Linux Kernel Open vSwitch CMD_DEL Races the RCU Grace Period Against ovs_flow_cmd_fill_info, Causing Use-After-Free on the Flow Mask

Status: NEW  |  Advisory ID: CVE-2026-80994

Key Details

What to Know

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

net: openvswitch: fix flow mask use-after-free on flow deletion

The commit in the Fixes tag below made so flow->mask free is scheduled

via RCU right after it is removed from the flow table. The pointer

stays in the flow structure and it can be accessible while in the same

RCU critical section. This is done to avoid requiring ovs_mutex for

the ovs_flow_free().

However, while removing the flow during processing of CMD_DEL, we do

not take RCU read lock before the removal, and ovs_flow_cmd_fill_info()

uses the flow->mask pointer afterwards. The RCU read lock is taken,

but it's already late at that point. The comment on that line

acknowledges that the lock is cosmetic and doesn't serve a real purpose.

This leads to use-after-free if the RCU grace period passes between

removal and the filling. It is a short race window, but it is there

and can lead to a real crash in case memory allocation for the info

takes a bit longer:

BUG: KASAN: slab-use-after-free in __ovs_nla_put_key

net/openvswitch/flow_netlink.c:1996

BUG: KASAN: slab-use-after-free in ovs_nla_put_key+0x2463/0x2e30

net/openvswitch/flow_netlink.c:2250

Read of size 4 at addr ffff88801ee89970 by task ovs_flow_del_ec/9487

Call Trace:

<TASK>

__ovs_nla_put_key net/openvswitch/flow_netlink.c:1996

ovs_nla_put_key+0x2463/0x2e30 net/openvswitch/flow_netlink.c:2250

ovs_flow_cmd_fill_info+0x420/0x9c0 net/openvswitch/datapath.c:930

ovs_flow_cmd_del+0x53a/0x970 net/openvswitch/datapath.c:1467

...

netlink_rcv_skb+0x156/0x420 net/netlink/af_netlink.c:2556

</TASK>

Allocated by task 9487:

mask_alloc net/openvswitch/flow_table.c:967

flow_mask_insert net/openvswitch/flow_table.c:1012

ovs_flow_tbl_insert+0xea2/0x1a90 net/openvswitch/flow_table.c:1084

ovs_flow_cmd_new+0x7e3/0xd90 net/openvswitch/datapath.c:1086

...

netlink_rcv_skb+0x156/0x420 net/netlink/af_netlink.c:2556

Freed by task 9485:

rcu_free_sheaf+0x1e/0x100 mm/slub.c:5978

rcu_do_batch kernel/rcu/tree.c:2645

rcu_core+0x59c/0x10c0 kernel/rcu/tree.c:2897

handle_softirqs+0x1e4/0x9a0 kernel/softirq.c:622

...

instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1062

ovs_flow_tbl_remove() must be called after the ovs_flow_cmd_fill_info()

to avoid this race. This also helps with cleaning up the forced cast

and the cosmetic RCU read lock. Before the commit in the Fixes tag the

order did not matter as long as the flow object itself was not freed.

A wider RCU critical section could be another option, but we have a

GFP_KERNEL allocation in the way.

Reported by Trend Micro's Zero Day Initiative as ZDI-CAN-32042. (NVD)

References