From: Ido Schimmel idosch@nvidia.com
commit 6ead38147ebb813f08be6ea8ef547a0e4c09559a upstream.
VXLAN FDB entries can point to either a remote destination or an FDB nexthop group. The latter is usually used in EVPN deployments where learning is disabled.
However, when learning is enabled, an incoming packet might try to refresh an FDB entry that points to an FDB nexthop group and therefore does not have a remote. Such packets should be dropped, but they are only dropped after dereferencing the non-existent remote, resulting in a NPD [1] which can be reproduced using [2].
Fix by dropping such packets earlier. Remove the misleading comment from first_remote_rcu().
[1] BUG: kernel NULL pointer dereference, address: 0000000000000000 [...] CPU: 13 UID: 0 PID: 361 Comm: mausezahn Not tainted 6.17.0-rc1-virtme-g9f6b606b6b37 #1 PREEMPT(voluntary) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014 RIP: 0010:vxlan_snoop+0x98/0x1e0 [...] Call Trace: <TASK> vxlan_encap_bypass+0x209/0x240 encap_bypass_if_local+0xb1/0x100 vxlan_xmit_one+0x1375/0x17e0 vxlan_xmit+0x6b4/0x15f0 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 address add 192.0.2.2/32 dev lo
ip nexthop add id 1 via 192.0.2.3 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 12345 localbypass ip link add name vx1 up type vxlan id 10020 local 192.0.2.2 dstport 54321 learning
bridge fdb add 00:11:22:33:44:55 dev vx0 self static dst 192.0.2.2 port 54321 vni 10020 bridge fdb add 00:aa:bb:cc:dd:ee dev vx1 self static nhid 10
mausezahn vx0 -a 00:aa:bb:cc:dd:ee -b 00:11:22:33:44:55 -c 1 -q
Fixes: 1274e1cc4226 ("vxlan: ecmp support for mac fdb entries") Reported-by: Marlin Cremers mcremers@cloudbear.nl 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/20250901065035.159644-2-idosch@nvidia.com Signed-off-by: Jakub Kicinski kuba@kernel.org [ kovalev: bp to fix CVE-2025-39851 ] Signed-off-by: Vasiliy Kovalev kovalev@altlinux.org --- drivers/net/vxlan/vxlan_core.c | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-)
diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c index 5a7008136100..8872cb7a2dbb 100644 --- a/drivers/net/vxlan/vxlan_core.c +++ b/drivers/net/vxlan/vxlan_core.c @@ -174,9 +174,7 @@ static inline struct hlist_head *vs_head(struct net *net, __be16 port) return &vn->sock_list[hash_32(ntohs(port), PORT_HASH_BITS)]; }
-/* First remote destination for a forwarding entry. - * Guaranteed to be non-NULL because remotes are never deleted. - */ +/* First remote destination for a forwarding entry. */ static inline struct vxlan_rdst *first_remote_rcu(struct vxlan_fdb *fdb) { if (rcu_access_pointer(fdb->nh)) @@ -1507,6 +1505,10 @@ static bool vxlan_snoop(struct net_device *dev, if (likely(f)) { struct vxlan_rdst *rdst = first_remote_rcu(f);
+ /* Don't override an fdb with nexthop with a learnt entry */ + if (rcu_access_pointer(f->nh)) + return true; + if (likely(vxlan_addr_equal(&rdst->remote_ip, src_ip) && rdst->remote_ifindex == ifindex)) return false; @@ -1515,10 +1517,6 @@ static bool vxlan_snoop(struct net_device *dev, if (f->state & (NUD_PERMANENT | NUD_NOARP)) return true;
- /* Don't override an fdb with nexthop with a learnt entry */ - if (rcu_access_pointer(f->nh)) - return true; - if (net_ratelimit()) netdev_info(dev, "%pM migrated from %pIS to %pIS\n",