netdev - Re: [PATCH net] ipv6: make icmp6_send() robust against null skb->dev

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Date:   Tue, 8 Jan 2019 09:57:46 +0100
From:   Piotr Sawicki <p.sawicki2@...tner.samsung.com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ipv6: make icmp6_send() robust against null
 skb->dev

dccp_v6_rcv() calls __sk_receive_skb() which calls sk_filter_trim_cap().

sk_filter_trim_cap() should return a value not equal to 0 and cause the skb to be dropped, since icmpv6_send() is called when smack_socket_sock_rcv_skb() returns -EACCES.

So, the packet shouldn't be put into the backlog queue.

How did it get there?


Regards,

Piotr


On 1/4/19 8:00 PM, Eric Dumazet wrote:
> syzbot was able to crash one host with the following stack trace :
>
> kasan: GPF could be caused by NULL-ptr deref or user memory access
> general protection fault: 0000 [#1] PREEMPT SMP KASAN
> CPU: 0 PID: 8625 Comm: syz-executor4 Not tainted 4.20.0+ #8
> RIP: 0010:dev_net include/linux/netdevice.h:2169 [inline]
> RIP: 0010:icmp6_send+0x116/0x2d30 net/ipv6/icmp.c:426
>  icmpv6_send
>  smack_socket_sock_rcv_skb
>  security_sock_rcv_skb
>  sk_filter_trim_cap
>  __sk_receive_skb
>  dccp_v6_do_rcv
>  release_sock
>
> This is because a RX packet found socket owned by user and
> was stored into socket backlog. Before leaving RCU protected section,
> skb->dev was cleared in __sk_receive_skb(). When socket backlog
> was finally handled at release_sock() time, skb was fed to
> smack_socket_sock_rcv_skb() then icmp6_send()
>
> We could fix the bug in smack_socket_sock_rcv_skb(), or simply
> make icmp6_send() more robust against such possibility.
>
> In the future we might provide to icmp6_send() the net pointer
> instead of infering it.
>
> Fixes: d66a8acbda92 ("Smack: Inform peer that IPv6 traffic has been blocked")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Piotr Sawicki <p.sawicki2@...tner.samsung.com>
> Cc: Casey Schaufler <casey@...aufler-ca.com>
> Reported-by: syzbot <syzkaller@...glegroups.com>
> ---
>  net/ipv6/icmp.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
> index 5d7aa2c2770ca2b4981d2dd211c3cf0a79a6f9e2..bbcdfd2996926a78c3ea0b274adfa9b5f297efbc 100644
> --- a/net/ipv6/icmp.c
> +++ b/net/ipv6/icmp.c
> @@ -423,10 +423,10 @@ static int icmp6_iif(const struct sk_buff *skb)
>  static void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
>  		       const struct in6_addr *force_saddr)
>  {
> -	struct net *net = dev_net(skb->dev);
>  	struct inet6_dev *idev = NULL;
>  	struct ipv6hdr *hdr = ipv6_hdr(skb);
>  	struct sock *sk;
> +	struct net *net;
>  	struct ipv6_pinfo *np;
>  	const struct in6_addr *saddr = NULL;
>  	struct dst_entry *dst;
> @@ -437,12 +437,16 @@ static void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
>  	int iif = 0;
>  	int addr_type = 0;
>  	int len;
> -	u32 mark = IP6_REPLY_MARK(net, skb->mark);
> +	u32 mark;
>  
>  	if ((u8 *)hdr < skb->head ||
>  	    (skb_network_header(skb) + sizeof(*hdr)) > skb_tail_pointer(skb))
>  		return;
>  
> +	if (!skb->dev)
> +		return;
> +	net = dev_net(skb->dev);
> +	mark = IP6_REPLY_MARK(net, skb->mark);
>  	/*
>  	 *	Make sure we respect the rules
>  	 *	i.e. RFC 1885 2.4(e)