CVE-2026-74742veth: fix queue index used to wake the peer txq in veth_poll

Official description

In the Linux kernel, the following vulnerability has been resolved: veth: fix queue index used to wake the peer txq in veth_poll veth_poll() derives the index of the peer TX queue to wake from rq->xdp_rxq.queue_index. That field is only initialized by xdp_rxq_info_reg() in veth_enable_xdp_range(), which runs only when an XDP program is attached. On the plain GRO/NAPI path (veth_napi_enable_range()) xdp_rxq_info_reg() is never called, so queue_index stays 0 for every queue, as priv->rq is zero-allocated. So in a multi-queue setup with GRO enabled and no XDP program attached, every NAPI instance looks at the peer's TX queue 0. If veth_xmit() stops peer TX queue 1 because the ptr_ring is full (NETDEV_TX_BUSY), nothing ever wakes it again: the poller draining queue 1 wakes queue 0 instead. veth implements no ndo_tx_timeout, so the netdev watchdog does not kick in either, and the queue stays stopped indefinitely. Derive the index from the position of the rq within priv->rq instead, which is correct regardless of whether XDP was ever enabled. Scripts to reproduce the stall are available at https://github.com/netoptimizer/veth-backpressure-performance-testing

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/b662a1fb4f3a5ea19bac24eea8315b1d05be51e7
• https://git.kernel.org/stable/c/73f8dd22b1e533a99ecc3f9b5de6c6daccaecace
• https://git.kernel.org/stable/c/90bb11fb29d3c55a2c46dc7c386d096b286e7fcf
• https://git.kernel.org/stable/c/60db47f02bfa2aa688938aa199117ec4f8e31d23