linux-kernel - Re: [patch V4 01/32] mm/slab: Fix broken stack trace storage

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Message-ID: <612f9b99-a75b-6aeb-cf92-7dc5421cd950@suse.cz>
Date:   Tue, 16 Apr 2019 13:37:48 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        Linux-MM <linux-mm@...ck.org>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [patch V4 01/32] mm/slab: Fix broken stack trace storage

On 4/15/19 11:22 PM, Thomas Gleixner wrote:
> On Mon, 15 Apr 2019, Andy Lutomirski wrote:
>> On Mon, Apr 15, 2019 at 9:17 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>>> On Mon, Apr 15, 2019 at 06:07:44PM +0200, Thomas Gleixner wrote:
>>>>> Looks like stack_trace.nr_entries isn't initialized?  (though this code
>>>>> gets eventually replaced by a later patch)
>>>>
>>>> struct initializer initialized the non mentioned fields to 0, if I'm not
>>>> totally mistaken.
>>>
>>> Hm, it seems you are correct.  And I thought I knew C.
>>>
>>>>> Who actually reads this stack trace?  I couldn't find a consumer.
>>>>
>>>> It's stored directly in the memory pointed to by @addr and that's the freed
>>>> cache memory. If that is used later (UAF) then the stack trace can be
>>>> printed to see where it was freed.
>>>
>>> Right... but who reads it?
>>
>> That seems like a reasonable question.  After some grepping and some
>> git searching, it looks like there might not be any users.  I found
> 
> Anymore. There was something 10y+ ago.

In theory it can be useful in a crash dump. But I don't see any related
debugging
check that would trigger a panic, in order to get one.

>> SLAB_STORE_USER, but that seems to be independent.
>>
>> So maybe the whole mess should just be deleted.  If anyone ever
>> notices, they can re-add it better.
> 
> No objections from my side, but the mm people might have opinions.

Anyone who wants to debug wrong slab usage probably uses SLUB anyway, so
I don't think it's a problem to remove broken SLAB debugging. Perhaps
even SLAB itself will be removed soon if there's performance data
supporting it [1].

[1]
https://lore.kernel.org/linux-mm/20190412112816.GD18914@techsingularity.net/T/#u

> Thanks,
> 
> 	tglx
>