CVE-2026-89790ipv6: avoid divide by zero in rt6_multipath_rebalance

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/f30cf8fd9872299c0c27f9916252ba2b9f422dce
• https://git.kernel.org/stable/c/f82b5dbb2fef65b52a62d5ffe05e0483c4385a83
• https://git.kernel.org/stable/c/d2c26c2911dd1a363c488add4fb63eb5f0f28f87