Re: [PATCH] net: initialize local variables in net/ipv6/mcast.c and net/ipv4/igmp.c

[Eric Dumazet]

On 4/2/21 7:36 PM, Phillip Potter wrote:
> Use memset to initialize two local buffers in net/ipv6/mcast.c,
> and another in net/ipv4/igmp.c. Fixes a KMSAN found uninit-value
> bug reported by syzbot at:
> https://syzkaller.appspot.com/bug?id=0766d38c656abeace60621896d705743aeefed51


According to this link, the bug no longer triggers.

Please explain why you think it is still there.

> 
> Reported-by: syzbot+001516d86dbe88862cec@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Phillip Potter <phil@xxxxxxxxxxxxxxxx>
> ---
>  net/ipv4/igmp.c  | 2 ++
>  net/ipv6/mcast.c | 4 ++++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
> index 7b272bbed2b4..bc8e358a9a2a 100644
> --- a/net/ipv4/igmp.c
> +++ b/net/ipv4/igmp.c
> @@ -1131,6 +1131,8 @@ static void ip_mc_filter_add(struct in_device *in_dev, __be32 addr)
>  	char buf[MAX_ADDR_LEN];
>  	struct net_device *dev = in_dev->dev;
>  
> +	memset(buf, 0, sizeof(buf));
> +
>  	/* Checking for IFF_MULTICAST here is WRONG-WRONG-WRONG.
>  	   We will get multicast token leakage, when IFF_MULTICAST
>  	   is changed. This check should be done in ndo_set_rx_mode
> diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
> index 6c8604390266..ad90dc28f318 100644
> --- a/net/ipv6/mcast.c
> +++ b/net/ipv6/mcast.c
> @@ -658,6 +658,8 @@ static void igmp6_group_added(struct ifmcaddr6 *mc)
>  	struct net_device *dev = mc->idev->dev;
>  	char buf[MAX_ADDR_LEN];
>  
> +	memset(buf, 0, sizeof(buf));
> +
>  	if (IPV6_ADDR_MC_SCOPE(&mc->mca_addr) <
>  	    IPV6_ADDR_SCOPE_LINKLOCAL)
>  		return;
> @@ -694,6 +696,8 @@ static void igmp6_group_dropped(struct ifmcaddr6 *mc)
>  	struct net_device *dev = mc->idev->dev;
>  	char buf[MAX_ADDR_LEN];
>  
> +	memset(buf, 0, sizeof(buf));
> +
>  	if (IPV6_ADDR_MC_SCOPE(&mc->mca_addr) <
>  	    IPV6_ADDR_SCOPE_LINKLOCAL)
>  		return;
>