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CVE-2026-45840 is a denial-of-service vulnerability in the Linux kernel's Open vSwitch implementation. The issue is in the vport netlink reply path: reply helpers allocate a fixed-size skb using nlmsg_new(NLMSG_DEFAULT_SIZE, ...) but then serialize the full upcall PID array via ovs_vport_get_upcall_portids(). The setter ovs_vport_set_upcall_portids() accepted any non-zero multiple of sizeof(u32) without an upper bound, allowing an attacker to configure an oversized PID array. When the kernel later attempts to include that array in a vport netlink reply, nla_put() can fail with -EMSGSIZE because the reply buffer is too small, and the code then hits BUG_ON(err < 0), causing a kernel BUG and panic. The fix rejects arrays larger than nr_cpu_ids and pre-computes a sufficiently large worst-case reply size in ovs_vport_cmd_msg_size().
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