netdev - Re: [Cocci] [PATCH] net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

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Message-ID: <alpine.DEB.2.21.1911212139130.2902@hadrien>
Date:   Thu, 21 Nov 2019 21:40:45 +0100 (CET)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Michal Kubecek <mkubecek@...e.cz>
cc:     Dan Carpenter <dan.carpenter@...cle.com>,
        jakub.kicinski@...ronome.com, ast@...nel.org,
        natechancellor@...il.com, jiang.xuexin@....com.cn,
        cocci <cocci@...teme.lip6.fr>, f.fainelli@...il.com,
        daniel@...earbox.net, john.fastabend@...il.com,
        lirongqing@...du.com, maxime.chevallier@...tlin.com,
        vivien.didelot@...il.com, pablo@...filter.org,
        wang.yi59@....com.cn, hawk@...nel.org, arnd@...db.de,
        jiri@...lanox.com, xue.zhihong@....com.cn,
        zhanglin <zhang.lin16@....com.cn>,
        Thomas Gleixner <tglx@...utronix.de>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linyunsheng@...wei.com, Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        davem@...emloft.net
Subject: Re: [Cocci] [PATCH] net: Zeroing the structure ethtool_wolinfo in
 ethtool_get_wol()

On Thu, 21 Nov 2019, Michal Kubecek wrote:

> On Thu, Nov 21, 2019 at 03:07:33PM +0300, Dan Carpenter wrote:
> > On Thu, Nov 21, 2019 at 12:19:17PM +0100, Michal Kubecek wrote:
> > > On Thu, Nov 21, 2019 at 11:23:34AM +0100, Enrico Weigelt, metux IT consult wrote:
> > > > On 26.10.19 21:40, Joe Perches wrote:
> > > > > On Sat, 2019-10-26 at 15:54 +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_wolinfo wol = { .cmd = ETHTOOL_GWOL };
> > > > >> +	struct ethtool_wolinfo wol;
> > > > >>
> > > > >>  	if (!dev->ethtool_ops->get_wol)
> > > > >>  		return -EOPNOTSUPP;
> > > > >>
> > > > >> +	memset(&wol, 0, sizeof(struct ethtool_wolinfo));
> > > > >> +	wol.cmd = ETHTOOL_GWOL;
> > > > >>  	dev->ethtool_ops->get_wol(dev, &wol);
> > > > >>
> > > > >>  	if (copy_to_user(useraddr, &wol, sizeof(wol)))
> > > > >
> > > > > It seems likely there are more of these.
> > > > >
> > > > > Is there any way for coccinelle to find them?
> > > >
> > > > Just curios: is static struct initialization (on stack) something that
> > > > should be avoided ? I've been under the impression that static
> > > > initialization allows thinner code and gives the compiler better chance
> > > > for optimizations.
> > >
> > > Not in general. The (potential) problem here is that the structure has
> > > padding and it is as a whole (i.e. including the padding) copied to
> > > userspace. While I'm not aware of a compiler that wouldn't actually
> > > initialize the whole data block including the padding in this case, the
> > > C standard provides no guarantee about that so that to be sure we cannot
> > > leak leftover kernel data to userspace, we need to explicitly initialize
> > > the whole block.
> >
> > GCC will not always initialize the struct holes.  This patch fixes a
> > real bug that GCC on my system (v7.4)
>
> Just checked (again) to be sure. No matter if the function is inlined or
> not, gcc 7.4.1 initializes the structure by one movl (of 0x5) and two
> movq (of 0x0), i.e. initializes all sizeof(struct ethtool_wolinfo) = 20
> bytes including the padding.
>
> One could certainly construct examples where a real life compiler would
> only initialize the fields. That's why I said "in this case".

Looking again at the case that I mentioned, I see:

# drivers/gpu/drm/amd/amdgpu/amdgpu_kms.c:691:          struct drm_amdgpu_info_device dev_info = {};
        call    __sanitizer_cov_trace_pc        #
        leaq    840(%rsp), %rdi #, tmp1126
        xorl    %eax, %eax      # tmp1127
        movl    $46, %ecx       #, tmp1128
        rep stosq

So I guess the rep stosq is doing the memset.

julia

>
> Michal Kubecek
>
>
> _______________________________________________
> Cocci mailing list
> Cocci@...teme.lip6.fr
> https://systeme.lip6.fr/mailman/listinfo/cocci
>

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