Re: [PATCH net] openvswitch: fix kernel panic from oversized vport upcall PID arrays

[David Laight <david.laight.linux@gmail.com>]

On Fri, 10 Apr 2026 22:59:16 -0700
Weiming Shi <bestswngs@gmail.com> wrote:

> The vport netlink reply helpers allocate a fixed-size skb with
> nlmsg_new(NLMSG_DEFAULT_SIZE, ...) but serialize the full upcall PID
> array via ovs_vport_get_upcall_portids(). Since
> ovs_vport_set_upcall_portids() accepts any non-zero multiple of
> sizeof(u32) with no upper bound, a CAP_NET_ADMIN user can install a
> PID array large enough to overflow the reply buffer. When the
> subsequent nla_put() fails with -EMSGSIZE, five BUG_ON(err < 0) sites
> fire and panic the kernel. On systems with unprivileged user namespaces
> enabled (e.g., Ubuntu default), this is reachable via unshare -Urn.
> 
>  kernel BUG at net/openvswitch/datapath.c:2414!
>  Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
>  CPU: 1 UID: 0 PID: 65 Comm: poc Not tainted 7.0.0-rc7-00195-geb216e422044 #1
>  RIP: 0010:ovs_vport_cmd_set (net/openvswitch/datapath.c:2414 (discriminator 1))
>  Call Trace:
>   <TASK>
>   genl_family_rcv_msg_doit (net/netlink/genetlink.c:1116)
>   genl_rcv_msg (net/netlink/genetlink.c:1194 net/netlink/genetlink.c:1209)
>   netlink_rcv_skb (net/netlink/af_netlink.c:2550)
>   genl_rcv (net/netlink/genetlink.c:1219)
>   netlink_unicast (net/netlink/af_netlink.c:1319 net/netlink/af_netlink.c:1344)
>   netlink_sendmsg (net/netlink/af_netlink.c:1894)
>   __sys_sendto (net/socket.c:2206 (discriminator 1))
>   __x64_sys_sendto (net/socket.c:2209)
>   do_syscall_64 (arch/x86/entry/syscall_64.c:63 (discriminator 1))
>   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
>   </TASK>
>  Kernel panic - not syncing: Fatal exception
> 
> Dynamically compute the reply skb size based on the vport's actual PID
> array length instead of using a fixed NLMSG_DEFAULT_SIZE, and replace
> the BUG_ON() calls with WARN_ON_ONCE() plus graceful error returns.

IIRC WARN_ON_ONCE() will still panic all the systems with panic-on_warn set.

	David