From 6619e01855e65ef35723685a557e3eb106f4ecf5 Mon Sep 17 00:00:00 2001 From: Daehyeon Ko <4ncienth@gmail.com> Date: Fri, 2 Oct 2026 13:50:58 +0900 Subject: [PATCH 1/3] ipv4: stop PMTU walk when nexthop group shrinks Commit 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths if multipath enabled") made __ip_rt_update_pmtu() update every path. For nexthop objects, fib_info_num_path() and fib_info_nhc() independently load nh->nh_grp. RCU protects each group's lifetime but does not make the loads observe the same group. If replacement shrinks the group after the loop accepts an index, fib_info_nhc() returns NULL and update_or_create_fnhe() dereferences it. A deterministic probe only widened the existing window between the real operations. On v7.2, an RTM_NEWNEXTHOP replacement published a one-member group after the PMTU reader accepted index 1 from a two-member group, producing: KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] RIP: update_or_create_fnhe+0x45/0x15b0 Stop when the indexed path is absent. Groups are dense, so no later index exists in that snapshot. The same real-writer window completed 114,423 PMTU iterations without an oops with this check. A reproducer is available on request. Replacement requires CAP_NET_ADMIN in the network namespace. Where unprivileged user namespaces are permitted, a local user can obtain it in a new user and network namespace. Fixes: 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths if multipath enabled") Cc: stable@vger.kernel.org Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> Reviewed-by: Ido Schimmel Link: https://patch.msgid.link/4fb87ed994dfc067178fc845cdd8bc9b5446b039.1790915964.git.4ncienth@gmail.com Signed-off-by: Jakub Kicinski --- net/ipv4/route.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/net/ipv4/route.c b/net/ipv4/route.c index 37674d76f90f..5f2197874beb 100644 --- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -1082,6 +1082,8 @@ static void __ip_rt_update_pmtu(struct rtable *rt, struct flowi4 *fl4, u32 mtu) for (nhsel = 0; nhsel < fib_info_num_path(res.fi); nhsel++) { nhc = fib_info_nhc(res.fi, nhsel); + if (!nhc) + break; update_or_create_fnhe(nhc, fl4->daddr, 0, mtu, lock, jiffies + net->ipv4.ip_rt_mtu_expires); } From 4f4afc07dcb2edd4f9afa36595373f8cdb9b58c2 Mon Sep 17 00:00:00 2001 From: Daehyeon Ko <4ncienth@gmail.com> Date: Fri, 2 Oct 2026 13:50:59 +0900 Subject: [PATCH 2/3] ipv4: stop exception dump when nexthop group shrinks 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 Link: https://lore.kernel.org/netdev/20261001170513.GA1657889@shredder/ Cc: stable@vger.kernel.org Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> Reviewed-by: Ido Schimmel Link: https://patch.msgid.link/5bb7bd97411eb9db454464bf449107395e760f26.1790915964.git.4ncienth@gmail.com Signed-off-by: Jakub Kicinski --- net/ipv4/route.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/net/ipv4/route.c b/net/ipv4/route.c index 5f2197874beb..268d8821f7e0 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; From d3daa33c7af92e38f2274bbd2576f58bfbaf89e6 Mon Sep 17 00:00:00 2001 From: Daehyeon Ko <4ncienth@gmail.com> Date: Fri, 2 Oct 2026 13:51:00 +0900 Subject: [PATCH 3/3] ipv4: stop route notification sizing when nexthop group shrinks 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 Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> Reviewed-by: Ido Schimmel Link: https://patch.msgid.link/1054fa46165bba1b473782fa41367804aa908e86.1790915964.git.4ncienth@gmail.com Signed-off-by: Jakub Kicinski --- 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 5c9021ea3a79..47469b775808 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)