netdev - Re: [PATCH] ip6_tunnel: Fix uninit-value in ip6_tnl

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Message-ID: <CANn89iJfAq5f3++3_yYpTtL0LPXXF4b1LOFH=6ZGwA5k7S5v6w@mail.gmail.com>
Date: Mon, 5 Jan 2026 10:35:06 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Florian Westphal <fw@...len.de>
Cc: Mazin Al Haddad <mazin@...state.dev>, davem@...emloft.net, dsahern@...nel.org, 
	kuba@...nel.org, pabeni@...hat.com, horms@...nel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 
	syzbot+6023ea32e206eef7920a@...kaller.appspotmail.com
Subject: Re: [PATCH] ip6_tunnel: Fix uninit-value in ip6_tnl_xmit

On Mon, Jan 5, 2026 at 10:26 AM Florian Westphal <fw@...len.de> wrote:
>
> Eric Dumazet <edumazet@...gle.com> wrote:
>
> Hi Eric
>
> > > When taking the branch with skb_realloc_headroom, pskb_expand_head is
> > > called, as such, pointers referencing content within the new skb's header
> > > are invalid. Currently, the assignment of hop_limit accesses the now
> > > invalid pointer in the network header of this "new" skb. Fix this by
> > > moving the logic to assign hop_limit earlier so that the assignment
> > > references the original un-resized skb instead.
> >
> > Unfortunately this is not fixing anything.
> >
> > If the IPv6 header was in the skb head before skb_realloc_headroom()
> > and/or pskb_expand_head(),
> > it would be copied in the new skb head.
> >
> > Note how the repro is sending a packet with vlan tag (88A8 : ETH_P_8021AD)
> >
> > endto$packet(r0, &(0x7f0000000180)="a6bea8a120e5f8320c30ce5088a8",
> > 0x12, 0x0, &(0x7f0000000140)={0x11, 0x0, r3, 0x1, 0x0, 0x6, @local},
> > 0x14)
> >
> > Current code, using pskb_inet_may_pull() is not ready yet.
> >
> > My patch has been tested by syzbot and I was about to submit it.
> >
> > diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
> > index 235808cfec705032b545d6f396f8e58f4693e8d8..c4f0383a136cf5f5e6846293078ec8b826c754c9
> > 100644
> > --- a/net/ipv6/ip6_gre.c
> > +++ b/net/ipv6/ip6_gre.c
> > @@ -910,7 +910,7 @@ static netdev_tx_t ip6gre_tunnel_xmit(struct sk_buff *skb,
> >         __be16 payload_protocol;
> >         int ret;
> >
> > -       if (!pskb_inet_may_pull(skb))
> > +       if (!skb_vlan_inet_prepare(skb, false))
> >                 goto tx_err;
> >
> >         if (!ip6_tnl_xmit_ctl(t, &t->parms.laddr, &t->parms.raddr))
> > @@ -958,7 +958,7 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct
> > sk_buff *skb,
> >         __u32 mtu;
> >         int nhoff;
> >
> > -       if (!pskb_inet_may_pull(skb))
> > +       if (!skb_vlan_inet_prepare(skb, false))
> >                 goto tx_err;
> >
> >         if (!ip6_tnl_xmit_ctl(t, &t->parms.laddr, &t->parms.raddr))
>
> Could you please submit this fix?  There appears to be an open
> syzbot report (https://syzkaller.appspot.com/bug?extid=6023ea32e206eef7920a)
> that would be addressed by this.
>
> Thanks!

Yes, I am back to work and was looking at a similar syzbot issue.