ipv4: fix a non-progressing fragmentation loop from undersized effective MTUs

From: Yong Wang <[email protected]>

Hi Linux kernel maintainers,

This series fixes a non-progressing IPv4 fragmentation loop in net/ipv4/ip_output.c. The bug is reachable by a non-root user with CAP_NET_ADMIN in a user-created user and network namespace.

---- details below ----

Bug details:

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.

Because the failure happens in the fragmentation path itself, this series fixes it in ip_do_fragment() by rejecting mtu < hlen + 8 with -EMSGSIZE, matching the existing IPv6 fragmentation check.

The bug is reachable through more than one userspace-controlled path. A user with CAP_NET_ADMIN in a user-created network namespace can trigger it either by installing an undersized locked route MTU or by forwarding through an egress device whose MTU is too small.

Tested in a 2 vCPU, 2 GB RAM x86 QEMU guest:

- the old kernel reproduces the softlockup both with "mtu lock 20" and with a dummy egress device whose MTU is 20
- the new kernel returns packet loss in the no-route-mtu-lock reproducer and the guest remains alive

Reproducer:

Run inside the guest as root:

bash poc.sh

The PoC sets up forwarding through an egress dummy device whose MTU is 20, without installing any route MTU lock, and injects one forwarded non-DF ping from a child netns.

We run the PoC in a 2 vCPU, 2 GB RAM x86 QEMU environment.

v2:
- move the fix from ip_metrics_convert() to ip_do_fragment()
- update the commit message and cover letter per reviewer and Sashiko feedback
- switch the cover-letter reproducer to the no-route-mtu-lock dummy-device case

v1: https://lore.kernel.org/netdev/[email protected]/

------BEGIN poc.sh------

#!/bin/bash

sysctl -w net.ipv4.ip_forward=1

ip link add name dummy1 up mtu 20 type dummy
ip address add 192.0.2.1/24 dev dummy1

ip link add veth0 type veth peer name veth1
ip addr add 198.51.100.1/24 dev veth0
ip link set veth0 up

ip netns add ns1
ip link set veth1 netns ns1
ip -n ns1 address add 198.51.100.2/24 dev veth1 ip -n ns1 link set veth1 up
ip -n ns1 route add default via 198.51.100.1 ip netns exec ns1 ping -M dont -s 1000 -c 1 192.0.2.2

------END poc.sh--------

Best regards,
Yong Wang

Yong Wang (1):
ipv4: reject undersized MTUs in ip_do_fragment()

 net/ipv4/ip_output.c | 4 ++++
 1 file changed, 4 insertions(+)

2.53.0