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

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Message-ID: <aVuD4kwMyyJlSxIV@strlen.de>
Date: Mon, 5 Jan 2026 10:26:58 +0100
From: Florian Westphal <fw@...len.de>
To: Eric Dumazet <edumazet@...gle.com>
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

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!