CVE-2026-31469: virtio_net: Fix UAF on dst_ops when IFF_XMIT_DST_RELEASE is cleared and napi_tx is false

In the Linux kernel, the following vulnerability has been resolved:

virtionet: Fix UAF on dstops when IFFXMITDSTRELEASE is cleared and napitx is false

A UAF issue occurs when the virtionet driver is configured with napitx=N and the device's IFFXMITDSTRELEASE flag is cleared (e.g., during the configuration of tc route filter rules).

When IFFXMITDSTRELEASE is removed from the netdevice, the network stack expects the driver to hold the reference to skb->dst until the packet is fully transmitted and freed. In virtionet with napitx=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 dstops structure has freed. When a subsequent packet is transmitted, freeoldxmit() is triggered to clean up old skbs. It then calls dstrelease() on the skb associated with the stale dstentry. Since the dstops (referenced by the dstentry) has already been freed, a UAF kernel paging request occurs.

fix it by adds skbdstdrop(skb) in startxmit to explicitly release the dst reference before the skb is queued in virtionet.

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 ... percpucounteraddbatch+0x3c/0x158 lib/percpucounter.c:98 (P) dstrelease+0xe0/0x110 net/core/dst.c:177 skbreleaseheadstate+0xe8/0x108 net/core/skbuff.c:1177 skskbreasondrop+0x54/0x2d8 net/core/skbuff.c:1255 devkfreeskbanyreason+0x64/0x78 net/core/dev.c:3469 napiconsumeskb+0x1c4/0x3a0 net/core/skbuff.c:1527 freeoldxmit+0x164/0x230 drivers/net/virtionet.c:611 [virtionet] freeoldxmit drivers/net/virtionet.c:1081 [virtionet] startxmit+0x7c/0x530 drivers/net/virtionet.c:3329 [virtionet] ...

Reproduction Steps: NETDEV="enp3s0"

configqdiscroutefilter() { 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 }

testns() { 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 }

configqdiscroutefilter

testns sleep 2 testns