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SubjectRe: [PATCH] sparc64: convert mdesc_handle.refcnt from atomic_t to refcount_t
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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@chromium.org>
> Reviewed-by: David Windsor <dwindsor@gmail.com>
> Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>

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

> ---
> 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);
>

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