[PATCH net] ip6_gre: use skb_vlan_inet_prepare() instead of pskb_inet_may_pull()

[PATCH net] ip6_gre: use skb_vlan_inet_prepare() instead of pskb_inet_may_pull()

[Eric Dumazet]

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@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6023ea32e206eef7920a
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Mazin Al Haddad <mazin@getstate.dev>
---
 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..8b06ba058bde 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, false))
 		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, false))
 		goto tx_err;
 
 	if (!ip6_tnl_xmit_ctl(t, &t->parms.laddr, &t->parms.raddr))
-- 
2.52.0.351.gbe84eed79e-goog

Re: [PATCH net] ip6_gre: use skb_vlan_inet_prepare() instead of pskb_inet_may_pull()

[Jakub Kicinski]

On Mon,  5 Jan 2026 10:03:30 +0000 Eric Dumazet wrote:
> I added skb_vlan_inet_prepare() helper in the cited commit, hinting
> that we would need to use it more broadly.

This appears to break all GRE tests, unfortunately.
Conditions need to be inverted?
-- 
pw-bot: cr

Re: [PATCH net] ip6_gre: use skb_vlan_inet_prepare() instead of pskb_inet_may_pull()

[Eric Dumazet]

On Mon, Jan 5, 2026 at 9:40 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Mon,  5 Jan 2026 10:03:30 +0000 Eric Dumazet wrote:
> > I added skb_vlan_inet_prepare() helper in the cited commit, hinting
> > that we would need to use it more broadly.
>
> This appears to break all GRE tests, unfortunately.
> Conditions need to be inverted?

Ah sorry, I will send a fixed V2 tomorrow.