Re: [PATCH] hugetlbfs: change put_page/unlock_page order in hugetlbfs_fallocate()

[Mike Kravetz]

Adding Andrew, Michal on CC

On 08/27/2017 01:08 PM, Nadav Amit wrote:
> Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> 
>> On 08/26/2017 12:11 PM, Nadav Amit wrote:
>>> hugetlfs_fallocate() currently performs put_page() before unlock_page().
>>> This scenario opens a small time window, from the time the page is added
>>> to the page cache, until it is unlocked, in which the page might be
>>> removed from the page-cache by another core. If the page is removed
>>> during this time windows, it might cause a memory corruption, as the
>>> wrong page will be unlocked.
>>>
>>> It is arguable whether this scenario can happen in a real system, and
>>> there are several mitigating factors. The issue was found by code
>>> inspection (actually grep), and not by actually triggering the flow.
>>> Yet, since putting the page before unlocking is incorrect it should be
>>> fixed, if only to prevent future breakage or someone copy-pasting this
>>> code.
>>>
>>> Fixes: 70c3547e36f5c ("hugetlbfs: add hugetlbfs_fallocate()")
>>>
>>> cc: Eric Biggers <ebiggers3@xxxxxxxxx>
>>> cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>>
>>> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
>>
>> Thank you Nadav.
> 
> No problem.
> 
>>
>> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>
>> Since hugetlbfs is an in memory filesystem, the only way one 'should' be
>> able to remove a page (file content) is through an inode operation such as
>> truncate, hole punch, or unlink.  That was the basis for my response that
>> the inode lock would be required for page freeing.
>>
>> Eric's question about sys_fadvise64(POSIX_FADV_DONTNEED) is interesting.
>> I was expecting to see a check for hugetlbfs pages and exit (without
>> modification) if encountered.  A quick review of the code did not find
>> any such checks.
>>
>> I'll take a closer look to determine exactly how hugetlbfs files are
>> handled.  IMO, there should be something similar to the DAX check where
>> the routine quickly exits.
> 
> I did not cc stable when submitting the patch, based on your previous
> response. Let me know if you want me to send v2 which does so.

I still do not believe there is a need to change this in stable.  Your patch
should be sufficient to ensure we do the right thing going forward.

Looking at and testing the sys_fadvise64(POSIX_FADV_DONTNEED) code with
hugetlbfs does indeed show a more general problem.  One can use
sys_fadvise64() to remove a huge page from a hugetlbfs file. :(  This does
not go through the special hugetlbfs page handling code, but rather the
normal mm paths.  As a result hugetlbfs accounting (like reserve counts)
gets out of sync and the hugetlbfs filesystem may become unusable.  Sigh!!!

I will address this issue in a separate patch.
-- 
Mike Kravetz