CVE-2026-99121

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

ipv4: avoid divide by zero in fib rebalance

fib rebalance() computes the total eligible nexthop weight in one pass and programs upper bounds in a second pass. A concurrent change to ignore routes with linkdown can make the first pass return zero while the second pass sees an eligible nexthop, resulting in division by zero.

If the first pass reports a zero total, set each nexthop upper bound to -1 and skip the division. This matches the IPv6 fix in commit d2c26c2911dd ("ipv6: avoid divide by zero in rt6 multipath rebalance") and preserves the lock-free rebalance path.