netdev - Re: [PATCH net v3] ip6_gre: use skb_vlan_inet_prepare() instead of pskb_inet_may

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Message-ID: <CANn89i+Akf5vcL2zzgWrfs7D=h2pov5DmKKckw+3XK9Sjav=yw@mail.gmail.com>
Date: Mon, 19 Jan 2026 12:38:19 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Florian Westphal <fw@...len.de>
Cc: netdev@...r.kernel.org, pabeni@...hat.com, 
	syzbot+6023ea32e206eef7920a@...kaller.appspotmail.com, kuba@...nel.org, 
	davem@...emloft.net, Mazin Al Haddad <mazin@...state.dev>
Subject: Re: [PATCH net v3] ip6_gre: use skb_vlan_inet_prepare() instead of pskb_inet_may_pull()

On Mon, Jan 19, 2026 at 12:25 PM Florian Westphal <fw@...len.de> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> I added skb_vlan_inet_prepare() helper in the cited commit, hinting
> that we would need to use it more broadly.
>
> syzbot confirmed this was the case in ip6_gre.
>
> uninit-value in ip6table_mangle_hook+0x97d/0x9c0 net/ipv6/netfilter/ip6table_mangle.c:72
>  ip6t_mangle_out net/ipv6/netfilter/ip6table_mangle.c:56 [inline]
>  ip6table_mangle_hook+0x97d/0x9c0 net/ipv6/netfilter/ip6table_mangle.c:72
>  nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline]
>  nf_hook_slow+0xf4/0x400 net/netfilter/core.c:626
>  nf_hook include/linux/netfilter.h:269 [inline]
>  __ip6_local_out+0x5ac/0x640 net/ipv6/output_core.c:143
>  ip6_local_out+0x4c/0x210 net/ipv6/output_core.c:153
>  ip6tunnel_xmit+0x129/0x460 include/net/ip6_tunnel.h:161
>  ip6_tnl_xmit+0x341a/0x3860 net/ipv6/ip6_tunnel.c:1281
>
> Uninit was stored to memory at:
>  ip6_tnl_xmit+0x34f7/0x3860 net/ipv6/ip6_tunnel.c:1277
>  __gre6_xmit+0x14b9/0x1550 net/ipv6/ip6_gre.c:815
>  ip6gre_xmit_ipv4 net/ipv6/ip6_gre.c:839 [inline]
>  ip6gre_tunnel_xmit+0x18f7/0x2030 net/ipv6/ip6_gre.c:922
>
> Uninit was created at:
>  slab_post_alloc_hook mm/slub.c:4091 [inline]
>  slab_alloc_node mm/slub.c:4134 [inline]
>  __do_kmalloc_node mm/slub.c:4263 [inline]
>  __kmalloc_node_track_caller_noprof+0x6c7/0xf90 mm/slub.c:4283
>  kmalloc_reserve+0x23e/0x4a0 net/core/skbuff.c:609
>  pskb_expand_head+0x226/0x1a60 net/core/skbuff.c:2275
>  skb_realloc_headroom+0x140/0x2b0 net/core/skbuff.c:2355
>  ip6_tnl_xmit+0x2106/0x3860 net/ipv6/ip6_tunnel.c:1227
>  __gre6_xmit+0x14b9/0x1550 net/ipv6/ip6_gre.c:815
>  ip6gre_xmit_ipv4 net/ipv6/ip6_gre.c:839 [inline]
>  ip6gre_tunnel_xmit+0x18f7/0x2030 net/ipv6/ip6_gre.c:922
>
> Fixes: d8a6213d70ac ("geneve: fix header validation in geneve[6]_xmit_skb")
> Reported-by: syzbot+6023ea32e206eef7920a@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=6023ea32e206eef7920a
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Mazin Al Haddad <mazin@...state.dev>
> Signed-off-by: Florian Westphal <fw@...len.de>
> ---
>  v3: pass 'true' argument to skb_vlan_inet_prepare to not change network
>  header offset.

Ah right, this escaped my radar, thanks Florian !