[PATCH net v6 1/1] net: gso: limit recursive IP-in-IP segmentation

[Zihan Xi <zihanx@nebusec.ai>]

IP-in-IP GSO can re-enter inet_gso_segment() or ipv6_gso_segment()
for each nested IP header. encap_level tracks header bytes, not callback
depth, so a deep chain can exhaust the kernel stack. Making
inet_gso_segment() stackable introduced unbounded IPv4 nesting; IPIP
GSO/TSO later made the path reachable. The IPv6 stackable path was
introduced separately and uses the same guard.

Count IPv4 and IPv6 GSO handler entries in skb_gso_cb, initialized once
per top-level GSO operation and preserved across GRE/UDP context changes.
Use the existing IP_TUNNEL_RECURSION_LIMIT for both handlers. The first
five entries pass, and the sixth returns -EINVAL before dispatching
another GSO callback.

Fixes: 3347c9602955 ("ipv4: gso: make inet_gso_segment() stackable")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Closes: https://lore.kernel.org/all/cover.1790157745.git.zihanx@nebusec.ai/
Assisted-by: LLM
Co-developed-by: Luxing Yin <root@tr0jan.top>
Signed-off-by: Luxing Yin <root@tr0jan.top>
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
---
changes in v6:
  - Use IP_TUNNEL_RECURSION_LIMIT directly for both IP GSO handler
    checks instead of defining a duplicate GSO_RECURSION_LIMIT.
  - Add Closes for the v5 cover letter.
  - v5 Link: https://lore.kernel.org/all/cover.1790157745.git.zihanx@nebusec.ai/
changes in v5:
  - Replace the cumulative header-offset check with a shared IP GSO
    recursion counter. Allow five handler entries and reject the sixth.
  - Initialize the counter for each top-level GSO operation and keep it
    across GRE/UDP context changes.
  - v4 Link: https://lore.kernel.org/all/cover.1790041241.git.zihanx@nebusec.ai/
changes in v4:
  - Keep the budget check only at the two IP GSO handler entries; remove
    the common callback wrapper and other GSO call-site checks.
  - Reuse skb_gso_cb->mac_offset and current skb headroom as the
    cumulative header-offset budget.
  - v3 Link: https://lore.kernel.org/all/cover.1789802623.git.zihanx@nebusec.ai/
changes in v3:
  - Treat an exhausted 256-byte budget as a callback-entry failure,
    including the zero-length check used by the common callback wrapper.
  - v2 Link: https://lore.kernel.org/all/cover.1789618203.git.zihanx@nebusec.ai/
changes in v2:
  - Replace the callback counter with a cumulative 256-byte header budget
    carried in skb_gso_cb.
  - Apply the budget at common callback entry and across IP, GRE, UDP,
    MPLS, NSH, ESP, and IPv6 extension dispatch, including context resets.
  - Rebase the UDP hunk and rerun both IPv4 PoCs; use decoded crash
    evidence from unpatched 88c17de85ddb.
  - v1 Link: https://lore.kernel.org/all/cover.1789302084.git.zihanx@nebusec.ai/
 include/net/gso.h      | 8 ++++++++
 net/core/gso.c         | 1 +
 net/ipv4/af_inet.c     | 3 +++
 net/ipv6/ip6_offload.c | 4 ++++
 4 files changed, 16 insertions(+)

diff --git a/include/net/gso.h b/include/net/gso.h
index 29975440cad51..0749230d414ec 100644
--- a/include/net/gso.h
+++ b/include/net/gso.h
@@ -19,10 +19,18 @@ struct skb_gso_cb {
 	int	encap_level;
 	__wsum	csum;
 	__u16	csum_start;
+	/* Number of IPv4/IPv6 GSO handler entries for this packet. */
+	u8	recursion_counter;
 };
 #define SKB_GSO_CB_OFFSET	32
 #define SKB_GSO_CB(skb) ((struct skb_gso_cb *)((skb)->cb + SKB_GSO_CB_OFFSET))
 
+static inline bool gso_recursion_inc_test(struct sk_buff *skb,
+					  unsigned int limit)
+{
+	return ++SKB_GSO_CB(skb)->recursion_counter > limit;
+}
+
 static inline int skb_tnl_header_len(const struct sk_buff *inner_skb)
 {
 	return (skb_mac_header(inner_skb) - inner_skb->head) -
diff --git a/net/core/gso.c b/net/core/gso.c
index bcd156372f4df..e96ef63500648 100644
--- a/net/core/gso.c
+++ b/net/core/gso.c
@@ -117,6 +117,7 @@ struct sk_buff *__skb_gso_segment(struct sk_buff *skb,
 
 	SKB_GSO_CB(skb)->mac_offset = skb_headroom(skb);
 	SKB_GSO_CB(skb)->encap_level = 0;
+	SKB_GSO_CB(skb)->recursion_counter = 0;
 
 	skb_reset_mac_header(skb);
 	skb_reset_mac_len(skb);
diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 32d006c1a8eed..4ce38c99fef9e 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -1375,6 +1375,9 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
 	int id;
 
 	skb_reset_network_header(skb);
+	if (unlikely(gso_recursion_inc_test(skb,
+					   IP_TUNNEL_RECURSION_LIMIT)))
+		goto out;
 	nhoff = skb_network_header(skb) - skb_mac_header(skb);
 	if (unlikely(!pskb_may_pull(skb, sizeof(*iph))))
 		goto out;
diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
index 78f50c93c536c..700f1861d8eb7 100644
--- a/net/ipv6/ip6_offload.c
+++ b/net/ipv6/ip6_offload.c
@@ -17,6 +17,7 @@
 #include <net/udp.h>
 #include <net/gro.h>
 #include <net/gso.h>
+#include <net/ip_tunnels.h>
 
 #include "ip6_offload.h"
 #include "tcpv6_offload.c"
@@ -104,6 +105,9 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
 	bool gso_partial;
 
 	skb_reset_network_header(skb);
+	if (unlikely(gso_recursion_inc_test(skb,
+					   IP_TUNNEL_RECURSION_LIMIT)))
+		goto out;
 	nhoff = skb_network_header(skb) - skb_mac_header(skb);
 	if (unlikely(!pskb_may_pull(skb, sizeof(*ipv6h))))
 		goto out;
-- 
2.43.0