net/sched: Fix UAF when resolving a clash

KASAN reports the following UAF:

 BUG: KASAN: slab-use-after-free in tcf_ct_flow_table_process_conn+0x12b/0x380 [act_ct]
 Read of size 1 at addr ffff888c07603600 by task handler130/6469

 Call Trace:
  <IRQ>
  dump_stack_lvl+0x48/0x70
  print_address_description.constprop.0+0x33/0x3d0
  print_report+0xc0/0x2b0
  kasan_report+0xd0/0x120
  __asan_load1+0x6c/0x80
  tcf_ct_flow_table_process_conn+0x12b/0x380 [act_ct]
  tcf_ct_act+0x886/0x1350 [act_ct]
  tcf_action_exec+0xf8/0x1f0
  fl_classify+0x355/0x360 [cls_flower]
  __tcf_classify+0x1fd/0x330
  tcf_classify+0x21c/0x3c0
  sch_handle_ingress.constprop.0+0x2c5/0x500
  __netif_receive_skb_core.constprop.0+0xb25/0x1510
  __netif_receive_skb_list_core+0x220/0x4c0
  netif_receive_skb_list_internal+0x446/0x620
  napi_complete_done+0x157/0x3d0
  gro_cell_poll+0xcf/0x100
  __napi_poll+0x65/0x310
  net_rx_action+0x30c/0x5c0
  __do_softirq+0x14f/0x491
  __irq_exit_rcu+0x82/0xc0
  irq_exit_rcu+0xe/0x20
  common_interrupt+0xa1/0xb0
  </IRQ>
  <TASK>
  asm_common_interrupt+0x27/0x40

 Allocated by task 6469:
  kasan_save_stack+0x38/0x70
  kasan_set_track+0x25/0x40
  kasan_save_alloc_info+0x1e/0x40
  __kasan_krealloc+0x133/0x190
  krealloc+0xaa/0x130
  nf_ct_ext_add+0xed/0x230 [nf_conntrack]
  tcf_ct_act+0x1095/0x1350 [act_ct]
  tcf_action_exec+0xf8/0x1f0
  fl_classify+0x355/0x360 [cls_flower]
  __tcf_classify+0x1fd/0x330
  tcf_classify+0x21c/0x3c0
  sch_handle_ingress.constprop.0+0x2c5/0x500
  __netif_receive_skb_core.constprop.0+0xb25/0x1510
  __netif_receive_skb_list_core+0x220/0x4c0
  netif_receive_skb_list_internal+0x446/0x620
  napi_complete_done+0x157/0x3d0
  gro_cell_poll+0xcf/0x100
  __napi_poll+0x65/0x310
  net_rx_action+0x30c/0x5c0
  __do_softirq+0x14f/0x491

 Freed by task 6469:
  kasan_save_stack+0x38/0x70
  kasan_set_track+0x25/0x40
  kasan_save_free_info+0x2b/0x60
  ____kasan_slab_free+0x180/0x1f0
  __kasan_slab_free+0x12/0x30
  slab_free_freelist_hook+0xd2/0x1a0
  __kmem_cache_free+0x1a2/0x2f0
  kfree+0x78/0x120
  nf_conntrack_free+0x74/0x130 [nf_conntrack]
  nf_ct_destroy+0xb2/0x140 [nf_conntrack]
  __nf_ct_resolve_clash+0x529/0x5d0 [nf_conntrack]
  nf_ct_resolve_clash+0xf6/0x490 [nf_conntrack]
  __nf_conntrack_confirm+0x2c6/0x770 [nf_conntrack]
  tcf_ct_act+0x12ad/0x1350 [act_ct]
  tcf_action_exec+0xf8/0x1f0
  fl_classify+0x355/0x360 [cls_flower]
  __tcf_classify+0x1fd/0x330
  tcf_classify+0x21c/0x3c0
  sch_handle_ingress.constprop.0+0x2c5/0x500
  __netif_receive_skb_core.constprop.0+0xb25/0x1510
  __netif_receive_skb_list_core+0x220/0x4c0
  netif_receive_skb_list_internal+0x446/0x620
  napi_complete_done+0x157/0x3d0
  gro_cell_poll+0xcf/0x100
  __napi_poll+0x65/0x310
  net_rx_action+0x30c/0x5c0
  __do_softirq+0x14f/0x491

The ct may be dropped if a clash has been resolved but is still passed to
the tcf_ct_flow_table_process_conn function for further usage. This issue
can be fixed by retrieving ct from skb again after confirming conntrack.

Fixes: 0cc254e ("net/sched: act_ct: Offload connections with commit action")
Co-developed-by: Gerald Yang <gerald.yang@canonical.com>
Signed-off-by: Gerald Yang <gerald.yang@canonical.com>
Signed-off-by: Chengen Du <chengen.du@canonical.com>
Link: https://patch.msgid.link/20240710053747.13223-1-chengen.du@canonical.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/sched/act_ct.c

@@ -1077,6 +1077,14 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a,
 		 */
 		if (nf_conntrack_confirm(skb) != NF_ACCEPT)
 			goto drop;
+
+		/* The ct may be dropped if a clash has been resolved,
+		 * so it's necessary to retrieve it from skb again to
+		 * prevent UAF.
+		 */
+		ct = nf_ct_get(skb, &ctinfo);
+		if (!ct)
+			skip_add = true;
 	}
 
 	if (!skip_add)