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 ()"?
> 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.
Yes. I have checked all callers of swap_type_to_swap_info(). One
suspecting caller is swap_cluster_readahead(). But after the following
patch in mmotm tree,
mm/swapfile: use percpu_ref to serialize against concurrent swapoff
get/put_swap_device() will enclose the swap_cluster_readahead() to
check the swap entry beforehand.
Best Regards,
Huang, Ying
