CVE-2026-80793 — ipv4: reject undersized MTUs in ip_do_fragment()

Affected: Linux

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.

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CVE-2026-80793 — ipv4: reject undersized MTUs in ip_do_fragment()