[PATCH 10/15] uprobes: convert uprobe.ref to refcount_t

From: Elena Reshetova
Date: Fri Aug 25 2017 - 05:42:32 EST


atomic_t variables are currently used to implement reference
counters with the following properties:
- counter is initialized to 1 using atomic_set()
- a resource is freed upon counter reaching zero
- once counter reaches zero, its further
increments aren't allowed
- counter schema uses basic atomic operations
(set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable uprobe.ref is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
---
kernel/events/uprobes.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 456596d..ca093957 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -66,7 +66,7 @@ static struct percpu_rw_semaphore dup_mmap_sem;

struct uprobe {
struct rb_node rb_node; /* node in the rb tree */
- atomic_t ref;
+ refcount_t ref;
struct rw_semaphore register_rwsem;
struct rw_semaphore consumer_rwsem;
struct list_head pending_list;
@@ -371,13 +371,13 @@ set_orig_insn(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long v

static struct uprobe *get_uprobe(struct uprobe *uprobe)
{
- atomic_inc(&uprobe->ref);
+ refcount_inc(&uprobe->ref);
return uprobe;
}

static void put_uprobe(struct uprobe *uprobe)
{
- if (atomic_dec_and_test(&uprobe->ref))
+ if (refcount_dec_and_test(&uprobe->ref))
kfree(uprobe);
}

@@ -459,7 +459,7 @@ static struct uprobe *__insert_uprobe(struct uprobe *uprobe)
rb_link_node(&uprobe->rb_node, parent, p);
rb_insert_color(&uprobe->rb_node, &uprobes_tree);
/* get access + creation ref */
- atomic_set(&uprobe->ref, 2);
+ refcount_set(&uprobe->ref, 2);

return u;
}
--
2.7.4