linux-kernel - Re: slab corruption with current -git

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Date:   Mon, 10 Oct 2016 04:24:01 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     torvalds@...ux-foundation.org
Cc:     aconole@...hat.com, fw@...len.de, viro@...iv.linux.org.uk,
        akpm@...ux-foundation.org, axboe@...com, tytso@....edu,
        cl@...ux.com, pablo@...filter.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org,
        netfilter-devel@...r.kernel.org
Subject: Re: slab corruption with current -git

From: David Miller <davem@...emloft.net>
Date: Sun, 09 Oct 2016 23:57:45 -0400 (EDT)

> From: Linus Torvalds <torvalds@...ux-foundation.org>
> Date: Sun, 9 Oct 2016 20:41:17 -0700
> 
>> Note that the "correct way" of doing list operations also almost
>> inevitably is the shortest way by far, since it gets rid of all the
>> special cases. So the patch looks nice. It gets rid of the magic
>> "nf_set_hooks_head()" thing too, because once you do list following
>> right, the head is no different from any other pointer in the list.
> 
> Perhaps we should have some "slist" primitives added to
> include/linux/list.h but since the comparison differs for each user I
> guess it's hard to abstract in a way that's generic and inlines
> properly.
> 
> I'll start taking a look at your patch and this stuff as well, thanks
> Linus.

So I've been reviewing this patch and it looks fine, but I also want
to figure out what is actually causing the OOPS and I can't spot it
yet.

One possible way to see that oops is to free the head entry of the
chain without unlinking it.  The next unregister will dereference a
POISON pointer.

Actually...

The POISON value comes not from a hook entry, but from the array of
pointers in the per-netns datastructure.

This means that the netns is possibly getting freed up before we
unregister the netfilter hooks.