ipv4: avoid divide by zero in fib_rebalance

Overview

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.

Affected

• Linux >= 0e884c78ee19e902f300ed147083c28a0c6302f0 < 5dac5c7ba8f5f3fb4bf29f2a2c3cb552f5b83c8f
• Linux >= 0e884c78ee19e902f300ed147083c28a0c6302f0 < d5520e815c4b1427104ce1428ee23821f2aa824d
• Linux >= 0e884c78ee19e902f300ed147083c28a0c6302f0 < 5046d2880fec7d49ebed2fd2866747ee1d06ad71
• Linux 4.4

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.