linux-kernel - Re: UBSAN: Undefined behaviour in mm/page

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Date:   Fri, 9 Nov 2018 19:24:48 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>
Cc:     Kyungtae Kim <kt0755@...il.com>, akpm@...ux-foundation.org,
        pavel.tatashin@...rosoft.com, osalvador@...e.de,
        rppt@...ux.vnet.ibm.com, aaron.lu@...el.com,
        iamjoonsoo.kim@....com, alexander.h.duyck@...ux.intel.com,
        mgorman@...hsingularity.net, lifeasageek@...il.com,
        threeearcat@...il.com, syzkaller@...glegroups.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Subject: Re: UBSAN: Undefined behaviour in mm/page_alloc.c

On 2018/11/09 19:10, Vlastimil Babka wrote:>>>> +	 * reclaim >= MAX_ORDER areas which will never succeed. Callers may
>>>> +	 * be using allocators in order of preference for an area that is
>>>> +	 * too large.
>>>> +	 */
>>>> +	if (order >= MAX_ORDER) {
>>>
>>> Also, why not to add BUG_ON(gfp_mask & __GFP_NOFAIL); here?
>>
>> Because we do not want to blow up the kernel just because of a stupid
>> usage of the allocator. Can you think of an example where it would
>> actually make any sense?
>>
>> I would argue that such a theoretical abuse would blow up on an
>> unchecked NULL ptr access. Isn't that enough?
> 
> Agreed.
> 

If someone has written a module with __GFP_NOFAIL for an architecture
where PAGE_SIZE == 2048KB, and someone else tried to use that module on
another architecture where PAGE_SIZE == 4KB. You are saying that
triggering NULL pointer dereference is a fault of that user's ignorance
about MM. You are saying that everyone knows internal of MM. Sad...