Re: [PATCH] mm/gup: don't permit users to call get_user_pages with FOLL_LONGTERM

[Ira Weiny]

On Wed, Aug 19, 2020 at 11:01:00PM +1200, Barry Song wrote:
> gug prohibits users from calling get_user_pages() with FOLL_PIN. But it
> allows users to call get_user_pages() with FOLL_LONGTERM only. It seems
> insensible.
> 
> since FOLL_LONGTERM is a stricter case of FOLL_PIN, we should prohibit
> users from calling get_user_pages() with FOLL_LONGTERM while not with
> FOLL_PIN.
> 
> mm/gup_benchmark.c used to be the only user who did this improperly.
> But it has been fixed by moving to use pin_user_pages().
> 
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Shuah Khan <shuah@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>

Seems reasonable to me.

Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx>

> ---
>  mm/gup.c | 37 ++++++++++++++++++++++---------------
>  1 file changed, 22 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index ae096ea7583f..4da669f79566 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1789,6 +1789,25 @@ static long __get_user_pages_remote(struct mm_struct *mm,
>  				       gup_flags | FOLL_TOUCH | FOLL_REMOTE);
>  }
>  
> +static bool is_valid_gup_flags(unsigned int gup_flags)
> +{
> +	/*
> +	 * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
> +	 * never directly by the caller, so enforce that with an assertion:
> +	 */
> +	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> +		return false;
> +	/*
> +	 * FOLL_PIN is a prerequisite to FOLL_LONGTERM. Another way of saying
> +	 * that is, FOLL_LONGTERM is a specific case, more restrictive case of
> +	 * FOLL_PIN.
> +	 */
> +	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
> +		return false;
> +
> +	return true;
> +}
> +
>  /**
>   * get_user_pages_remote() - pin user pages in memory
>   * @mm:		mm_struct of target mm
> @@ -1854,11 +1873,7 @@ long get_user_pages_remote(struct mm_struct *mm,
>  		unsigned int gup_flags, struct page **pages,
>  		struct vm_area_struct **vmas, int *locked)
>  {
> -	/*
> -	 * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
> -	 * never directly by the caller, so enforce that with an assertion:
> -	 */
> -	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> +	if (!is_valid_gup_flags(gup_flags))
>  		return -EINVAL;
>  
>  	return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
> @@ -1904,11 +1919,7 @@ long get_user_pages(unsigned long start, unsigned long nr_pages,
>  		unsigned int gup_flags, struct page **pages,
>  		struct vm_area_struct **vmas)
>  {
> -	/*
> -	 * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
> -	 * never directly by the caller, so enforce that with an assertion:
> -	 */
> -	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> +	if (!is_valid_gup_flags(gup_flags))
>  		return -EINVAL;
>  
>  	return __gup_longterm_locked(current->mm, start, nr_pages,
> @@ -2810,11 +2821,7 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast_only);
>  int get_user_pages_fast(unsigned long start, int nr_pages,
>  			unsigned int gup_flags, struct page **pages)
>  {
> -	/*
> -	 * FOLL_PIN must only be set internally by the pin_user_pages*() APIs,
> -	 * never directly by the caller, so enforce that:
> -	 */
> -	if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> +	if (!is_valid_gup_flags(gup_flags))
>  		return -EINVAL;
>  
>  	/*
> -- 
> 2.27.0
> 
> 
>