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:

<TASK>

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)

</TASK>

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