[PATCH net v2 3/3] ipv4: stop route notification sizing when nexthop group shrinks

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

Commit 680aea08e78c ("net: ipv4: Emit notification when fib
hardware flags are changed") added an RCU-only fib_nlmsg_size() call
for asynchronous hardware flag notifications.

fib_nlmsg_size() checks fib_info_num_path() before each iteration,
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 fib_nexthop_nlmsg_size() dereferences
it.

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

Fixes: 680aea08e78c ("net: ipv4: Emit notification when fib hardware flags are changed")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
 net/ipv4/fib_semantics.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 5c9021ea3a799..47469b7758082 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -542,6 +542,9 @@ size_t fib_nlmsg_size(struct fib_info *fi)
 			struct fib_nh_common *nhc = fib_info_nhc(fi, i);
 			size_t nhsize;
 
+			if (!nhc)
+				break;
+
 			nhsize = fib_nexthop_nlmsg_size(nhc, nhs != 1);
 
 			if (nhs != 1)
-- 
2.55.0