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

[Will Deacon]

On Mon, Jun 07, 2021 at 09:49:35AM +0800, Huang, Ying wrote:
> Hi, Will,
> 
> Will Deacon <will@xxxxxxxxxx> writes:
> 
> > 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?
> 
>  	for (type = 0; (si = swap_type_to_swap_info(type)); type++) {
>                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Because this is the second sub-statement inside "for ()", I think that "si"
> will be checked to be non-NULL before executing the statements inside
> "{}" follows "for ()"?

Sorry, yes, you're right. I misread the loop condition.

Will