[PATCH 6.19 053/311] ipv6: icmp: clear skb2->cb[] in ip6_err_gen_icmpv6_unreach()

Greg Kroah-Hartman
2026-04-08T18:52:04Z

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit 86ab3e55673a7a49a841838776f1ab18d23a67b5 ]

Sashiko AI-review observed:

In ip6_err_gen_icmpv6_unreach(), the skb is an outer IPv4 ICMP error packet
where its cb contains an IPv4 inet_skb_parm. When skb is cloned into skb2
and passed to icmp6_send(), it uses IP6CB(skb2).

IP6CB interprets the IPv4 inet_skb_parm as an inet6_skb_parm. The cipso
offset in inet_skb_parm.opt directly overlaps with dsthao in inet6_skb_parm
at offset 18.

If an attacker sends a forged ICMPv4 error with a CIPSO IP option, dsthao
would be a non-zero offset. Inside icmp6_send(), mip6_addr_swap() is called
and uses ipv6_find_tlv(skb, opt->dsthao, IPV6_TLV_HAO).

This would scan the inner, attacker-controlled IPv6 packet starting at that
offset, potentially returning a fake TLV without checking if the remaining
packet length can hold the full 18-byte struct ipv6_destopt_hao.

Could mip6_addr_swap() then perform a 16-byte swap that extends past the end
of the packet data into skb_shared_info?

Should the cb array also be cleared in ip6_err_gen_icmpv6_unreach() and
ip6ip6_err() to prevent this?

This patch implements the first suggestion.

I am not sure if ip6ip6_err() needs to be changed.
A separate patch would be better anyway.

Fixes: ca15a078bd90 ("sit: generate icmpv6 error when receiving icmpv4 error")
Reported-by: Ido Schimmel <idosch@nvidia.com>
Closes: https://sashiko.dev/#/patchset/20260326155138.2429480-1-edumazet%40google.com
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Oskar Kjos <oskar.kjos@hotmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260326202608.2976021-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/ipv6/icmp.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
index 0f41ca6f3d83e..9844758252718 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -871,6 +871,9 @@ int ip6_err_gen_icmpv6_unreach(struct sk_buff *skb, int nhs, int type,
if (!skb2)
return 1;

+ /* Remove debris left by IPv4 stack. */
+ memset(IP6CB(skb2), 0, sizeof(*IP6CB(skb2)));
+
skb_dst_drop(skb2);
skb_pull(skb2, nhs);
skb_reset_network_header(skb2);
--
2.53.0