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

[Michal Hocko]

[CC Andrew]

On Sat 26-08-17 12:11:24, 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.

Even if this a theoretical problem it is definitely worth fixing because
it is a anti-pattern of how the reference counted object should be treated.
 
> Fixes: 70c3547e36f5c ("hugetlbfs: add hugetlbfs_fallocate()")
> 
> cc: Eric Biggers <ebiggers3@xxxxxxxxx>
> cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> 
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks!

> ---
>  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)
> -- 
> 2.11.0

-- 
Michal Hocko
SUSE Labs