netdev - Re: [PATCH net] udp: fix a skb extensions leak

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Date:   Tue, 31 Mar 2020 00:13:34 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Steffen Klassert <steffen.klassert@...unet.com>
Cc:     network dev <netdev@...r.kernel.org>, davem <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Florian Westphal <fw@...len.de>
Subject: Re: [PATCH net] udp: fix a skb extensions leak

On Mon, Mar 30, 2020 at 4:29 PM Steffen Klassert
<steffen.klassert@...unet.com> wrote:
>
> On Thu, Mar 26, 2020 at 05:06:25PM +0800, Xin Long wrote:
> > On udp rx path udp_rcv_segment() may do segment where the frag skbs
> > will get the header copied from the head skb in skb_segment_list()
> > by calling __copy_skb_header(), which could overwrite the frag skbs'
> > extensions by __skb_ext_copy() and cause a leak.
> >
> > This issue was found after loading esp_offload where a sec path ext
> > is set in the skb.
> >
> > On udp tx gso path, it works well as the frag skbs' extensions are
> > not set. So this issue should be fixed on udp's rx path only and
> > release the frag skbs' extensions before going to do segment.
>
> Are you sure that this affects only the RX path? What if such
> a packet is forwarded? Also, I think TCP has the same problem.
You're right, just confirm it exists on the forwarded path.
__copy_skb_header() is also called by skb_segment(), but
I don't have tests to reproduce it on other protocols like TCP.

>
> >
> > Reported-by: Xiumei Mu <xmu@...hat.com>
> > Fixes: cf329aa42b66 ("udp: cope with UDP GRO packet misdirection")
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > ---
> >  include/net/udp.h | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/include/net/udp.h b/include/net/udp.h
> > index e55d5f7..7bf0ca5 100644
> > --- a/include/net/udp.h
> > +++ b/include/net/udp.h
> > @@ -486,6 +486,10 @@ static inline struct sk_buff *udp_rcv_segment(struct sock *sk,
> >       if (skb->pkt_type == PACKET_LOOPBACK)
> >               skb->ip_summed = CHECKSUM_PARTIAL;
> >
> > +     if (skb_has_frag_list(skb) && skb_has_extensions(skb))
> > +             skb_walk_frags(skb, segs)
> > +                     skb_ext_put(segs);
>
> If a skb in the fraglist has a secpath, it is still valid.
> So maybe instead of dropping it here and assign the one
> from the head skb, we could just keep the secpath. But
> I don't know about other extensions. I've CCed Florian,
> he might know a bit more about other extensions. Also,
> it might be good to check if the extensions of the GRO
> packets are all the same before merging.
>
Not sure if we can improve __copy_skb_header() or add
a new function to copy these members ONLY when nskb's
are not set.