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++;
} else {
defer = p;
p = swap_info[type];
--
2.30.2
