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Date:   Fri, 5 Oct 2018 11:58:55 -0300
From:   Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
To:     syzbot <syzbot+c7dd55d7aec49d48e49a@syzkaller.appspotmail.com>
Cc:     davem@davemloft.net, linux-kernel@vger.kernel.org,
        linux-sctp@vger.kernel.org, netdev@vger.kernel.org,
        nhorman@tuxdriver.com, syzkaller-bugs@googlegroups.com,
        vyasevich@gmail.com
Subject: Re: KASAN: use-after-free Read in sctp_id2assoc
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On Thu, Oct 04, 2018 at 01:48:03AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    4e6d47206c32 tls: Add support for inplace records encryption
> git tree:       net-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=13834b81400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=e569aa5632ebd436
> dashboard link: https://syzkaller.appspot.com/bug?extid=c7dd55d7aec49d48e49a
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> 
> Unfortunately, I don't have any reproducer for this crash yet.
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+c7dd55d7aec49d48e49a@syzkaller.appspotmail.com
> 
> netlink: 'syz-executor1': attribute type 1 has an invalid length.
> ==================================================================
> BUG: KASAN: use-after-free in sctp_id2assoc+0x3a7/0x3e0
> net/sctp/socket.c:276
> Read of size 8 at addr ffff880195b3eb20 by task syz-executor2/15454
> 
> CPU: 1 PID: 15454 Comm: syz-executor2 Not tainted 4.19.0-rc5+ #242
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x1c4/0x2b4 lib/dump_stack.c:113
>  print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
>  kasan_report_error mm/kasan/report.c:354 [inline]
>  kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
>  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
>  sctp_id2assoc+0x3a7/0x3e0 net/sctp/socket.c:276

I'm not seeing yet how this could happen.
All sockopts here are serialized by sock_lock.
do_peeloff here would create another socket, but the issue was
triggered before that.
The same function that freed this memory, also removes the entry from
idr mapping, so this entry shouldn't be there anymore.

I have only two theories so far:
- an issue with IDR/RCU.
- something else happened that just the call stacks are not revealing.