[PATCH 04/16] sched: convert user_struct.__count to refcount_t

[Elena Reshetova]

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 user_struct.__count 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 user_struct.__count it might make a difference
in following places:
 - free_uid(): decrement in refcount_dec_and_lock() only
   provides RELEASE ordering, control dependency on success
   and will hold a spin lock on success vs. fully ordered
   atomic counterpart. Note there is no changes in spin lock
   locking here.

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>
---
 include/linux/sched/user.h | 5 +++--
 kernel/user.c              | 8 ++++----
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/include/linux/sched/user.h b/include/linux/sched/user.h
index 0dcf4e4..2ca7cf4 100644
--- a/include/linux/sched/user.h
+++ b/include/linux/sched/user.h
@@ -4,6 +4,7 @@
 
 #include <linux/uidgid.h>
 #include <linux/atomic.h>
+#include <linux/refcount.h>
 
 struct key;
 
@@ -11,7 +12,7 @@ struct key;
  * Some day this will be a full-fledged user tracking system..
  */
 struct user_struct {
-	atomic_t __count;	/* reference count */
+	refcount_t __count;	/* reference count */
 	atomic_t processes;	/* How many processes does this user have? */
 	atomic_t sigpending;	/* How many pending signals does this user have? */
 #ifdef CONFIG_FANOTIFY
@@ -55,7 +56,7 @@ extern struct user_struct root_user;
 extern struct user_struct * alloc_uid(kuid_t);
 static inline struct user_struct *get_uid(struct user_struct *u)
 {
-	atomic_inc(&u->__count);
+	refcount_inc(&u->__count);
 	return u;
 }
 extern void free_uid(struct user_struct *);
diff --git a/kernel/user.c b/kernel/user.c
index 9a20acc..f104474 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -96,7 +96,7 @@ static DEFINE_SPINLOCK(uidhash_lock);
 
 /* root_user.__count is 1, for init task cred */
 struct user_struct root_user = {
-	.__count	= ATOMIC_INIT(1),
+	.__count	= REFCOUNT_INIT(1),
 	.processes	= ATOMIC_INIT(1),
 	.sigpending	= ATOMIC_INIT(0),
 	.locked_shm     = 0,
@@ -122,7 +122,7 @@ static struct user_struct *uid_hash_find(kuid_t uid, struct hlist_head *hashent)
 
 	hlist_for_each_entry(user, hashent, uidhash_node) {
 		if (uid_eq(user->uid, uid)) {
-			atomic_inc(&user->__count);
+			refcount_inc(&user->__count);
 			return user;
 		}
 	}
@@ -169,7 +169,7 @@ void free_uid(struct user_struct *up)
 		return;
 
 	local_irq_save(flags);
-	if (atomic_dec_and_lock(&up->__count, &uidhash_lock))
+	if (refcount_dec_and_lock(&up->__count, &uidhash_lock))
 		free_user(up, flags);
 	else
 		local_irq_restore(flags);
@@ -190,7 +190,7 @@ struct user_struct *alloc_uid(kuid_t uid)
 			goto out_unlock;
 
 		new->uid = uid;
-		atomic_set(&new->__count, 1);
+		refcount_set(&new->__count, 1);
 
 		/*
 		 * Before adding this, check whether we raced
-- 
2.7.4