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

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Date:   Tue, 8 Jan 2019 02:06:00 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     p.sawicki2@...tner.samsung.com
Cc:     David Miller <davem@...emloft.net>,
        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

On Tue, Jan 8, 2019 at 1:47 AM Piotr Sawicki
<p.sawicki2@...tner.samsung.com> wrote:
>
>
> On 1/8/19 10:21 AM, Eric Dumazet wrote:
> > On Tue, Jan 8, 2019 at 12:57 AM Piotr Sawicki
> > <p.sawicki2@...tner.samsung.com> wrote:
> >> 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?
> >>
> > I do not believe crash involved a BPF filter at all (My changelog said
> > nothing about sk_filter_trim_cap()
>
> Not only BPF but also the LSM subsystem is involved (in this case Smack).
>
> dccp_v6_rcv()
>         __sk_receive_skb()
>
>                 sk_filter_trim_cap()
>                         security_sock_rcv_skb()
>                                 smack_sock_rcv_skb()
>
> So, before putting this skb into the backlog queue,
>
> a network packet is checked against Smack rules. If Smack denies access,
>
> the packet is discarded.
>
> __sk_receive_skb()
> ...
>         if (sk_filter_trim_cap <https://elixir.bootlin.com/linux/latest/ident/sk_filter_trim_cap>(sk, skb, trim_cap))
>                 goto discard_and_relse; ...
>

Crash did not involve sk_filter_trim_cap() here...

Not sure what you are trying to say.


> > After packet is queued to backlog, the packet circulates, reaching the
> > smack_socket_sock_rcv_skb() point.
> >
> > The stack trace shows only the 2nd phase of the packet, when the user
> > process calls release_sock()
> >
> >
> >> 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)
> >