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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Vladimir Vdovin , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Daehyeon Ko <4ncienth@gmail.com> Subject: [PATCH net] ipv4: stop PMTU walk when nexthop group shrinks Date: Thu, 1 Oct 2026 10:05:50 +0900 Message-ID: <20261001010550.2742297-1-4ncienth@gmail.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 Assisted-by: LLM Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> --- net/ipv4/route.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/net/ipv4/route.c b/net/ipv4/route.c index 37674d76f90f0..5f2197874bebc 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); } base-commit: 4f1da630d13de0370e2f6361f961f1c8b384af1c -- 2.55.0