CVE-2026-80793

In the Linux kernel, the following vulnerability has been resolved:

ipv4: reject undersized MTUs in ip do fragment()

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.