CVE-2026-31531

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

ipv4: nexthop: allocate skb dynamically in rtm_get_nexthop()

When querying a nexthop object via RTM_GETNEXTHOP, the kernel currently allocates a fixed-size skb using NLMSG_GOODSIZE. While sufficient for single nexthops and small Equal-Cost Multi-Path groups, this fixed allocation fails for large nexthop groups like 512 nexthops.

This results in the following warning splat:

WARNING: net/ipv4/nexthop.c:3395 at rtm_get_nexthop+0x176/0x1c0, CPU#20: rep/4608 […] RIP: 0010:rtm_get_nexthop (net/ipv4/nexthop.c:3395) […] Call Trace: rtnetlink_rcv_msg (net/core/rtnetlink.c:6989) netlink_rcv_skb (net/netlink/af_netlink.c:2550) netlink_unicast (net/netlink/af_netlink.c:1319 net/netlink/af_netlink.c:1344) netlink_sendmsg (net/netlink/af_netlink.c:1894) ____sys_sendmsg (net/socket.c:721 net/socket.c:736 net/socket.c:2585) ___sys_sendmsg (net/socket.c:2641) __sys_sendmsg (net/socket.c:2671) do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

Fix this by allocating the size dynamically using nh_nlmsg_size() and using nlmsg_new(), this is consistent with nexthop_notify() behavior. In addition, adjust nh_nlmsg_size_grp() so it calculates the size needed based on flags passed. While at it, also add the size of NHA_FDB for nexthop group size calculation as it was missing too.

This cannot be reproduced via iproute2 as the group size is currently limited and the command fails as follows:

addattr_l ERROR: message exceeded bound of 1048

Affected Software

References

• https://git.kernel.org/stable/c/14cf0cd35361f4e94824bf8a42f72713d7702a73
• https://git.kernel.org/stable/c/40bd39e383a0478fd5c221f393df05fd9d70cfbc
• https://git.kernel.org/stable/c/615517f3f8d53b0cf41507c7599971e17adfdfa5
• https://git.kernel.org/stable/c/635038fe19db391117e66b46bdc2b6e447ac801d