With IFF_NAPI_FRAGS, tun_napi_alloc_frags() gets its skb from
napi_get_frags(). If GRO merged the previous frame with
GRO_MERGED_FREE, that skb was recycled by napi_reuse_skb(), which does
not reset skb->protocol set by napi_frags_skb() for the previous frame.
On the IFF_TAP path, tun_get_user() calls tun_vnet_hdr_tnl_to_skb()
before eth_type_trans(), so the virtio header checks see the protocol
of the previous frame: the outer protocol check of UDP tunnel GSO
packets rejects a valid frame when the two ethertypes differ, and
GSO packets without VIRTIO_NET_HDR_F_NEEDS_CSUM are dissected with
the wrong protocol.
This needs GRO to hold packets across writes, for example with
napi_defer_hard_irqs and gro_flush_timeout set on the device.
Clear skb->protocol for IFF_TAP frames, as for skbs allocated by
tun_alloc_skb(). eth_type_trans() sets it right after.
Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/netdev/179386753589.488856.11455181721004759454@kernel.org/ (local)
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Sashiko has reviewed this patch and found no issues.
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Sashiko AI review · https://sashiko.dev/#/patchset/20261105115715.1466924-1-edumazet@kernel.org?part=3