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Message-ID: <20250730131738.3385939-1-edumazet@google.com>
Date: Wed, 30 Jul 2025 13:17:38 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Willem de Bruijn <willemb@...gle.com>,
David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org, eric.dumazet@...il.com,
Eric Dumazet <edumazet@...gle.com>, syzbot+af43e647fd835acc02df@...kaller.appspotmail.com
Subject: [PATCH net] ipv6: reject malicious packets in ipv6_gso_segment()
syzbot was able to craft a packet with very long IPv6 extension headers
leading to an overflow of skb->transport_header.
This 16bit field has a limited range.
Add skb_reset_transport_header_careful() helper and use it
from ipv6_gso_segment()
WARNING: CPU: 0 PID: 5871 at ./include/linux/skbuff.h:3032 skb_reset_transport_header include/linux/skbuff.h:3032 [inline]
WARNING: CPU: 0 PID: 5871 at ./include/linux/skbuff.h:3032 ipv6_gso_segment+0x15e2/0x21e0 net/ipv6/ip6_offload.c:151
Modules linked in:
CPU: 0 UID: 0 PID: 5871 Comm: syz-executor211 Not tainted 6.16.0-rc6-syzkaller-g7abc678e3084 #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
RIP: 0010:skb_reset_transport_header include/linux/skbuff.h:3032 [inline]
RIP: 0010:ipv6_gso_segment+0x15e2/0x21e0 net/ipv6/ip6_offload.c:151
Call Trace:
<TASK>
skb_mac_gso_segment+0x31c/0x640 net/core/gso.c:53
nsh_gso_segment+0x54a/0xe10 net/nsh/nsh.c:110
skb_mac_gso_segment+0x31c/0x640 net/core/gso.c:53
__skb_gso_segment+0x342/0x510 net/core/gso.c:124
skb_gso_segment include/net/gso.h:83 [inline]
validate_xmit_skb+0x857/0x11b0 net/core/dev.c:3950
validate_xmit_skb_list+0x84/0x120 net/core/dev.c:4000
sch_direct_xmit+0xd3/0x4b0 net/sched/sch_generic.c:329
__dev_xmit_skb net/core/dev.c:4102 [inline]
__dev_queue_xmit+0x17b6/0x3a70 net/core/dev.c:4679
Fixes: d1da932ed4ec ("ipv6: Separate ipv6 offload support")
Reported-by: syzbot+af43e647fd835acc02df@...kaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/688a1a05.050a0220.5d226.0008.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
include/linux/skbuff.h | 23 +++++++++++++++++++++++
net/ipv6/ip6_offload.c | 4 +++-
2 files changed, 26 insertions(+), 1 deletion(-)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index b8b06e71b73ea3fb5210239f585f4ba714395fd7..14b923ddb6dfcc3c031a7c9e4cb5fc4171424616 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3033,6 +3033,29 @@ static inline void skb_reset_transport_header(struct sk_buff *skb)
skb->transport_header = offset;
}
+/**
+ * skb_reset_transport_header_careful - conditionally reset transport header
+ * @skb: buffer to alter
+ *
+ * Hardened version of skb_reset_transport_header().
+ *
+ * Returns: true if the operation was a success.
+ */
+static inline bool __must_check
+skb_reset_transport_header_careful(struct sk_buff *skb)
+{
+ long offset = skb->data - skb->head;
+
+ if (unlikely(offset != (typeof(skb->transport_header))offset))
+ return false;
+
+ if (unlikely(offset == (typeof(skb->transport_header))~0U))
+ return false;
+
+ skb->transport_header = offset;
+ return true;
+}
+
static inline void skb_set_transport_header(struct sk_buff *skb,
const int offset)
{
diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
index 9822163428b028d6dbf3f48abe4674bd6c581725..fce91183797a60fcbf271c73e086aeb0aa9d40c6 100644
--- a/net/ipv6/ip6_offload.c
+++ b/net/ipv6/ip6_offload.c
@@ -148,7 +148,9 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
ops = rcu_dereference(inet6_offloads[proto]);
if (likely(ops && ops->callbacks.gso_segment)) {
- skb_reset_transport_header(skb);
+ if (!skb_reset_transport_header_careful(skb))
+ goto out;
+
segs = ops->callbacks.gso_segment(skb, features);
if (!segs)
skb->network_header = skb_mac_header(skb) + nhoff - skb->head;
--
2.50.1.552.g942d659e1b-goog
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