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

Vlastimil Babka
2019-04-16T11:41:11Z

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@redhat.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
>