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.

References

• https://nvd.nist.gov/vuln/detail/CVE-2026-80793
• https://git.kernel.org/stable/c/3556beb8ca86677af2aca5bfbed6f8e790fccc83
• https://git.kernel.org/stable/c/36e0741833bd823866f8cb9f112f37cea1a70b60
• https://git.kernel.org/stable/c/515b6816ba0c12d8415c88e5f41e6ec029e35cfa
• https://git.kernel.org/stable/c/74ce7389f8f562d39015f59a00ef7ad6acd37803
• https://git.kernel.org/stable/c/a716a64a4ba68cd46f2745fba2b1099fe8e0aa59
• https://git.kernel.org/stable/c/b0ea911453ce7210e8200a07a94d2458bd1e6430
• https://git.kernel.org/stable/c/c0726f0caf8c6b3208552949e17d23634a2f3129
• https://git.kernel.org/stable/c/c8a74adccaf028223054633b532593101dfcc581
• https://git.kernel.org/stable/c/d9d1a676b033acabf8e5645f730486d1f8204a3f