Re: [PATCH 5.15] mm: validate buddy page before using

[Zi Yan]

On 20 Jun 2022, at 8:54, Greg KH wrote:

> On Mon, Jun 20, 2022 at 08:45:13AM -0400, Zi Yan wrote:
>> On 20 Jun 2022, at 8:24, Greg KH wrote:
>>
>>> On Mon, Jun 20, 2022 at 08:18:40PM +0800, Xianting Tian wrote:
>>>>
>>>> 在 2022/6/20 下午8:06, Greg KH 写道:
>>>>> On Mon, Jun 20, 2022 at 07:57:05PM +0800, Xianting Tian wrote:
>>>>>> 在 2022/6/20 下午7:42, Greg KH 写道:
>>>>>>> On Mon, Jun 20, 2022 at 06:54:44PM +0800, Xianting Tian wrote:
>>>>>>>> 在 2022/6/20 下午6:17, Greg KH 写道:
>>>>>>>>> On Fri, Jun 17, 2022 at 12:17:45AM +0800, Xianting Tian wrote:
>>>>>>>>>> Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.")
>>>>>>>>>> fixes a bug in 1dd214b8f21c and there is a similar bug in d9dddbf55667 that
>>>>>>>>>> can be fixed in a similar way too.
>>>>>>>>>>
>>>>>>>>>> In unset_migratetype_isolate(), we also need the fix, so move page_is_buddy()
>>>>>>>>>> from mm/page_alloc.c to mm/internal.h
>>>>>>>>>>
>>>>>>>>>> In addition, for RISC-V arch the first 2MB RAM could be reserved for opensbi,
>>>>>>>>>> so it would have pfn_base=512 and mem_map began with 512th PFN when
>>>>>>>>>> CONFIG_FLATMEM=y.
>>>>>>>>>> But __find_buddy_pfn algorithm thinks the start pfn 0, it could get 0 pfn or
>>>>>>>>>> less than the pfn_base value. We need page_is_buddy() to verify the buddy to
>>>>>>>>>> prevent accessing an invalid buddy.
>>>>>>>>>>
>>>>>>>>>> Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks")
>>>>>>>>>> Cc: stable@xxxxxxxxxxxxxxx
>>>>>>>>>> Reported-by: zjb194813@xxxxxxxxxxxxxxx
>>>>>>>>>> Reported-by: tianhu.hh@xxxxxxxxxxxxxxx
>>>>>>>>>> Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
>>>>>>>>>> ---
>>>>>>>>>>     mm/internal.h       | 34 ++++++++++++++++++++++++++++++++++
>>>>>>>>>>     mm/page_alloc.c     | 37 +++----------------------------------
>>>>>>>>>>     mm/page_isolation.c |  3 ++-
>>>>>>>>>>     3 files changed, 39 insertions(+), 35 deletions(-)
>>>>>>>>> What is the commit id of this in Linus's tree?
>>>>>>>> It is also this one，
>>>>>>>>
>>>>>>>> commit 787af64d05cd528aac9ad16752d11bb1c6061bb9
>>>>>>>> Author: Zi Yan <ziy@xxxxxxxxxx>
>>>>>>>> Date:   Wed Mar 30 15:45:43 2022 -0700
>>>>>>>>
>>>>>>>>       mm: page_alloc: validate buddy before check its migratetype.
>>>>>>>>
>>>>>>>>       Whenever a buddy page is found, page_is_buddy() should be called to
>>>>>>>>       check its validity.  Add the missing check during pageblock merge check.
>>>>>>>>
>>>>>>>>       Fixes: 1dd214b8f21c ("mm: page_alloc: avoid merging non-fallbackable
>>>>>>>> pageblocks with others")
>>>>>>>>       Link:
>>>>>>>> https://lore.kernel.org/all/20220330154208.71aca532@xxxxxxxxxxxxxxxxxx/
>>>>>>>>       Reported-and-tested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
>>>>>>>>       Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>>>>>>>>       Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>>>>>>> This commit looks nothing like what you posted here.
>>>>>>>
>>>>>>> Why the vast difference with no explaination as to why these are so
>>>>>>> different from the other backports you provided here?  Also why is the
>>>>>>> subject lines changed?
>>>>>> Yes, the changes of 5.15 are not same with others branches, because we need
>>>>>> additional fix for 5.15,
>>>>>>
>>>>>> You can check it in the thread:
>>>>>>
>>>>>> https://lore.kernel.org/linux-mm/435B45C3-E6A5-43B2-A5A2-318C748691FC@xxxxxxxxxx/ <https://lore.kernel.org/linux-mm/435B45C3-E6A5-43B2-A5A2-318C748691FC@xxxxxxxxxx/>
>>>>>>
>>>>>> Right. But pfn_valid_within() was removed since 5.15. So your fix is
>>>>>> required for kernels between 5.15 and 5.17 (inclusive).
>>>>> What is "your fix" here?
>>>>>
>>>>> This change differs a lot from what is in Linus's tree now, so this all
>>>>> needs to be resend and fixed up as I mention above if we are going to be
>>>>> able to take this.  As-is, it's all not correct so are dropped.
>>>>
>>>> I think, for branches except 5.15,  you can just backport Zi Yan's commit
>>>> 787af64d05cd in Linus tree. I won't send more patches further,
>>>
>>> So just for 5.18?  I am confused.
>>>
>>>> For 5.15, because it need additional fix except commit 787af64d05cd,  I will
>>>> send a new patch as your comments.
>>>>
>>>> Is it ok for you?
>>>
>>> No, please send fixed up patches for all branches you want them applied
>>> to as I do not understand what to do here at all, sorry.
>>
>> Hi Greg,
>>
>> The fixes sent by Xianting do not exist in Linus’s tree, since the bug is
>> fixed by another commit, which was not intended to fix the bug from the commit
>> d9dddbf55667. These fixes only target the stable branches.
>
> Then that all needs to be documented very very very well as to why we
> can't just take the commit that is in Linus's tree.
>
> Why can't we take that commit instead?

The situation is a little complicated.

The bug from commit d9dddbf55667 was not discovered back then. The commit 1dd214b8f21c
was trying to get migratetype merging more rigid and made the bug easy to get
hit, but none of us were aware of that the bug also exists in commit d9dddbf55667.
Then the commit 787af64d05cd fixed the bug, but since the original code was
changed by commit 1dd214b8f21c, thus, it does not directly apply to
commit d9dddbf55667. So I do not think it makes sense to use the original commits
1dd214b8f21c and 787af64d05cd, since the former makes a non bug fixing change and
the latter fixes the bug revealed by the former.

As a result, Xianting's patches fix the bug directly, looking more reasonable to me.

--
Best Regards,
Yan, Zi

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