netdev - Re: [PATCH] net: fix a data race when get vlan device

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Date:   Fri, 16 Apr 2021 03:27:36 +0000
From:   "zhudi (J)" <zhudi21@...wei.com>
To:     linyunsheng <linyunsheng@...wei.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Chenxiang (EulerOS)" <rose.chen@...wei.com>
Subject: Re: [PATCH] net: fix a data race when get vlan device

> dependencyOn 2021/4/15 11:35, zhudi wrote:
> > From: Di Zhu <zhudi21@...wei.com>
> >
> > We encountered a crash: in the packet receiving process, we got an
> > illegal VLAN device address, but the VLAN device address saved in
> > vmcore is correct. After checking the code, we found a possible data
> > competition:
> > CPU 0:                             CPU 1:
> >     (RCU read lock)                  (RTNL lock)
> >     vlan_do_receive()		       register_vlan_dev()
> >       vlan_find_dev()
> >
> >         ->__vlan_group_get_device()	 ->vlan_group_prealloc_vid()
> >
> > In vlan_group_prealloc_vid(), We need to make sure that kzalloc is
> > executed before assigning a value to vlan devices array, otherwise we
> 
> As my understanding, there is a dependency between calling kzalloc() and
> assigning the address(returned from kzalloc()) to vg->vlan_devices_arrays,
> CPU and compiler can see the dependency, why can't it handling the
> dependency before adding the smp_wmb()?
> 
> See CONTROL DEPENDENCIES section in Documentation/memory-
> barriers.txt:
> 
> However, stores are not speculated.  This means that ordering -is- provided
> for load-store control dependencies, as in the following example:
> 
>         q = READ_ONCE(a);
>         if (q) {
>                 WRITE_ONCE(b, 1);
>         }
> 

 Maybe I didn't make it clear.  This memory isolation is to ensure the order of
 memset(object, 0, size) in kzalloc() operations and the subsequent array assignment statements.

kzalloc()
    ->memset(object, 0, size)

smp_wmb()

vg->vlan_devices_arrays[pidx][vidx] = array;

Because __vlan_group_get_device() function depends on this order

> 
> 
> > may get a wrong address from the hardware cache on another cpu.
> >
> > So fix it by adding memory barrier instruction to ensure the order of
> > memory operations.
> >
> > Signed-off-by: Di Zhu <zhudi21@...wei.com>
> > ---
> >  net/8021q/vlan.c | 2 ++
> >  net/8021q/vlan.h | 3 +++
> >  2 files changed, 5 insertions(+)
> >
> > diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c index
> > 8b644113715e..4f541e05cd3f 100644
> > --- a/net/8021q/vlan.c
> > +++ b/net/8021q/vlan.c
> > @@ -71,6 +71,8 @@ static int vlan_group_prealloc_vid(struct vlan_group
> *vg,
> >  	if (array == NULL)
> >  		return -ENOBUFS;
> >
> > +	smp_wmb();
> > +
> >  	vg->vlan_devices_arrays[pidx][vidx] = array;
> >  	return 0;
> >  }
> > diff --git a/net/8021q/vlan.h b/net/8021q/vlan.h index
> > 953405362795..7408fda084d3 100644
> > --- a/net/8021q/vlan.h
> > +++ b/net/8021q/vlan.h
> > @@ -57,6 +57,9 @@ static inline struct net_device
> > *__vlan_group_get_device(struct vlan_group *vg,
> >
> >  	array = vg->vlan_devices_arrays[pidx]
> >  				       [vlan_id /
> VLAN_GROUP_ARRAY_PART_LEN];
> > +
> > +	smp_rmb();
> > +
> >  	return array ? array[vlan_id % VLAN_GROUP_ARRAY_PART_LEN] :
> NULL;  }
> >
> >