigne for tracking this issue.This old bug became visible after the blamed commit, using UDP sockets.
