Re: [PATCH] sparc64: convert mdesc_handle.refcnt from atomic_t to refcount_t

[Shannon Nelson]

On 10/20/2017 12:57 AM, Elena Reshetova wrote:
> 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 mdesc_handle.refcnt is used as pure reference counter.
> Convert it to refcount_t and fix up the operations.
>
> 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>

Acked-by: Shannon Nelson <shannon.nelson@xxxxxxxxxx>

> ---
>   arch/sparc/kernel/mdesc.c | 17 +++++++++--------
>   1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/arch/sparc/kernel/mdesc.c b/arch/sparc/kernel/mdesc.c
> index fa466ce..821a724 100644
> --- a/arch/sparc/kernel/mdesc.c
> +++ b/arch/sparc/kernel/mdesc.c
> @@ -12,6 +12,7 @@
>   #include <linux/miscdevice.h>
>   #include <linux/bootmem.h>
>   #include <linux/export.h>
> +#include <linux/refcount.h>
>   
>   #include <asm/cpudata.h>
>   #include <asm/hypervisor.h>
> @@ -70,7 +71,7 @@ struct mdesc_handle {
>   	struct list_head	list;
>   	struct mdesc_mem_ops	*mops;
>   	void			*self_base;
> -	atomic_t		refcnt;
> +	refcount_t		refcnt;
>   	unsigned int		handle_size;
>   	struct mdesc_hdr	mdesc;
>   };
> @@ -152,7 +153,7 @@ static void mdesc_handle_init(struct mdesc_handle *hp,
>   	memset(hp, 0, handle_size);
>   	INIT_LIST_HEAD(&hp->list);
>   	hp->self_base = base;
> -	atomic_set(&hp->refcnt, 1);
> +	refcount_set(&hp->refcnt, 1);
>   	hp->handle_size = handle_size;
>   }
>   
> @@ -182,7 +183,7 @@ static void __init mdesc_memblock_free(struct mdesc_handle *hp)
>   	unsigned int alloc_size;
>   	unsigned long start;
>   
> -	BUG_ON(atomic_read(&hp->refcnt) != 0);
> +	BUG_ON(refcount_read(&hp->refcnt) != 0);
>   	BUG_ON(!list_empty(&hp->list));
>   
>   	alloc_size = PAGE_ALIGN(hp->handle_size);
> @@ -220,7 +221,7 @@ static struct mdesc_handle *mdesc_kmalloc(unsigned int mdesc_size)
>   
>   static void mdesc_kfree(struct mdesc_handle *hp)
>   {
> -	BUG_ON(atomic_read(&hp->refcnt) != 0);
> +	BUG_ON(refcount_read(&hp->refcnt) != 0);
>   	BUG_ON(!list_empty(&hp->list));
>   
>   	kfree(hp->self_base);
> @@ -259,7 +260,7 @@ struct mdesc_handle *mdesc_grab(void)
>   	spin_lock_irqsave(&mdesc_lock, flags);
>   	hp = cur_mdesc;
>   	if (hp)
> -		atomic_inc(&hp->refcnt);
> +		refcount_inc(&hp->refcnt);
>   	spin_unlock_irqrestore(&mdesc_lock, flags);
>   
>   	return hp;
> @@ -271,7 +272,7 @@ void mdesc_release(struct mdesc_handle *hp)
>   	unsigned long flags;
>   
>   	spin_lock_irqsave(&mdesc_lock, flags);
> -	if (atomic_dec_and_test(&hp->refcnt)) {
> +	if (refcount_dec_and_test(&hp->refcnt)) {
>   		list_del_init(&hp->list);
>   		hp->mops->free(hp);
>   	}
> @@ -513,7 +514,7 @@ void mdesc_update(void)
>   	if (status != HV_EOK || real_len > len) {
>   		printk(KERN_ERR "MD: mdesc reread fails with %lu\n",
>   		       status);
> -		atomic_dec(&hp->refcnt);
> +		refcount_dec(&hp->refcnt);
>   		mdesc_free(hp);
>   		goto out;
>   	}
> @@ -526,7 +527,7 @@ void mdesc_update(void)
>   	mdesc_notify_clients(orig_hp, hp);
>   
>   	spin_lock_irqsave(&mdesc_lock, flags);
> -	if (atomic_dec_and_test(&orig_hp->refcnt))
> +	if (refcount_dec_and_test(&orig_hp->refcnt))
>   		mdesc_free(orig_hp);
>   	else
>   		list_add(&orig_hp->list, &mdesc_zombie_list);