linux-kernel - RE: [PATCH 4/4] kvm: convert kvm.users_count from atomic_t to refcount

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Date:   Mon, 20 Feb 2017 12:22:35 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "hpa@...or.com" <hpa@...or.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Windsor <dwindsor@...il.com>
Subject: RE: [PATCH 4/4] kvm: convert kvm.users_count from atomic_t to
 refcount_t

> On 20/02/2017 12:06, Elena Reshetova 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.
> >
> > Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> > Signed-off-by: Hans Liljestrand <ishkamiel@...il.com>
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > Signed-off-by: David Windsor <dwindsor@...il.com>
> 
> Looks good, will apply after the merge window.
> 
> Paolo

Thank you very much!

Best Regards,
Elena.

> 
> > ---
> >  include/linux/kvm_host.h | 3 ++-
> >  virt/kvm/kvm_main.c      | 8 ++++----
> >  2 files changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index cda457b..7fa05a5 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -26,6 +26,7 @@
> >  #include <linux/context_tracking.h>
> >  #include <linux/irqbypass.h>
> >  #include <linux/swait.h>
> > +#include <linux/refcount.h>
> >  #include <asm/signal.h>
> >
> >  #include <linux/kvm.h>
> > @@ -402,7 +403,7 @@ struct kvm {
> >  #endif
> >  	struct kvm_vm_stat stat;
> >  	struct kvm_arch arch;
> > -	atomic_t users_count;
> > +	refcount_t users_count;
> >  #ifdef KVM_COALESCED_MMIO_PAGE_OFFSET
> >  	struct kvm_coalesced_mmio_ring *coalesced_mmio_ring;
> >  	spinlock_t ring_lock;
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index dcd1c12..6ae5775 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -622,7 +622,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
> >  	mutex_init(&kvm->lock);
> >  	mutex_init(&kvm->irq_lock);
> >  	mutex_init(&kvm->slots_lock);
> > -	atomic_set(&kvm->users_count, 1);
> > +	refcount_set(&kvm->users_count, 1);
> >  	INIT_LIST_HEAD(&kvm->devices);
> >
> >  	r = kvm_arch_init_vm(kvm, type);
> > @@ -745,13 +745,13 @@ static void kvm_destroy_vm(struct kvm *kvm)
> >
> >  void kvm_get_kvm(struct kvm *kvm)
> >  {
> > -	atomic_inc(&kvm->users_count);
> > +	refcount_inc(&kvm->users_count);
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_get_kvm);
> >
> >  void kvm_put_kvm(struct kvm *kvm)
> >  {
> > -	if (atomic_dec_and_test(&kvm->users_count))
> > +	if (refcount_dec_and_test(&kvm->users_count))
> >  		kvm_destroy_vm(kvm);
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_put_kvm);
> > @@ -3640,7 +3640,7 @@ static int kvm_debugfs_open(struct inode *inode,
> struct file *file,
> >  	 * To avoid the race between open and the removal of the debugfs
> >  	 * directory we test against the users count.
> >  	 */
> > -	if (!atomic_add_unless(&stat_data->kvm->users_count, 1, 0))
> > +	if (!refcount_inc_not_zero(&stat_data->kvm->users_count))
> >  		return -ENOENT;
> >
> >  	if (simple_attr_open(inode, file, get, set, fmt)) {
> >