CVE-2026-89790

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

ipv6: avoid divide by zero in rt6_multipath_rebalance

rt6_multipath_rebalance() calculates the total eligible nexthop weight

in one pass and programs upper bounds in a second pass. Since

RTM_NEWROUTE is RTNL-free, a concurrent

ignore_routes_with_linkdown update can make the first pass return zero

while the second sees an eligible nexthop, causing

rt6_upper_bound_set() to divide by zero.

UBSAN: division-overflow in net/ipv6/route.c:4845:17

Oops: divide error: 0000 [#1] SMP KASAN NOPTI

rt6_upper_bound_set() net/ipv6/route.c:4845

rt6_multipath_rebalance()

fib6_add_rt2node()

ip6_route_multipath_add()

inet6_rtm_newroute()

Skip upper-bound calculation when the first pass reports a zero total.

This respects the lock-free performance considerations here and solves

insecure scenarios.