linux-kernel - Re: [PATCH 5/6] userns: Use refcount_t for reference counting instead atomic

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Message-ID: <20180716123918.g7kkm5nivtk7ldlt@linutronix.de>
Date:   Mon, 16 Jul 2018 14:39:18 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     tglx@...utronix.de, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 5/6] userns: Use refcount_t for reference counting
 instead atomic_t

On 2018-07-03 22:01:40 [+0200], To linux-kernel@...r.kernel.org wrote:
> 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.

Ingo, Andrew: who of you two feels most comfortable to apply this one
(and 6/6 of this series, both `userns')? If none, who would you suggest?

> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.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

Sebastian