On Fri, Jan 4, 2019 at 11:36 AM Casey Schaufler <ca...@schaufler-ca.com> wrote:
>
> On 1/4/2019 11:00 AM, 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.
>
> The Smack patch would be a trivial check for skb->dev == NULL,
> in which case it wouldn't call icmp6_send(). Unless there's a
> timing issue, of course. If there are no known timing issues I
> would be happy to create a Smack patch to address this problem.
>
> Or, I'm happy with the patch below if you like it.
>
Well, doing the check in icmp6_send() is more generic, this is the path I took,
thanks ;)
> >
> > 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 <eduma...@google.com>
> > Cc: Piotr Sawicki <p.sawic...@partner.samsung.com>
> > Cc: Casey Schaufler <ca...@schaufler-ca.com>
> > Reported-by: syzbot <syzkal...@googlegroups.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)
>
