Re: [PATCH aio-next] aio: fix race in ring buffer page lookup introducedby page migration support

[Gu Zheng]

Hi Ben, Al,

On 09/10/2013 12:02 AM, Benjamin LaHaise wrote:

> Hi Al, Gu,
> 
> I've added this patch to my tree at git://git.kvack.org/~bcrl/aio-next.git 
> to fix the get_user_pages() issue introduced by Gu's changes in the page 
> migration patch.  Thanks Al for spotting this.

Thanks very much for spotting and fixing this issue.

Best regards,
Gu

> 
> 		-ben
> 
> commit d6c355c7dabcd753a75bc77d150d36328a355267
> Author: Benjamin LaHaise <bcrl@xxxxxxxxx>
> Date:   Mon Sep 9 11:57:59 2013 -0400
> 
>     aio: fix race in ring buffer page lookup introduced by page migration support
>     
>     Prior to the introduction of page migration support in "fs/aio: Add support
>     to aio ring pages migration" / 36bc08cc01709b4a9bb563b35aa530241ddc63e3,
>     mapping of the ring buffer pages was done via get_user_pages() while
>     retaining mmap_sem held for write.  This avoided possible races with userland
>     racing an munmap() or mremap().  The page migration patch, however, switched
>     to using mm_populate() to prime the page mapping.  mm_populate() cannot be
>     called with mmap_sem held.
>     
>     Instead of dropping the mmap_sem, revert to the old behaviour and simply
>     drop the use of mm_populate() since get_user_pages() will cause the pages to
>     get mapped anyways.  Thanks to Al Viro for spotting this issue.
>     
>     Signed-off-by: Benjamin LaHaise <bcrl@xxxxxxxxx>
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 6e26755..f4a27af 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -307,16 +307,25 @@ static int aio_setup_ring(struct kioctx *ctx)
>  		aio_free_ring(ctx);
>  		return -EAGAIN;
>  	}
> -	up_write(&mm->mmap_sem);
> -
> -	mm_populate(ctx->mmap_base, populate);
>  
>  	pr_debug("mmap address: 0x%08lx\n", ctx->mmap_base);
> +
> +	/* We must do this while still holding mmap_sem for write, as we
> +	 * need to be protected against userspace attempting to mremap()
> +	 * or munmap() the ring buffer.
> +	 */
>  	ctx->nr_pages = get_user_pages(current, mm, ctx->mmap_base, nr_pages,
>  				       1, 0, ctx->ring_pages, NULL);
> +
> +	/* Dropping the reference here is safe as the page cache will hold
> +	 * onto the pages for us.  It is also required so that page migration
> +	 * can unmap the pages and get the right reference count.
> +	 */
>  	for (i = 0; i < ctx->nr_pages; i++)
>  		put_page(ctx->ring_pages[i]);
>  
> +	up_write(&mm->mmap_sem);
> +
>  	if (unlikely(ctx->nr_pages != nr_pages)) {
>  		aio_free_ring(ctx);
>  		return -EAGAIN;


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