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Tue, 27 Oct 2026 03:47:28 -0700 (PDT) Received: from localhost.localdomain ([14.116.239.36]) by smtp.gmail.com with ESMTPSA id 586e51a60fabf-4a842e89b77sm6597299fac.2.2026.10.27.03.47.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Oct 2026 03:47:26 -0700 (PDT) From: Henry Martin To: Ido Schimmel , Ilya Maximets , "David S . Miller" , Jakub Kicinski , Eric Dumazet , Paolo Abeni , Andrew Lunn , Simon Horman , Kuniyuki Iwashima , Petr Machata , Ferruh Yigit , Antoine Tenart , Seungwon Bae Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Henry Martin , stable@vger.kernel.org Subject: [PATCH net v4 1/2] vxlan: reject adding a remote to a nexthop fdb entry Date: Tue, 27 Oct 2026 18:47:12 +0800 Message-ID: <20261027104713.328028-1-bsdhenrymartin@gmail.com> X-Mailer: git-send-email 2.43.7 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit FDB entries backed by a nexthop group must not carry a remote destination list. vxlan_fdb_update_existing() currently appends a remote via vxlan_fdb_append() on an NLM_F_APPEND request with a multicast or zero MAC even when the entry is backed by a nexthop, breaking that invariant through three different callers: - "bridge fdb append" with a dst on a nexthop entry; - "ip link set dev vx0 type vxlan remote " (vxlan_changelink()); - "bridge vni add" on a vnifilter device (vxlan_update_default_fdb_entry()); this includes a plain "bridge vni add vni N" without a group, which falls back to the device's default remote, and a unicast group. All three reach the append block with nhid == 0 and a PERMANENT state, so they pass the earlier nexthop checks. There is no crash and no leak: the appended remote is freed with the entry, and the transmit path ignores it (vxlan_xmit() follows the nexthop whenever f->nh is set). But the stray remote changes deletion semantics: __vxlan_fdb_delete() destroys the whole fdb entry when the matched remote is the only one on the list, so removing it via "bridge fdb del ... dst " or "ip link set ... remote 0.0.0.0" tears down the nexthop entry, not just the stray remote. The entry state is also misreported: vxlan_fdb_notify(), RTM_GETNEIGH dumps and the switchdev FDB programming all prefer the remote list over the nexthop, so user space sees the dead remote while the datapath follows the nexthop group. Reject the combination at the top of vxlan_fdb_update_existing(), next to the existing nexthop guards, so a refused request has no side effects. Also move the non-unicast NLM_F_REPLACE rejection to the same place; it previously returned -EOPNOTSUPP only after the entry state and flags had been rewritten, skipping both the notification pass and the err_notify rollback. Note the resulting behavior change for the rtnetlink callers: operations that previously succeeded while silently attaching a remote to a nexthop entry now fail with extack "Cannot add a remote to a nexthop fdb entry". Add test cases to fib_nexthops.sh in the same fashion as the sibling fix, commit 98fc57d16744 ("vxlan: reject dynamic fdb entries that reference a nexthop id"). This issue was discovered by Tencent CodeBuddy Security. Cc: stable@vger.kernel.org Fixes: 1274e1cc4226 ("vxlan: ecmp support for mac fdb entries") Signed-off-by: Henry Martin --- v4: Also move the non-unicast REPLACE rejection before the entry is modified (the same half-update class, pre-existing), describe the stray remote's effect on __vxlan_fdb_delete() instead of calling it "unused", and note that a plain "bridge vni add" or a unicast group hits the check too (sashiko review of v2). v3: Describe the actual consequence and add fib_nexthops.sh test cases as requested by Ilya Maximets. v2: Move the check before the entry state/flags update, next to the other nexthop guards (reported by sashiko, confirmed by Jakub). drivers/net/vxlan/vxlan_core.c | 26 +++++++++++++-------------- tools/testing/selftests/net/fib_nexthops.sh | 18 ++++++++++++++++++ 2 files changed, 40 insertions(+), 4 deletions(-) diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c index 3390e341d1e58..13143085065f6 100644 --- a/drivers/net/vxlan/vxlan_core.c +++ b/drivers/net/vxlan/vxlan_core.c @@ -1002,6 +1016,22 @@ static int vxlan_fdb_update_existing(struct vxlan_dev *vxlan, return -EOPNOTSUPP; } + if (rcu_access_pointer(f->nh) && + (flags & NLM_F_APPEND) && + (is_multicast_ether_addr(f->key.eth_addr) || + is_zero_ether_addr(f->key.eth_addr))) { + NL_SET_ERR_MSG(extack, + "Cannot add a remote to a nexthop fdb entry"); + return -EOPNOTSUPP; + } + + if ((flags & NLM_F_REPLACE) && + (is_multicast_ether_addr(f->key.eth_addr) || + is_zero_ether_addr(f->key.eth_addr))) { + NL_SET_ERR_MSG(extack, "Cannot replace non-unicast fdb entries"); + return -EOPNOTSUPP; + } + /* Do not allow an externally learned entry to take over an entry added * by the user. */ @@ -1018,22 +1048,18 @@ static int vxlan_fdb_update_existing(struct vxlan_dev *vxlan, } if ((flags & NLM_F_REPLACE)) { - /* Only change unicasts */ - if (!(is_multicast_ether_addr(f->key.eth_addr) || - is_zero_ether_addr(f->key.eth_addr))) { - if (nhid) { - rc = vxlan_fdb_nh_update(vxlan, f, nhid, extack); - if (rc < 0) - return rc; - } else { - rc = vxlan_fdb_replace(f, ip, port, vni, - ifindex, &oldrd); - } - notify |= rc; + /* Only change unicasts; non-unicast replace is rejected + * before any state is modified. + */ + if (nhid) { + rc = vxlan_fdb_nh_update(vxlan, f, nhid, extack); + if (rc < 0) + return rc; } else { - NL_SET_ERR_MSG(extack, "Cannot replace non-unicast fdb entries"); - return -EOPNOTSUPP; + rc = vxlan_fdb_replace(f, ip, port, vni, + ifindex, &oldrd); } + notify |= rc; } if ((flags & NLM_F_APPEND) && (is_multicast_ether_addr(f->key.eth_addr) || diff --git a/tools/testing/selftests/net/fib_nexthops.sh b/tools/testing/selftests/net/fib_nexthops.sh index 431d7bed76224..bdc23205bde4c 100755 --- a/tools/testing/selftests/net/fib_nexthops.sh +++ b/tools/testing/selftests/net/fib_nexthops.sh @@ -547,6 +547,15 @@ ipv6_fdb_grp_fcnal() run_cmd "$BRIDGE fdb append 02:02:00:00:00:17 dev vx10 dst 2001:db8:91::11 self dynamic" log_test $? 255 "Fdb mac append with nexthop group and dynamic state" + # fdb entries with a nexthop group cannot take a remote append + run_cmd "$BRIDGE fdb add 00:00:00:00:00:00 dev vx10 nhid 102 self static" + run_cmd "$BRIDGE fdb append 00:00:00:00:00:00 dev vx10 dst 2001:db8:91::11 self" + log_test $? 255 "Fdb append with nexthop group" + + # fdb entries with a nexthop group cannot be replaced with a non-unicast MAC + run_cmd "$BRIDGE fdb replace 00:00:00:00:00:00 dev vx10 dst 2001:db8:91::11 self permanent" + log_test $? 255 "Fdb replace with nexthop group" + run_cmd "$IP -6 ro add 2001:db8:101::1/128 nhid 66" log_test $? 2 "Route add with fdb nexthop" @@ -697,6 +706,15 @@ ipv4_fdb_grp_fcnal() run_cmd "$BRIDGE fdb append 02:02:00:00:00:17 dev vx10 dst 10.0.0.3 self dynamic" log_test $? 255 "Fdb mac append with nexthop group and dynamic state" + # fdb entries with a nexthop group cannot take a remote append + run_cmd "$BRIDGE fdb add 00:00:00:00:00:00 dev vx10 nhid 102 self static" + run_cmd "$BRIDGE fdb append 00:00:00:00:00:00 dev vx10 dst 10.0.0.3 self" + log_test $? 255 "Fdb append with nexthop group" + + # fdb entries with a nexthop group cannot be replaced with a non-unicast MAC + run_cmd "$BRIDGE fdb replace 00:00:00:00:00:00 dev vx10 dst 10.0.0.3 self permanent" + log_test $? 255 "Fdb replace with nexthop group" + run_cmd "$IP ro add 172.16.0.0/22 nhid 16" log_test $? 2 "Route add with fdb nexthop" -- 2.43.7