linux-kernel - Re: [PATCH] hugetlbfs: change put_page/unlock_page order in hugetlbfs

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Date:   Sun, 27 Aug 2017 20:08:58 +0000
From:   Nadav Amit <namit@...are.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     Nadia Yvette Chambers <nyc@...omorphy.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Eric Biggers <ebiggers3@...il.com>
Subject: Re: [PATCH] hugetlbfs: change put_page/unlock_page order in
 hugetlbfs_fallocate()

Mike Kravetz <mike.kravetz@...cle.com> 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@...il.com>
>> cc: Mike Kravetz <mike.kravetz@...cle.com>
>> 
>> Signed-off-by: Nadav Amit <namit@...are.com>
> 
> Thank you Nadav.

No problem.

> 
> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
> 
> 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.

Thanks,
Nadav