From: Florian Westphal <fw@strlen.de>
Date: Tue, 30 May 2017 11:38:12 +0200

Quoting Joe Stringer:
  If a user loads nf_conntrack_ftp, sends FTP traffic through a network
  namespace, destroys that namespace then unloads the FTP helper module,
  then the kernel will crash.

Events that lead to the crash:
1. conntrack is created with ftp helper in netns x
2. This netns is destroyed
3. netns destruction is scheduled
4. netns destruction wq starts, removes netns from global list
5. ftp helper is unloaded, which resets all helpers of the conntracks
via for_each_net()

but because netns is already gone from list the for_each_net() loop
doesn't include it, therefore all of these conntracks are unaffected.

6. helper module unload finishes
7. netns wq invokes destructor for rmmod'ed helper

CC: "Eric W. Biederman" <redacted>
Reported-by: Joe Stringer <redacted>
Signed-off-by: Florian Westphal <fw@strlen.de>
---
 Eric, I'd like an explicit (n)ack from you for this one.

Indeed, Eric, please do.

Otherwise I'm fine with the generic parts:

Acked-by: David S. Miller <davem@davemloft.net>