[PATCH net v2 1/1] ipv4: reject undersized MTUs in ip_do_fragment()

From: Yong Wang <[email protected]>

ip_do_fragment() subtracts the IPv4 header length from the effective
MTU and passes the resulting payload MTU to ip_frag_next().

If the effective MTU is smaller than hlen + 8, ip_frag_next() rounds
the fragment payload length down to zero. The fragmentation state then
never makes forward progress: state->left, state->ptr and state->offset
stay unchanged while ip_do_fragment() keeps allocating and transmitting
header-only fragments until the softlockup detector fires.

This is reproducible with a route installed using "mtu lock 20", but it
is also reproducible without route MTU lock, for example by forwarding a
packet to a device whose MTU is 20.

Fix it in ip_do_fragment() by rejecting mtu < hlen + 8 with -EMSGSIZE,
matching the existing IPv6 fragmentation check.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: [email protected]
Reported-by: Vega <[email protected]>
Assisted-by: Codex:GPT-5.4
Signed-off-by: Yong Wang <[email protected]>
Signed-off-by: Ren Wei <[email protected]>
---
 net/ipv4/ip_output.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index e6dd1e5b8c32..74e095b6b7ca 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -790,6 +790,10 @@ int ip_do_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 	 */
 
 	hlen = iph->ihl * 4;
+	if (mtu < hlen + 8) {
+		err = -EMSGSIZE;
+		goto fail;
+	}
 	mtu = mtu - hlen;	/* Size of data space */
 	IPCB(skb)->flags |= IPSKB_FRAG_COMPLETE;
 	ll_rs = LL_RESERVED_SPACE(rt->dst.dev);
-- 
2.53.0