Re: [PATCH] virtio_net: Fix UAF on dst_ops when IFF_XMIT_DST_RELEASE is cleared and napi_tx is false

[xietangxin <xietangxin@yeah.net>]

On 3/9/2026 11:42 AM, Jason Wang wrote:
> On Sat, Mar 7, 2026 at 11:53 AM xietangxin <xietangxin@yeah.net> 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.
>>
>> 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
>>
>> Signed-off-by: xietangxin <xietangxin@yeah.net>
>> ---
> 
> This is needed for stable I think.
> 
> And do we need to fix tun_net_xmit() as well?
> 
> Thanks
Hi Jason,

I have analyzed the tun driver and concluded that it don't suffer from this UAF issue.

The netns containing a tun interface cannot be destroyed until all processes
holding the tun file descriptor have exited. When the file descriptor is closed,
tun_chr_close() is called, which immediately free all skbs in the tx_ring.
This happens before cleanup_net() destroys the dst_ops. Therefore, no skbs referencing freeed dst_ops.
Unlike virtio_net, where skbs can remain in the TX queue even if the ns is deleted.

I believe the fix is only necessary for virtio_net. What do you think?

Best regards,

Tangxin Xie