Re: [PATCH] mm/gup: fix omission of check on FOLL_LONGTERM in get_user_pages_fast()

[Ira Weiny]

On Thu, May 30, 2019 at 06:54:04AM +0800, Pingfan Liu wrote:
> As for FOLL_LONGTERM, it is checked in the slow path
> __gup_longterm_unlocked(). But it is not checked in the fast path, which
> means a possible leak of CMA page to longterm pinned requirement through
> this crack.
> 
> Place a check in the fast path.
> 
> Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>
> Cc: Keith Busch <keith.busch@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  mm/gup.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index f173fcb..00feab3 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2235,6 +2235,18 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
>  		local_irq_enable();
>  		ret = nr;
>  	}
> +#if defined(CONFIG_CMA)
> +	if (unlikely(gup_flags & FOLL_LONGTERM)) {
> +		int i, j;
> +
> +		for (i = 0; i < nr; i++)
> +			if (is_migrate_cma_page(pages[i])) {
> +				for (j = i; j < nr; j++)
> +					put_page(pages[j]);

Should be put_user_page() now.  For now that just calls put_page() but it is
slated to change soon.

I also wonder if this would be more efficient as a check as we are walking the
page tables and bail early.

Perhaps the code complexity is not worth it?

> +				nr = i;

Why not just break from the loop here?

Or better yet just use 'i' in the inner loop...

Ira

> +			}
> +	}
> +#endif
>  
>  	if (nr < nr_pages) {
>  		/* Try to get the remaining pages with get_user_pages */
> -- 
> 2.7.5
>