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

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Date:   Fri, 4 Jan 2019 11:38:32 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        p.sawicki2@...tner.samsung.com, syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] ipv6: make icmp6_send() robust against null skb->dev

On Fri, Jan 4, 2019 at 11:36 AM Casey Schaufler <casey@...aufler-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 <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)
>