From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id C48556B0266 for ; Wed, 15 Nov 2017 09:08:21 -0500 (EST) Received: by mail-pf0-f198.google.com with SMTP id p2so21005728pfk.13 for ; Wed, 15 Nov 2017 06:08:21 -0800 (PST) Received: from mga06.intel.com (mga06.intel.com. [134.134.136.31]) by mx.google.com with ESMTPS id a6si8549271plt.76.2017.11.15.06.08.20 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 15 Nov 2017 06:08:20 -0800 (PST) From: Elena Reshetova Subject: [PATCH 06/16] sched/task_struct: convert task_struct.usage to refcount_t Date: Wed, 15 Nov 2017 16:03:30 +0200 Message-Id: <1510754620-27088-7-git-send-email-elena.reshetova@intel.com> In-Reply-To: <1510754620-27088-1-git-send-email-elena.reshetova@intel.com> References: <1510754620-27088-1-git-send-email-elena.reshetova@intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: mingo@redhat.com Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, peterz@infradead.org, gregkh@linuxfoundation.org, viro@zeniv.linux.org.uk, tj@kernel.org, hannes@cmpxchg.org, lizefan@huawei.com, acme@kernel.org, alexander.shishkin@linux.intel.com, eparis@redhat.com, akpm@linux-foundation.org, arnd@arndb.de, luto@kernel.org, keescook@chromium.org, tglx@linutronix.de, dvhart@infradead.org, ebiederm@xmission.com, linux-mm@kvack.org, axboe@kernel.dk, 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 task_struct.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 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 task_struct.usage it might make a difference in following places: - put_task_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 Reviewed-by: David Windsor Reviewed-by: Hans Liljestrand Signed-off-by: Elena Reshetova --- include/linux/init_task.h | 2 +- include/linux/sched.h | 3 ++- include/linux/sched/task.h | 4 ++-- kernel/fork.c | 4 ++-- 4 files changed, 7 insertions(+), 6 deletions(-) diff --git a/include/linux/init_task.h b/include/linux/init_task.h index 9eb2ce8..1e35fce 100644 --- a/include/linux/init_task.h +++ b/include/linux/init_task.h @@ -227,7 +227,7 @@ extern struct cred init_cred; INIT_TASK_TI(tsk) \ .state = 0, \ .stack = init_stack, \ - .usage = ATOMIC_INIT(2), \ + .usage = REFCOUNT_INIT(2), \ .flags = PF_KTHREAD, \ .prio = MAX_PRIO-20, \ .static_prio = MAX_PRIO-20, \ diff --git a/include/linux/sched.h b/include/linux/sched.h index 44f9df5..924a812 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -21,6 +21,7 @@ #include #include #include +#include #include #include #include @@ -536,7 +537,7 @@ struct task_struct { randomized_struct_fields_start void *stack; - atomic_t usage; + refcount_t usage; /* Per task flags (PF_*), defined further below: */ unsigned int flags; unsigned int ptrace; diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h index 5be31eb..dae8d04 100644 --- a/include/linux/sched/task.h +++ b/include/linux/sched/task.h @@ -86,13 +86,13 @@ extern void sched_exec(void); #define sched_exec() {} #endif -#define get_task_struct(tsk) do { atomic_inc(&(tsk)->usage); } while(0) +#define get_task_struct(tsk) do { refcount_inc(&(tsk)->usage); } while(0) extern void __put_task_struct(struct task_struct *t); static inline void put_task_struct(struct task_struct *t) { - if (atomic_dec_and_test(&t->usage)) + if (refcount_dec_and_test(&t->usage)) __put_task_struct(t); } diff --git a/kernel/fork.c b/kernel/fork.c index a65ec7d..16df4f5 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -649,7 +649,7 @@ static inline void put_signal_struct(struct signal_struct *sig) void __put_task_struct(struct task_struct *tsk) { WARN_ON(!tsk->exit_state); - WARN_ON(atomic_read(&tsk->usage)); + WARN_ON(refcount_read(&tsk->usage)); WARN_ON(tsk == current); cgroup_free(tsk); @@ -824,7 +824,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node) * One for us, one for whoever does the "release_task()" (usually * parent) */ - atomic_set(&tsk->usage, 2); + refcount_set(&tsk->usage, 2); #ifdef CONFIG_BLK_DEV_IO_TRACE tsk->btrace_seq = 0; #endif -- 2.7.4 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org