linux-kernel - Re: [PATCH 12/16] nsproxy: convert nsproxy.count to refcount

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Date:   Wed, 15 Nov 2017 10:36:52 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Elena Reshetova <elena.reshetova@...el.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, peterz@...radead.org,
        gregkh@...uxfoundation.org, viro@...iv.linux.org.uk, tj@...nel.org,
        hannes@...xchg.org, lizefan@...wei.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, eparis@...hat.com,
        akpm@...ux-foundation.org, arnd@...db.de, luto@...nel.org,
        keescook@...omium.org, tglx@...utronix.de, dvhart@...radead.org,
        linux-mm@...ck.org, axboe@...nel.dk
Subject: Re: [PATCH 12/16] nsproxy: convert nsproxy.count to refcount_t


The middle of the merge window is the wrong time to send patches as
maintaner attention is going to making certain the merge goes smoothly
and nothing is missed.

Eric



Elena Reshetova <elena.reshetova@...el.com> writes:

> 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 nsproxy.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 nsproxy.count it might make a difference
> in following places:
>  - put_nsproxy() and switch_task_namespaces(): decrement in
>    refcount_dec_and_test() only provides RELEASE ordering
>    and control dependency on success vs. fully ordered
>    atomic counterpart
>
> Suggested-by: Kees Cook <keescook@...omium.org>
> Reviewed-by: David Windsor <dwindsor@...il.com>
> Reviewed-by: Hans Liljestrand <ishkamiel@...il.com>
> Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> ---
>  include/linux/nsproxy.h | 6 +++---
>  kernel/nsproxy.c        | 6 +++---
>  2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
> index 2ae1b1a..90f09ff 100644
> --- a/include/linux/nsproxy.h
> +++ b/include/linux/nsproxy.h
> @@ -29,7 +29,7 @@ struct fs_struct;
>   * nsproxy is copied.
>   */
>  struct nsproxy {
> -	atomic_t count;
> +	refcount_t count;
>  	struct uts_namespace *uts_ns;
>  	struct ipc_namespace *ipc_ns;
>  	struct mnt_namespace *mnt_ns;
> @@ -75,14 +75,14 @@ int __init nsproxy_cache_init(void);
>  
>  static inline void put_nsproxy(struct nsproxy *ns)
>  {
> -	if (atomic_dec_and_test(&ns->count)) {
> +	if (refcount_dec_and_test(&ns->count)) {
>  		free_nsproxy(ns);
>  	}
>  }
>  
>  static inline void get_nsproxy(struct nsproxy *ns)
>  {
> -	atomic_inc(&ns->count);
> +	refcount_inc(&ns->count);
>  }
>  
>  #endif
> diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
> index f6c5d33..5bfe691 100644
> --- a/kernel/nsproxy.c
> +++ b/kernel/nsproxy.c
> @@ -31,7 +31,7 @@
>  static struct kmem_cache *nsproxy_cachep;
>  
>  struct nsproxy init_nsproxy = {
> -	.count			= ATOMIC_INIT(1),
> +	.count			= REFCOUNT_INIT(1),
>  	.uts_ns			= &init_uts_ns,
>  #if defined(CONFIG_POSIX_MQUEUE) || defined(CONFIG_SYSVIPC)
>  	.ipc_ns			= &init_ipc_ns,
> @@ -52,7 +52,7 @@ static inline struct nsproxy *create_nsproxy(void)
>  
>  	nsproxy = kmem_cache_alloc(nsproxy_cachep, GFP_KERNEL);
>  	if (nsproxy)
> -		atomic_set(&nsproxy->count, 1);
> +		refcount_set(&nsproxy->count, 1);
>  	return nsproxy;
>  }
>  
> @@ -225,7 +225,7 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
>  	p->nsproxy = new;
>  	task_unlock(p);
>  
> -	if (ns && atomic_dec_and_test(&ns->count))
> +	if (ns && refcount_dec_and_test(&ns->count))
>  		free_nsproxy(ns);
>  }