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 <[email protected]>
> Cc: Piotr Sawicki <[email protected]>
> Cc: Casey Schaufler <[email protected]>
> Reported-by: syzbot <[email protected]>
> ---
> 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)
