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ipv4: avoid divide by zero in fib_rebalance

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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
d2c26c2911 ("ipv6: avoid divide by zero in rt6_multipath_rebalance")
and preserves the lock-free rebalance path.

Fixes: 0e884c78ee ("ipv4: L3 hash-based multipath")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260827182514.4667-2-zihanx@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Zihan Xi 2026-08-27 18:25:14 +00:00 committed by Jakub Kicinski
parent 1b78070aae
commit 5046d2880f
1 changed files with 1 additions and 1 deletions
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2
net/ipv4/fib_semantics.c
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@ -874,7 +874,7 @@ static void fib_rebalance(struct fib_info *fi)
change_nexthops(fi) {
int upper_bound;
if (nexthop_nh->fib_nh_flags & RTNH_F_DEAD) {
if (!total || nexthop_nh->fib_nh_flags & RTNH_F_DEAD) {
upper_bound = -1;
} else if (ip_ignore_linkdown(nexthop_nh->fib_nh_dev) &&
nexthop_nh->fib_nh_flags & RTNH_F_LINKDOWN) {