Re: [PATCH 1/2] hugetlb: fix update_and_free_page contig page struct assumption

[Mike Kravetz]

On 2/17/21 11:02 AM, Andrew Morton wrote:
> On Wed, 17 Feb 2021 10:49:25 -0800 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> 
>> page structs are not guaranteed to be contiguous for gigantic pages.  The
>> routine update_and_free_page can encounter a gigantic page, yet it assumes
>> page structs are contiguous when setting page flags in subpages.
>>
>> If update_and_free_page encounters non-contiguous page structs, we can
>> see “BUG: Bad page state in process …” errors.
>>
>> Non-contiguous page structs are generally not an issue.  However, they can
>> exist with a specific kernel configuration and hotplug operations.  For
>> example: Configure the kernel with CONFIG_SPARSEMEM and
>> !CONFIG_SPARSEMEM_VMEMMAP.  Then, hotplug add memory for the area where the
>> gigantic page will be allocated.
>> Zi Yan outlined steps to reproduce here [1].
>>
>> [1] https://lore.kernel.org/linux-mm/16F7C58B-4D79-41C5-9B64-A1A1628F4AF2@xxxxxxxxxx/
>>
>> Fixes: 944d9fec8d7a ("hugetlb: add support for gigantic page allocation at runtime")
> 
> June 2014.  That's a long lurk time for a bug.  I wonder if some later
> commit revealed it.
> 
> I guess it doesn't matter a lot, but some -stable kernel maintainers
> might wonder if they really need this fix...

I am not sure how common a CONFIG_SPARSEMEM and !CONFIG_SPARSEMEM_VMEMMAP
config is.  On the more popular architectures, this is not the default.
But, you can build a kernel with such options.  And, then you need to
hotplug memory add and allocate a gigantic page there.

It is unlikely to happen, but possible since Zi could force the BUG.

The copy_huge_page_from_user bug requires the same non-normal configuration
and is just as unlikely to occurr.  But, since it can overwrite somewhat
random pages I would feel better if it was fixed.
-- 
Mike Kravetz