From: Ido Schimmel idosch@nvidia.com

commit 1f5d2fd1ca04a23c18b1bde9a43ce2fa2ffa1bce upstream.

When the "proxy" option is enabled on a VXLAN device, the device will suppress ARP requests and IPv6 Neighbor Solicitation messages if it is able to reply on behalf of the remote host. That is, if a matching and valid neighbor entry is configured on the VXLAN device whose MAC address is not behind the "any" remote (0.0.0.0 / ::).

The code currently assumes that the FDB entry for the neighbor's MAC address points to a valid remote destination, but this is incorrect if the entry is associated with an FDB nexthop group. This can result in a NPD [1][3] which can be reproduced using [2][4].

Fix by checking that the remote destination exists before dereferencing it.

[1] BUG: kernel NULL pointer dereference, address: 0000000000000000 [...] CPU: 4 UID: 0 PID: 365 Comm: arping Not tainted 6.17.0-rc2-virtme-g2a89cb21162c #2 PREEMPT(voluntary) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014 RIP: 0010:vxlan_xmit+0xb58/0x15f0 [...] Call Trace: <TASK> dev_hard_start_xmit+0x5d/0x1c0 __dev_queue_xmit+0x246/0xfd0 packet_sendmsg+0x113a/0x1850 __sock_sendmsg+0x38/0x70 __sys_sendto+0x126/0x180 __x64_sys_sendto+0x24/0x30 do_syscall_64+0xa4/0x260 entry_SYSCALL_64_after_hwframe+0x4b/0x53

[2] #!/bin/bash

ip address add 192.0.2.1/32 dev lo

ip nexthop add id 1 via 192.0.2.2 fdb ip nexthop add id 10 group 1 fdb

ip link add name vx0 up type vxlan id 10010 local 192.0.2.1 dstport 4789 proxy

ip neigh add 192.0.2.3 lladdr 00:11:22:33:44:55 nud perm dev vx0

bridge fdb add 00:11:22:33:44:55 dev vx0 self static nhid 10

arping -b -c 1 -s 192.0.2.1 -I vx0 192.0.2.3

[3] BUG: kernel NULL pointer dereference, address: 0000000000000000 [...] CPU: 13 UID: 0 PID: 372 Comm: ndisc6 Not tainted 6.17.0-rc2-virtmne-g6ee90cb26014 #3 PREEMPT(voluntary) Hardware name: QEMU Standard PC (i440FX + PIIX, 1v996), BIOS 1.17.0-4.fc41 04/01/2x014 RIP: 0010:vxlan_xmit+0x803/0x1600 [...] Call Trace: <TASK> dev_hard_start_xmit+0x5d/0x1c0 __dev_queue_xmit+0x246/0xfd0 ip6_finish_output2+0x210/0x6c0 ip6_finish_output+0x1af/0x2b0 ip6_mr_output+0x92/0x3e0 ip6_send_skb+0x30/0x90 rawv6_sendmsg+0xe6e/0x12e0 __sock_sendmsg+0x38/0x70 __sys_sendto+0x126/0x180 __x64_sys_sendto+0x24/0x30 do_syscall_64+0xa4/0x260 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x7f383422ec77

[4] #!/bin/bash

ip address add 2001:db8:1::1/128 dev lo

ip nexthop add id 1 via 2001:db8:1::1 fdb ip nexthop add id 10 group 1 fdb

ip link add name vx0 up type vxlan id 10010 local 2001:db8:1::1 dstport 4789 proxy

ip neigh add 2001:db8:1::3 lladdr 00:11:22:33:44:55 nud perm dev vx0

bridge fdb add 00:11:22:33:44:55 dev vx0 self static nhid 10

ndisc6 -r 1 -s 2001:db8:1::1 -w 1 2001:db8:1::3 vx0

Fixes: 1274e1cc4226 ("vxlan: ecmp support for mac fdb entries") Reviewed-by: Petr Machata petrm@nvidia.com Signed-off-by: Ido Schimmel idosch@nvidia.com Reviewed-by: Nikolay Aleksandrov razor@blackwall.org Link: https://patch.msgid.link/20250901065035.159644-3-idosch@nvidia.com Signed-off-by: Jakub Kicinski kuba@kernel.org [ kovalev: bp to fix CVE-2025-39850 ] Signed-off-by: Vasiliy Kovalev kovalev@altlinux.org --- drivers/net/vxlan/vxlan_core.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c index 1b6b6acd3489..57606891e413 100644 --- a/drivers/net/vxlan/vxlan_core.c +++ b/drivers/net/vxlan/vxlan_core.c @@ -1883,6 +1883,7 @@ static int arp_reduce(struct net_device *dev, struct sk_buff *skb, __be32 vni) n = neigh_lookup(&arp_tbl, &tip, dev);

if (n) { + struct vxlan_rdst *rdst = NULL; struct vxlan_fdb *f; struct sk_buff *reply;

@@ -1892,7 +1893,9 @@ static int arp_reduce(struct net_device *dev, struct sk_buff *skb, __be32 vni) }

f = vxlan_find_mac(vxlan, n->ha, vni); - if (f && vxlan_addr_any(&(first_remote_rcu(f)->remote_ip))) { + if (f) + rdst = first_remote_rcu(f); + if (rdst && vxlan_addr_any(&rdst->remote_ip)) { /* bridge-local neighbor */ neigh_release(n); goto out; @@ -2047,6 +2050,7 @@ static int neigh_reduce(struct net_device *dev, struct sk_buff *skb, __be32 vni) n = neigh_lookup(ipv6_stub->nd_tbl, &msg->target, dev);

if (n) { + struct vxlan_rdst *rdst = NULL; struct vxlan_fdb *f; struct sk_buff *reply;

@@ -2056,7 +2060,9 @@ static int neigh_reduce(struct net_device *dev, struct sk_buff *skb, __be32 vni) }

f = vxlan_find_mac(vxlan, n->ha, vni); - if (f && vxlan_addr_any(&(first_remote_rcu(f)->remote_ip))) { + if (f) + rdst = first_remote_rcu(f); + if (rdst && vxlan_addr_any(&rdst->remote_ip)) { /* bridge-local neighbor */ neigh_release(n); goto out;