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Subject: Re: [PATCH NET v3 5/7] vrf: use skb_expand_head in vrf_finish_output
To:     Vasily Averin <vvs@virtuozzo.com>,
        "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <15eba3b2-80e2-5547-8ad9-167d810ad7e3@virtuozzo.com>
 <cover.1627891754.git.vvs@virtuozzo.com>
 <e4ca1ef1-56f3-5bce-eec8-617e24bc7b1a@virtuozzo.com>
From:   Julian Wiedmann <jwi@linux.ibm.com>
Message-ID: <ccce7edb-54dd-e6bf-1e84-0ec320d8886c@linux.ibm.com>
Date:   Thu, 5 Aug 2021 14:55:50 +0300
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Precedence: bulk

On 02.08.21 11:52, Vasily Averin wrote:
> Unlike skb_realloc_headroom, new helper skb_expand_head
> does not allocate a new skb if possible.
> 
> Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
> ---
>  drivers/net/vrf.c | 21 +++++++--------------
>  1 file changed, 7 insertions(+), 14 deletions(-)
> 

[...]

>  	/* Be paranoid, rather than too clever. */
>  	if (unlikely(skb_headroom(skb) < hh_len && dev->header_ops)) {
> -		struct sk_buff *skb2;
> -
> -		skb2 = skb_realloc_headroom(skb, LL_RESERVED_SPACE(dev));
> -		if (!skb2) {
> -			ret = -ENOMEM;
> -			goto err;
> +		skb = skb_expand_head(skb, hh_len);
> +		if (!skb) {
> +			skb->dev->stats.tx_errors++;
> +			return -ENOMEM;

Hello Vasily,

FYI, Coverity complains that we check skb != NULL here but then
still dereference skb->dev:


*** CID 1506214:  Null pointer dereferences  (FORWARD_NULL)
/drivers/net/vrf.c: 867 in vrf_finish_output()
861     	nf_reset_ct(skb);
862     
863     	/* Be paranoid, rather than too clever. */
864     	if (unlikely(skb_headroom(skb) < hh_len && dev->header_ops)) {
865     		skb = skb_expand_head(skb, hh_len);
866     		if (!skb) {
>>>     CID 1506214:  Null pointer dereferences  (FORWARD_NULL)
>>>     Dereferencing null pointer "skb".
867     			skb->dev->stats.tx_errors++;
868     			return -ENOMEM;
869     		}
870     	}
871     
872     	rcu_read_lock_bh();