From 60d4de3ff7f775509deba94b3db3c1abe55bf7a5 Mon Sep 17 00:00:00 2001 From: Elena Reshetova Date: Fri, 18 Jan 2019 14:27:27 +0200 Subject: [PATCH] sched/core: Convert signal_struct.sigcnt 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 signal_struct.sigcnt 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 signal_struct.sigcnt it might make a difference in following places: - put_signal_struct(): decrement in refcount_dec_and_test() only provides RELEASE ordering and control dependency on success vs. fully ordered atomic counterpart Suggested-by: Kees Cook Signed-off-by: Elena Reshetova Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: David Windsor Reviewed-by: Hans Liljestrand Reviewed-by: Andrea Parri Reviewed-by: Oleg Nesterov Cc: Linus Torvalds Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: akpm@linux-foundation.org Cc: viro@zeniv.linux.org.uk Link: https://lkml.kernel.org/r/1547814450-18902-3-git-send-email-elena.reshetova@intel.com Signed-off-by: Ingo Molnar --- include/linux/sched/signal.h | 2 +- init/init_task.c | 2 +- kernel/fork.c | 6 +++--- 3 files changed, 5 insertions(+), 5 deletions(-) diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index 37eeb1a28eba..ae5655197698 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -83,7 +83,7 @@ struct multiprocess_signals { * the locking of signal_struct. */ struct signal_struct { - atomic_t sigcnt; + refcount_t sigcnt; atomic_t live; int nr_threads; struct list_head thread_head; diff --git a/init/init_task.c b/init/init_task.c index 5aebe3be4d7c..9aa3ebc74970 100644 --- a/init/init_task.c +++ b/init/init_task.c @@ -44,7 +44,7 @@ static struct signal_struct init_signals = { }; static struct sighand_struct init_sighand = { - .count = ATOMIC_INIT(1), + .count = REFCOUNT_INIT(1), .action = { { { .sa_handler = SIG_DFL, } }, }, .siglock = __SPIN_LOCK_UNLOCKED(init_sighand.siglock), .signalfd_wqh = __WAIT_QUEUE_HEAD_INITIALIZER(init_sighand.signalfd_wqh), diff --git a/kernel/fork.c b/kernel/fork.c index 370856d4c0b3..935a42d5f8ff 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -710,7 +710,7 @@ static inline void free_signal_struct(struct signal_struct *sig) static inline void put_signal_struct(struct signal_struct *sig) { - if (atomic_dec_and_test(&sig->sigcnt)) + if (refcount_dec_and_test(&sig->sigcnt)) free_signal_struct(sig); } @@ -1527,7 +1527,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk) sig->nr_threads = 1; atomic_set(&sig->live, 1); - atomic_set(&sig->sigcnt, 1); + refcount_set(&sig->sigcnt, 1); /* list_add(thread_node, thread_head) without INIT_LIST_HEAD() */ sig->thread_head = (struct list_head)LIST_HEAD_INIT(tsk->thread_node); @@ -2082,7 +2082,7 @@ static __latent_entropy struct task_struct *copy_process( } else { current->signal->nr_threads++; atomic_inc(¤t->signal->live); - atomic_inc(¤t->signal->sigcnt); + refcount_inc(¤t->signal->sigcnt); task_join_group_stop(p); list_add_tail_rcu(&p->thread_group, &p->group_leader->thread_group);