[PATCH net v2 2/3] ipv4: stop exception dump when nexthop group shrinks

[Daehyeon Ko <4ncienth@gmail.com>]

Commit 4ce5dc9316de ("inet: switch inet_dump_fib() to RCU
protection") allowed IPv4 FIB dumps to run concurrently with nexthop
replacement.

fib_dump_info_fnhe() uses fib_info_num_path() as the loop bound,
then fib_info_nhc() independently reloads nh->nh_grp. If
RTM_NEWNEXTHOP replaces a group with fewer paths between those loads,
fib_info_nhc() returns NULL and the dump dereferences it through
nhc_flags.

Stop the walk when the indexed path is absent. Nexthop groups are
dense, so the current snapshot has no later path.

Fixes: 4ce5dc9316de ("inet: switch inet_dump_fib() to RCU protection")
Reported-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/netdev/20261001170513.GA1657889@shredder/
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
 net/ipv4/route.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 5f2197874bebc..268d8821f7e09 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -3170,6 +3170,9 @@ int fib_dump_info_fnhe(struct sk_buff *skb, struct netlink_callback *cb,
 		struct fnhe_hash_bucket *bucket;
 		int err;
 
+		if (!nhc)
+			break;
+
 		if (nhc->nhc_flags & RTNH_F_DEAD)
 			continue;
 
-- 
2.55.0