On Thu, 12 Mar 2026 10:54:06 +0800, xietangxin <[email protected]> wrote:
> A UAF issue occurs when the virtio_net driver is configured with napi_tx=N
> and the device's IFF_XMIT_DST_RELEASE flag is cleared
> (e.g., during the configuration of tc route filter rules).
>
> When IFF_XMIT_DST_RELEASE is removed from the net_device, the network stack
> expects the driver to hold the reference to skb->dst until the packet
> is fully transmitted and freed. In virtio_net with napi_tx=N,
> skbs may remain in the virtio transmit ring for an extended period.
>
> If the network namespace is destroyed while these skbs are still pending,
> the corresponding dst_ops structure has freed. When a subsequent packet
> is transmitted, free_old_xmit() is triggered to clean up old skbs.
> It then calls dst_release() on the skb associated with the stale dst_entry.
> Since the dst_ops (referenced by the dst_entry) has already been freed,
> a UAF kernel paging request occurs.
Sorry, this sounds a bit off to me. We know that napi_tx=N merely prolongs the
presence of the skb on the device side. However, even without napi_tx=N, there
is no guarantee that the skb will be freed within any specific timeframe.
Therefore, napi_tx=N just makes the issue more reproducible; it is not the root
cause. Also, I'm surprised that the dst could be freed while it is still
referenced/held. I have a feeling that something is being overlooked here.
Thanks.
>
> fix it by adds skb_dst_drop(skb) in start_xmit to explicitly release
> the dst reference before the skb is queued in virtio_net.
>
> Call Trace:
> Unable to handle kernel paging request at virtual address ffff80007e150000
> CPU: 2 UID: 0 PID: 6236 Comm: ping Kdump: loaded Not tainted 7.0.0-rc1+ #6
> PREEMPT
> ...
> percpu_counter_add_batch+0x3c/0x158 lib/percpu_counter.c:98 (P)
> dst_release+0xe0/0x110 net/core/dst.c:177
> skb_release_head_state+0xe8/0x108 net/core/skbuff.c:1177
> sk_skb_reason_drop+0x54/0x2d8 net/core/skbuff.c:1255
> dev_kfree_skb_any_reason+0x64/0x78 net/core/dev.c:3469
> napi_consume_skb+0x1c4/0x3a0 net/core/skbuff.c:1527
> __free_old_xmit+0x164/0x230 drivers/net/virtio_net.c:611 [virtio_net]
> free_old_xmit drivers/net/virtio_net.c:1081 [virtio_net]
> start_xmit+0x7c/0x530 drivers/net/virtio_net.c:3329 [virtio_net]
> ...
>
> Reproduction Steps:
> NETDEV="enp3s0"
>
> config_qdisc_route_filter() {
> tc qdisc del dev $NETDEV root
> tc qdisc add dev $NETDEV root handle 1: prio
> tc filter add dev $NETDEV parent 1:0 \
> protocol ip prio 100 route to 100 flowid 1:1
> ip route add 192.168.1.100/32 dev $NETDEV realm 100
> }
>
> test_ns() {
> ip netns add testns
> ip link set $NETDEV netns testns
> ip netns exec testns ifconfig $NETDEV 10.0.32.46/24
> ip netns exec testns ping -c 1 10.0.32.1
> ip netns del testns
> }
>
> config_qdisc_route_filter
>
> test_ns
> sleep 2
> test_ns
>
> Fixes: f2fc6a54585a ("[NETNS][IPV6] route6 - move ip6_dst_ops inside the
> network namespace")
> Cc: [email protected]
> Signed-off-by: xietangxin <[email protected]>
> ---
> change in v2: add cc stable and fix tag
>
> v1: https://lore.kernel.org/all/[email protected]/
> ---
> drivers/net/virtio_net.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 72d6a9c6a..5b13a61b3 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3351,6 +3351,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb,
> struct net_device *dev)
> /* Don't wait up for transmitted skbs to be freed. */
> if (!use_napi) {
> skb_orphan(skb);
> + skb_dst_drop(skb);
> nf_reset_ct(skb);
> }
>
> --
> 2.43.0
>
