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

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Message-ID: <20260119112512.28196-1-fw@strlen.de>
Date: Mon, 19 Jan 2026 12:24:57 +0100
From: Florian Westphal <fw@...len.de>
To: <netdev@...r.kernel.org>
Cc: pabeni@...hat.com,
	syzbot+6023ea32e206eef7920a@...kaller.appspotmail.com,
	kuba@...nel.org,
	davem@...emloft.net,
	Eric Dumazet <edumazet@...gle.com>,
	Mazin Al Haddad <mazin@...state.dev>,
	Florian Westphal <fw@...len.de>
Subject: [PATCH net v3] ip6_gre: use skb_vlan_inet_prepare() instead of pskb_inet_may_pull()

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.

 net/ipv6/ip6_gre.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index d19d86ed4376..9e214c355e6c 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -881,7 +881,7 @@ 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, true))
 		goto tx_err;
 
 	if (!ip6_tnl_xmit_ctl(t, &t->parms.laddr, &t->parms.raddr))
@@ -929,7 +929,7 @@ 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, true))
 		goto tx_err;
 
 	if (!ip6_tnl_xmit_ctl(t, &t->parms.laddr, &t->parms.raddr))
-- 
2.52.0