[PATCH v2] groups: Convert group_info.usage to refcount_t

[Kees Cook]

From: Elena Reshetova <elena.reshetova@xxxxxxxxx>

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 group_info.usage 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 refcount.h have different
memory ordering guarantees than their atomic counterparts. Please check
Documentation/core-api/refcount-vs-atomic.rst for more information.

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 group_info.usage it might make a difference in following places:
 - put_group_info(): decrement in refcount_dec_and_test() only
   provides RELEASE ordering and ACQUIRE ordering on success vs. fully
   ordered atomic counterpart

Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v2: rebase
v1: https://lore.kernel.org/lkml/20200612183450.4189588-3-keescook@xxxxxxxxxxxx/
---
 include/linux/cred.h | 7 ++++---
 kernel/cred.c        | 2 +-
 kernel/groups.c      | 2 +-
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 9ed9232af934..8661f6294ad4 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -12,6 +12,7 @@
 #include <linux/init.h>
 #include <linux/key.h>
 #include <linux/atomic.h>
+#include <linux/refcount.h>
 #include <linux/uidgid.h>
 #include <linux/sched.h>
 #include <linux/sched/user.h>
@@ -23,7 +24,7 @@ struct inode;
  * COW Supplementary groups list
  */
 struct group_info {
-	atomic_t	usage;
+	refcount_t	usage;
 	int		ngroups;
 	kgid_t		gid[];
 } __randomize_layout;
@@ -39,7 +40,7 @@ struct group_info {
  */
 static inline struct group_info *get_group_info(struct group_info *gi)
 {
-	atomic_inc(&gi->usage);
+	refcount_inc(&gi->usage);
 	return gi;
 }
 
@@ -49,7 +50,7 @@ static inline struct group_info *get_group_info(struct group_info *gi)
  */
 #define put_group_info(group_info)			\
 do {							\
-	if (atomic_dec_and_test(&(group_info)->usage))	\
+	if (refcount_dec_and_test(&(group_info)->usage))	\
 		groups_free(group_info);		\
 } while (0)
 
diff --git a/kernel/cred.c b/kernel/cred.c
index 811ad654abd1..bed458cfb812 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -33,7 +33,7 @@ do {									\
 static struct kmem_cache *cred_jar;
 
 /* init to 2 - one for init_task, one to ensure it is never freed */
-static struct group_info init_groups = { .usage = ATOMIC_INIT(2) };
+static struct group_info init_groups = { .usage = REFCOUNT_INIT(2) };
 
 /*
  * The initial credentials for the initial task
diff --git a/kernel/groups.c b/kernel/groups.c
index 9aaed2a31073..9b43da22647d 100644
--- a/kernel/groups.c
+++ b/kernel/groups.c
@@ -19,7 +19,7 @@ struct group_info *groups_alloc(int gidsetsize)
 	if (!gi)
 		return NULL;
 
-	atomic_set(&gi->usage, 1);
+	refcount_set(&gi->usage, 1);
 	gi->ngroups = gidsetsize;
 	return gi;
 }
-- 
2.34.1