netdev - Re: [PATCH net 2/2] net/sched: sch_frag: fix stack OOB read while fragmenting IPv4 packets

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Date:   Tue, 20 Apr 2021 10:59:13 +0200
From:   Davide Caratti <dcaratti@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Jamal Hadi Salim <jhs@...atatu.com>, Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, wenxu <wenxu@...oud.cn>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: [PATCH net 2/2] net/sched: sch_frag: fix stack OOB read while
 fragmenting IPv4 packets

hello Cong, thanks for looking at this!

On Mon, 2021-04-19 at 11:46 -0700, Cong Wang wrote:
> On Mon, Apr 19, 2021 at 8:24 AM Davide Caratti <dcaratti@...hat.com> wrote:
> > diff --git a/net/sched/sch_frag.c b/net/sched/sch_frag.c
> > index e1e77d3fb6c0..8c06381391d6 100644
> > --- a/net/sched/sch_frag.c
> > +++ b/net/sched/sch_frag.c
> > @@ -90,16 +90,16 @@ static int sch_fragment(struct net *net, struct sk_buff *skb,
> >         }
> > 
> >         if (skb_protocol(skb, true) == htons(ETH_P_IP)) {
> > -               struct dst_entry sch_frag_dst;
> > +               struct rtable sch_frag_rt = { 0 };
> 
> Is setting these fields 0 sufficient here? Because normally a struct table
> is initialized by rt_dst_alloc() which sets several of them to non-zero,
> notably, rt->rt_type and rt->rt_uncached.
> 
> Similar for the IPv6 part, which is initialized by rt6_info_init().

for what we do now in openvswitch and sch_frag, that should be
sufficient: a similar thing is done by br_netfilter [1], apparently for
the same "refragmentation" purposes. On a fedora host (running 5.10, but
it shouldn't be much different than current Linux), I just dumped
'fake_rtable' from a bridge device:

# ip link add name test-br0 type bridge
# ip link set dev test-br0 up
# ip link add name test-port0 type dummy
# ip link set dev test-port0 master test-br0 up
# crash 
[...]
crash> net                                                             
   NET_DEVICE     NAME   IP ADDRESS(ES)
[...]
ffff89fb44ed8000  test-br0
ffff89fbfc45c000  test-port0
crash> p sizeof(struct net_device)
$12 = 3200
crash> p ((struct net_bridge*)(0xffff89fb44ed8000 + 3200))->fake_rtable
$13 = {
  dst = {
    dev = 0xffff89fb44ed8000, 
    ops = 0xffffffffc0afef40, 
    _metrics = 18446744072647256257, 
    expires = 0, 
    xfrm = 0x0, 
    input = 0x0, 
    output = 0x0, 
    flags = 18, <-- that should be DST_NOXFRM | DST_FAKE_RTABLE
    obsolete = 0, 
    header_len = 0, 
    trailer_len = 0, 
    __refcnt = {
      counter = 1
    }, 
    __use = 0, 
    lastuse = 0, 
    lwtstate = 0x0, 
    callback_head = {
      next = 0x0, 
      func = 0x0
    }, 
    error = 0, 
    __pad = 0, 
    tclassid = 0
  }, 
  rt_genid = 0, 
  rt_flags = 0, 
  rt_type = 0, 
  rt_is_input = 0 '\000', 
  rt_uses_gateway = 0 '\000', 
  rt_iif = 0, 
  rt_gw_family = 0 '\000', 
  {
    rt_gw4 = 0, 
    rt_gw6 = {
      in6_u = {
        u6_addr8 = "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000", 
        u6_addr16 = {0, 0, 0, 0, 0, 0, 0, 0}, 
        u6_addr32 = {0, 0, 0, 0}
      }
    }
  }, 
  rt_mtu_locked = 0, 
  rt_pmtu = 0, 
  rt_uncached = {
    next = 0x0, 
    prev = 0x0
  }, 
  rt_uncached_list = 0x0                                               
}                 

only fake_rtable.dst members are set to something, the remaining are all zero-ed.

-- 
davide

[1] https://elixir.bootlin.com/linux/latest/source/net/bridge/br_nf_core.c#L62