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.

Fixes: bd11ff421d36 ("ipv6: Get rid of RTNL for SIOCDELRT and RTM_DELROUTE.")
Reported-by: AutonomousCodeSecurity@microsoft.com
Reported-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
Reported-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260817013237.2797-1-blbllhy@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions