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Subject[PATCH 5/6] userns: Use refcount_t for reference counting instead atomic_t
Date
refcount_t type and corresponding API should be used instead of atomic_t when
the variable is used as a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free situations.

Cc: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
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 96fe289c4c6e..39ad98c09c58 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>
#include <linux/ratelimit.h>

struct key;
@@ -12,7 +13,7 @@ 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
@@ -59,7 +60,7 @@ 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 36288d840675..5f65ef195259 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,
@@ -123,7 +123,7 @@ 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;
}
}
@@ -170,7 +170,7 @@ 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);
@@ -191,7 +191,7 @@ struct user_struct *alloc_uid(kuid_t uid)
goto out_unlock;

new->uid = uid;
- atomic_set(&new->__count, 1);
+ refcount_set(&new->__count, 1);
ratelimit_state_init(&new->ratelimit, HZ, 100);
ratelimit_set_flags(&new->ratelimit, RATELIMIT_MSG_ON_RELEASE);

--
2.18.0
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