uprobes: convert uprobe.ref to refcount_t

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.

**Important note for maintainers:

Some functions from refcount_t API defined in lib/refcount.c
have different memory ordering guarantees than their atomic
counterparts.
The full comparison can be seen in
https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
in state to be merged to the documentation tree.
Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in
some rare cases it might matter.
Please double check that you don't have some undocumented
memory guarantees for this variable usage.

For the uprobe.ref it might make a difference
in following places:
 - put_uprobe(): decrement in refcount_dec_and_test() only
   provides RELEASE ordering and control dependency on success
   vs. fully ordered atomic counterpart

Link: http://lkml.kernel.org/r/1547637627-29526-1-git-send-email-elena.reshetova@intel.com
Suggested-by: Kees Cook <keescook@chromium.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 8aef47ee7bfad62f6147412caa9af28314bd7758..0a8bf7a4fc5ed773b7906eb5d684b1679bd22b80 100644 (file)
--- 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;
@@ -560,13 +560,13 @@ 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)) {
                /*
                 * If application munmap(exec_vma) before uprobe_unregister()
                 * gets called, we don't get a chance to remove uprobe from
@@ -657,7 +657,7 @@ 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;
 }