[PATCH v2] lib/stackdepot: Fix global out-of-bounds in stackdepot

[Walter Wu]

If the depot_index = STACK_ALLOC_MAX_SLABS - 2 and next_slab_inited = 0,
then it will cause array out-of-bounds access, so that we should modify
the detection to avoid this array out-of-bounds bug.

Assume depot_index = STACK_ALLOC_MAX_SLABS - 3
Consider following call flow sequence:

stack_depot_save()
   depot_alloc_stack()
      if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) //pass
      depot_index++  //depot_index = STACK_ALLOC_MAX_SLABS - 2
      if (depot_index + 1 < STACK_ALLOC_MAX_SLABS) //enter
         smp_store_release(&next_slab_inited, 0); //next_slab_inited = 0
      init_stack_slab()
	     if (stack_slabs[depot_index] == NULL) //enter

stack_depot_save()
   depot_alloc_stack()
      if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) //pass
      depot_index++  //depot_index = STACK_ALLOC_MAX_SLABS - 1
      init_stack_slab(&prealloc)
         stack_slabs[depot_index + 1]  //here get global out-of-bounds

Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
---
changes in v2:
modify call flow sequence and preconditon

---
 lib/stackdepot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index ed717dd08ff3..7e8a15e41600 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -106,7 +106,7 @@ static struct stack_record *depot_alloc_stack(unsigned long *entries, int size,
 	required_size = ALIGN(required_size, 1 << STACK_ALLOC_ALIGN);
 
 	if (unlikely(depot_offset + required_size > STACK_ALLOC_SIZE)) {
-		if (unlikely(depot_index + 1 >= STACK_ALLOC_MAX_SLABS)) {
+		if (unlikely(depot_index + 2 >= STACK_ALLOC_MAX_SLABS)) {
 			WARN_ONCE(1, "Stack depot reached limit capacity");
 			return NULL;
 		}
-- 
2.18.0