Re: [PATCH -V2] mm, swap: Remove unnecessary smp_rmb() in swap_type_to_swap_info()

[Will Deacon]

On Thu, May 20, 2021 at 03:33:01PM +0800, Huang Ying wrote:
> Before commit c10d38cc8d3e ("mm, swap: bounds check swap_info array
> accesses to avoid NULL derefs"), the typical code to reference the
> swap_info[] is as follows,
> 
>   type = swp_type(swp_entry);
>   if (type >= nr_swapfiles)
>           /* handle invalid swp_entry */;
>   p = swap_info[type];
>   /* access fields of *p.  OOPS! p may be NULL! */
> 
> Because the ordering isn't guaranteed, it's possible that
> swap_info[type] is read before "nr_swapfiles".  And that may result
> in NULL pointer dereference.
> 
> So after commit c10d38cc8d3e, the code becomes,
> 
>   struct swap_info_struct *swap_type_to_swap_info(int type)
>   {
> 	  if (type >= READ_ONCE(nr_swapfiles))
> 		  return NULL;
> 	  smp_rmb();
> 	  return READ_ONCE(swap_info[type]);
>   }
> 
>   /* users */
>   type = swp_type(swp_entry);
>   p = swap_type_to_swap_info(type);
>   if (!p)
> 	  /* handle invalid swp_entry */;
>   /* dereference p */
> 
> Where the value of swap_info[type] (that is, "p") is checked to be
> non-zero before being dereferenced.  So, the NULL deferencing
> becomes impossible even if "nr_swapfiles" is read after
> swap_info[type].  Therefore, the "smp_rmb()" becomes unnecessary.
> 
> And, we don't even need to read "nr_swapfiles" here.  Because the
> non-zero checking for "p" is sufficient.  We just need to make sure we
> will not access out of the boundary of the array.  With the change,
> nr_swapfiles will only be accessed with swap_lock held, except in
> swapcache_free_entries().  Where the absolute correctness of the value
> isn't needed, as described in the comments.
> 
> We still need to guarantee swap_info[type] is read before being
> dereferenced.  That can be satisfied via the data dependency ordering
> enforced by READ_ONCE(swap_info[type]).  This needs to be paired with
> proper write barriers.  So smp_store_release() is used in
> alloc_swap_info() to guarantee the fields of *swap_info[type] is
> initialized before swap_info[type] itself being written.  Note that
> the fields of *swap_info[type] is initialized to be 0 via kvzalloc()
> firstly.  The assignment and deferencing of swap_info[type] is like
> rcu_assign_pointer() and rcu_dereference().
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> Cc: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Omar Sandoval <osandov@xxxxxx>
> Cc: Paul McKenney <paulmck@xxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> 
> v2:
> 
> - Revise the patch description and comments per Peter's comments.
> 
> ---
>  mm/swapfile.c | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 2aad85751991..65dd979a0f94 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -100,11 +100,10 @@ atomic_t nr_rotate_swap = ATOMIC_INIT(0);
>  
>  static struct swap_info_struct *swap_type_to_swap_info(int type)
>  {
> -	if (type >= READ_ONCE(nr_swapfiles))
> +	if (type >= MAX_SWAPFILES)
>  		return NULL;
>  
> -	smp_rmb();	/* Pairs with smp_wmb in alloc_swap_info. */
> -	return READ_ONCE(swap_info[type]);
> +	return READ_ONCE(swap_info[type]); /* rcu_dereference() */
>  }
>  
>  static inline unsigned char swap_count(unsigned char ent)
> @@ -2884,14 +2883,12 @@ static struct swap_info_struct *alloc_swap_info(void)
>  	}
>  	if (type >= nr_swapfiles) {
>  		p->type = type;
> -		WRITE_ONCE(swap_info[type], p);
>  		/*
> -		 * Write swap_info[type] before nr_swapfiles, in case a
> -		 * racing procfs swap_start() or swap_next() is reading them.
> -		 * (We never shrink nr_swapfiles, we never free this entry.)
> +		 * Publish the swap_info_struct after initializing it.
> +		 * Note that kvzalloc() above zeroes all its fields.
>  		 */
> -		smp_wmb();
> -		WRITE_ONCE(nr_swapfiles, nr_swapfiles + 1);
> +		smp_store_release(&swap_info[type], p); /* rcu_assign_pointer() */
> +		nr_swapfiles++;

Although I like this change, I comment you are removing refers to some
dodgy-looking code. For example, swap_start() has this loop:

	for (type = 0; (si = swap_type_to_swap_info(type)); type++) {
		if (!(si->flags & SWP_USED) || !si->swap_map)
			continue;

so won't this just end up dereferencing NULL if nr_swapfiles < MAX_SWAPFILES?

I think you need to check all callers of swap_type_to_swap_info() are
either validating the 'type' they pass in or check the returned pointer.

Will