CVE-2026-43254: ovpn: tcp - fix packet extraction from stream

In the Linux kernel, the following vulnerability has been resolved:

ovpn: tcp - fix packet extraction from stream

When processing TCP stream data in ovpntcprecv, we receive large cloned skbs from strprcv that may contain multiple coalesced packets. The current implementation has two bugs:

1. Header offset overflow: Using pskbpull with large offsets on coalesced skbs causes skb->data - skb->head to exceed the u16 storage of skb->networkheader. This causes skbresetnetworkheader to fail on the inner decapsulated packet, resulting in packet drops.

2. Unaligned protocol headers: Extracting packets from arbitrary positions within the coalesced TCP stream provides no alignment guarantees for the packet data causing performance penalties on architectures without efficient unaligned access. Additionally, openvpn's 2-byte length prefix on TCP packets causes the subsequent 4-byte opcode and packet ID fields to be inherently misaligned.

Fix both issues by allocating a new skb for each openvpn packet and using skbcopybits to extract only the packet content into the new buffer, skipping the 2-byte length prefix. Also, check the length before invoking the function that performs the allocation to avoid creating an invalid skb.

If the packet has to be forwarded to userspace the 2-byte prefix can be pushed to the head safely, without misalignment.

As a side effect, this approach also avoids the expensive linearization that pskbpull triggers on cloned skbs with page fragments. In testing, this resulted in TCP throughput improvements of up to 74%.