Re: [PATCH net v4 1/2] tun: bound the NAPI receive queue

[sashiko-bot@kernel.org]

> With IFF_NAPI, tun_get_user() queues every written packet for NAPI on
> an unbounded list. When the poll runs asynchronously (threaded NAPI)
> and is slower than the writer, the queue grows until the system runs
> out of memory. An unprivileged user can do this from its own user and
> network namespaces, and the queued skbs are not charged to its memcg:
> in a test VM, a uid 1000 process limited to 128 MB caused a global OOM
> in ~6 seconds. The socket send buffer cannot bound this: tun sets it to
> INT_MAX, and the build_skb() and XDP batch paths do not check it.
> 
> Drop packets once more than netdev_max_backlog of them are queued, like
> enqueue_to_backlog() does, and count them as rx_dropped: before
> allocating and copying them where possible, and again under the queue
> lock before queueing. This covers write(), sendmsg() and the batches of
> XDP buffers vhost-net passes to tun_sendmsg(), which share the queue.
> Zerocopy buffers of packets dropped early are completed.
> [ ... ]
> by the poll), and the queued skbs are still not charged to the writer's
> memcg.
> 
> Fixes: 943170998b20 ("tun: enable NAPI for TUN/TAP driver")
> Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261103142232.1869185-1-antonio@mandelbit.com?part=1