Re: [PATCH 1/1] net: tap: set skb->dev before parsing virtio net header in tap_get_user_xdp()

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Dongli Zhang wrote:
> The commit 4f61f133f354 ("net: tap: NULL pointer derefence in
> dev_parse_header_protocol when skb->dev is null") fixed a crash in
> tap_get_user() by assigning skb->dev before calling tun_vnet_hdr_to_skb().
> This is required because virtio_net_hdr_to_skb() may invoke
> dev_parse_header_protocol(), which dereferences skb->dev. Without the
> assignment, a NULL pointer dereference can occur.
> 
> However, tap_get_user_xdp() still parses the virtio-net header before
> assigning skb->dev. When the vhost TX path passes an XDP buffer containing
> a GSO virtio-net header but the protocol is set to zero on purpose,
> tun_vnet_hdr_to_skb() can reach dev_parse_header_protocol() while skb->dev
> is still NULL, resulting in a crash.
> 
> Fix this by looking up the tap device and assigning skb->dev before calling
> tun_vnet_hdr_to_skb(), matching the ordering already used in
> tap_get_user(). Preserve the existing RCU read-side critical section across
> dev_queue_xmit().
> 
> Fixes: 924a9bc362a5 ("net: check if protocol extracted by virtio_net_hdr_set_proto is correct")
> Cc: stable@vger.kernel.org
> Assisted-by: Codex:GPT-5.5
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>