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

[Mike Kravetz]

[CC Andrew, Michal]

On 11/28/2017 06:37 PM, Eric Biggers wrote:
> On Sat, Aug 26, 2017 at 12:11:24PM -0700, 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>
>> ---
>>  fs/hugetlbfs/inode.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>> index 28d2753be094..9475fee79cee 100644
>> --- a/fs/hugetlbfs/inode.c
>> +++ b/fs/hugetlbfs/inode.c
>> @@ -655,11 +655,11 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
>>  		mutex_unlock(&hugetlb_fault_mutex_table[hash]);
>>  
>>  		/*
>> -		 * page_put due to reference from alloc_huge_page()
>>  		 * unlock_page because locked by add_to_page_cache()
>> +		 * page_put due to reference from alloc_huge_page()
>>  		 */
>> -		put_page(page);
>>  		unlock_page(page);
>> +		put_page(page);
>>  	}
>>  
>>  	if (!(mode & FALLOC_FL_KEEP_SIZE) && offset + len > inode->i_size)
>> -- 
> 
> This patch wasn't ever applied.  Nadia, do you take patches for hugetlbfs, or
> does this need to go through Andrew Morton?
> 
> Eric

Nadia has not been active for some time on hugetlbfs, so best to go through
Andrew.  Added Andrew and Michal on CC.

This patch has a:
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>

I am still of the opinion that this does not need to be sent to stable.
Although the ordering is current code is incorrect, there is no way for
this to be a problem with current locking.   In addition, I verified
that the perhaps bigger issue with sys_fadvise64(POSIX_FADV_DONTNEED)
for hugetlbfs and other filesystems is addressed in commit 3a77d214807c.
-- 
Mike Kravetz