netdev - Re: [PATCH] net: Zeroing the structure ethtool_wolinfo in ethtool_get

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Date:   Sat, 26 Oct 2019 08:52:35 -0700
From:   Joe Perches <joe@...ches.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        zhanglin <zhang.lin16@....com.cn>
Cc:     davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        jakub.kicinski@...ronome.com, hawk@...nel.org,
        john.fastabend@...il.com, mkubecek@...e.cz, jiri@...lanox.com,
        pablo@...filter.org, f.fainelli@...il.com,
        maxime.chevallier@...tlin.com, lirongqing@...du.com,
        vivien.didelot@...il.com, linyunsheng@...wei.com,
        natechancellor@...il.com, arnd@...db.de, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        xue.zhihong@....com.cn, wang.yi59@....com.cn,
        jiang.xuexin@....com.cn
Subject: Re: [PATCH] net: Zeroing the structure ethtool_wolinfo in
 ethtool_get_wol()

On Sat, 2019-10-26 at 17:24 +0300, Dan Carpenter wrote:
> On Sat, Oct 26, 2019 at 03:54:16PM +0800, zhanglin wrote:
> > memset() the structure ethtool_wolinfo that has padded bytes
> > but the padded bytes have not been zeroed out.
[]
> > diff --git a/net/core/ethtool.c b/net/core/ethtool.c
[]
> > @@ -1471,11 +1471,13 @@ static int ethtool_reset(struct net_device *dev, char __user *useraddr)
> >  
> >  static int ethtool_get_wol(struct net_device *dev, char __user *useraddr)
> >  {
> > -	struct ethtool_wolinf wol = { .cmd = ETHTOOL_GWOL };
> > +	struct ethtool_wolinfo wol;
> >  
> 
> How did you detect that they weren't initialized?  Is this a KASAN
> thing?
> 
> Most of the time GCC will zero out the padding bytes when you have an
> initializer like this, but sometimes it just makes the intialization a
> series of assignments which leaves the holes uninitialized.  I wish I
> knew the rules so that I could check for it in Smatch.  Or even better,
> I wish that there were an option to always zero the holes in this
> situation...

The standard doesn't specify what happens to the padding so
it's not just for gcc, it's compiler dependent.

So anything that's used in a copy_to_user with any possible
padding should either be zalloc'd or memset before assigned.

In this case:

include/uapi/linux/ethtool.h:#define SOPASS_MAX 6

and

include/uapi/linux/ethtool.h:struct ethtool_wolinfo {
include/uapi/linux/ethtool.h-   __u32   cmd;
include/uapi/linux/ethtool.h-   __u32   supported;
include/uapi/linux/ethtool.h-   __u32   wolopts;
include/uapi/linux/ethtool.h-   __u8    sopass[SOPASS_MAX];
include/uapi/linux/ethtool.h-};

so there's likely a couple bytes of trailing padding.