syzbot has bisected this bug to:

commit c981f254cc82f50f8cb864ce6432097b23195b9c
Author: Al Viro [off-list ref]
Date:   Sun Jan 7 18:19:09 2018 +0000

     sctp: use vmemdup_user() rather than badly open-coding memdup_user()

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=137bcecf200000
start commit:   c981f254 sctp: use vmemdup_user() rather than badly open-c..
git tree:       upstream
final crash:    https://syzkaller.appspot.com/x/report.txt?x=10fbcecf200000
console output: https://syzkaller.appspot.com/x/log.txt?x=177bcecf200000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5e7dc790609552d7
dashboard link: https://syzkaller.appspot.com/bug?extid=ec1b7575afef85a0e5ca
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16a9a84b400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17199bb3400000

Reported-by: syzbot+ec1b7575afef85a0e5ca@syzkaller.appspotmail.com
Fixes: c981f254 ("sctp: use vmemdup_user() rather than badly open-coding  
memdup_user()")