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
