linux-kernel - RE: [PATCH] sparc64: convert mdesc_handle.refcnt from atomic_t to refcount

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Date:   Mon, 23 Oct 2017 07:10:55 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Shannon Nelson <shannon.nelson@...cle.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "jag.raman@...cle.com" <jag.raman@...cle.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>
Subject: RE: [PATCH] sparc64: convert mdesc_handle.refcnt from atomic_t to
 refcount_t

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

Thank you Shannon! Would you be able to take this patch into the respective tree
to propagate normally from there?

Best Regards,
Elena.
> 
> > ---
> >   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);
> >