Re: [PATCH] mm: page_alloc: validate buddy before check its migratetype.

Zi Yan
2022-03-31T22:07:36Z

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On 31 Mar 2022, at 4:57, Vlastimil Babka wrote:

> On 3/31/22 02:10, Zi Yan wrote:
>> On 30 Mar 2022, at 19:48, Zi Yan wrote:
>>
>>> On 30 Mar 2022, at 19:03, Linus Torvalds wrote:
>>>
>>>> On Wed, Mar 30, 2022 at 3:12 PM Zi Yan <zi.yan@sent.com> wrote:
>>>>>
>>>>> Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable pageblocks with others")
>>>>
>>>> Oh, btw - should this perhaps be backported further back than that
>>>> alleged "fixes" commit?
>>>>
>>>> It does look like maybe the problem potentially existed before too,
>>>> and was just much harder to trigger.
>>>>
>>>> That said, google doesn't find any other reports that look like
>>>> Steven's oops, so maybe it really never happened and backporting isn't
>>>> called for.
>>>>
>>>> Or possibly my google-fu is just bad.
>>>>
>>>
>>> There might not be any issue with the original code because this bug
>>> could only be triggered when CONFIG_FLATMEM and CONFIG_MEMORY_ISOLATION
>>> are both set, which never happens, since CONFIG_MEMORY_ISOLATION
>>> depends on CONFIG_SPARSEMEM.
>
> Good point. Which means unset_migratetype_isolate() that Linus pointed
> out, is currently also safe as it's a CONFIG_MEMORY_ISOLATION code.
> We could still implement the suggested page_find_buddy() wrapper using
> page_is_buddy() internally, as well as the cleanup of __free_one_page(),
> but it's not urgent.
>
Sure. Will do that.

>>> By checking Steven's boot log, it should be PFN 0x21ee00 that triggers
>>> the bug, since the physical memory range ends at PFN 0x21edff.
>>> PFN 0x21ee00 is 2MB aligned instead of MAX_ORDER-1 (4MB) aligned.
>>> The original code assumes all physical memory ranges are at least
>>> MAX_ORDER-1 aligned, which is true when CONFIG_SPARSEMEM is set
>>> (CONFIG_MEMORY_ISOLATION depends on it), since CONFIG_SPARSEMEM
>>> allocates pageblock_flags array (the NULL-deferenced bitmap points
>>> to) at section size granularity (128MB > 4MB). However, CONFIG_FLATMEM
>>> does not do this. It allocates pageblock_flags array at the exact size
>>> of the physical memory. So checking 0x21ee00 will not cause NULL
>>> dereferencing when CONFIG_MEMORY_ISOLATION is set and the original
>>> if statement can be true.

For the record, the actual situation is that struct page of PFN 0x21ee00
is allocated but uninitialized. In CONFIG_FLATMEM, pageblock flags are
stored in zone->pageblock_flags. When reading PFN 0x21ee00's pageblock
flags, the page's zone is determined to be ZONE_MOVABLE, since page->flags
is -1UL and page_zonenum() is 3. The system does not have ZONE_MOVABLE,
so zone->pageblock_flags is NULL and reading it caused NULL pointer
dereferencing.

>>>
>>> Now I am wondering if the page_is_buddy() check is correct for
>>> CONFIG_FLATMEM. Is mem_map allocation aligned to MAX_ORDER-1
>>> or just the present physical memory range? Is PageBuddy(0x21ee00)
>>> accessing some random memory location?
>>
>> OK. mem_map seems to be MAX_ORDER-1 aligned, so there is no
>> problem with PageBuddy(0x21ee00).
>
> Yeah mem_map has to be in all config variants, otherwise buddy merging
> would have been blowing up in page_is_buddy() even prior to all the
> "sometimes avoid merging pageblock" changes.
>>
>> --
>> Best Regards,
>> Yan, Zi

--
Best Regards,
Yan, Zi

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