net/sched: act_csum: fix NULL dereference when 'goto chain' is used

the control action in the common member of struct tcf_csum must be a valid
value, as it can contain the chain index when 'goto chain' is used. Ensure
that the control action can be read as x->tcfa_action, when x is a pointer
to struct tc_action and x->ops->type is TCA_ACT_CSUM, to prevent the
following command:

  # tc filter add dev $h2 ingress protocol ip pref 1 handle 101 flower \
  > $tcflags dst_mac $h2mac action csum ip or tcp or udp or sctp goto chain 1

from triggering a NULL pointer dereference when a matching packet is
received.

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
 PGD 800000010416b067 P4D 800000010416b067 PUD 1041be067 PMD 0
 Oops: 0000 [#1] SMP PTI
 CPU: 0 PID: 3072 Comm: mausezahn Tainted: G            E     4.18.0-rc2.auguri+ #421
 Hardware name: Hewlett-Packard HP Z220 CMT Workstation/1790, BIOS K51 v01.58 02/07/2013
 RIP: 0010:tcf_action_exec+0xb8/0x100
 Code: 00 00 00 20 74 1d 83 f8 03 75 09 49 83 c4 08 4d 39 ec 75 bc 48 83 c4 10 5b 5d 41 5c 41 5d 41 5e 41 5f c3 49 8b 97 a8 00 00 00 <48> 8b 12 48 89 55 00 48 83 c4 10 5b 5d 41 5c 41 5d 41 5e 41 5f c3
 RSP: 0018:ffffa020dea03c40 EFLAGS: 00010246
 RAX: 0000000020000001 RBX: ffffa020d7ccef00 RCX: 0000000000000054
 RDX: 0000000000000000 RSI: ffffa020ca5ae000 RDI: ffffa020d7ccef00
 RBP: ffffa020dea03e60 R08: 0000000000000000 R09: ffffa020dea03c9c
 R10: ffffa020dea03c78 R11: 0000000000000008 R12: ffffa020d3fe4f00
 R13: ffffa020d3fe4f08 R14: 0000000000000001 R15: ffffa020d53ca300
 FS:  00007f5a46942740(0000) GS:ffffa020dea00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 0000000104218002 CR4: 00000000001606f0
 Call Trace:
  <IRQ>
  fl_classify+0x1ad/0x1c0 [cls_flower]
  ? arp_rcv+0x121/0x1b0
  ? __x2apic_send_IPI_dest+0x40/0x40
  ? smp_reschedule_interrupt+0x1c/0xd0
  ? reschedule_interrupt+0xf/0x20
  ? reschedule_interrupt+0xa/0x20
  ? device_is_rmrr_locked+0xe/0x50
  ? iommu_should_identity_map+0x49/0xd0
  ? __intel_map_single+0x30/0x140
  ? e1000e_update_rdt_wa.isra.52+0x22/0xb0 [e1000e]
  ? e1000_alloc_rx_buffers+0x233/0x250 [e1000e]
  ? kmem_cache_alloc+0x38/0x1c0
  tcf_classify+0x89/0x140
  __netif_receive_skb_core+0x5ea/0xb70
  ? enqueue_task_fair+0xb6/0x7d0
  ? process_backlog+0x97/0x150
  process_backlog+0x97/0x150
  net_rx_action+0x14b/0x3e0
  __do_softirq+0xde/0x2b4
  do_softirq_own_stack+0x2a/0x40
  </IRQ>
  do_softirq.part.18+0x49/0x50
  __local_bh_enable_ip+0x49/0x50
  __dev_queue_xmit+0x4ab/0x8a0
  ? wait_woken+0x80/0x80
  ? packet_sendmsg+0x38f/0x810
  ? __dev_queue_xmit+0x8a0/0x8a0
  packet_sendmsg+0x38f/0x810
  sock_sendmsg+0x36/0x40
  __sys_sendto+0x10e/0x140
  ? do_vfs_ioctl+0xa4/0x630
  ? syscall_trace_enter+0x1df/0x2e0
  ? __audit_syscall_exit+0x22a/0x290
  __x64_sys_sendto+0x24/0x30
  do_syscall_64+0x5b/0x180
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
 RIP: 0033:0x7f5a45cbec93
 Code: 48 8b 0d 18 83 20 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 83 3d 59 c7 20 00 00 75 13 49 89 ca b8 2c 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 34 c3 48 83 ec 08 e8 2b f7 ff ff 48 89 04 24
 RSP: 002b:00007ffd0ee6d748 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
 RAX: ffffffffffffffda RBX: 0000000001161010 RCX: 00007f5a45cbec93
 RDX: 0000000000000062 RSI: 0000000001161322 RDI: 0000000000000003
 RBP: 00007ffd0ee6d780 R08: 00007ffd0ee6d760 R09: 0000000000000014
 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000062
 R13: 0000000001161322 R14: 00007ffd0ee6d760 R15: 0000000000000003
 Modules linked in: act_csum act_gact cls_flower sch_ingress vrf veth act_tunnel_key(E) xt_CHECKSUM iptable_mangle ipt_MASQUERADE iptable_nat nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack nf_conntrack ipt_REJECT nf_reject_ipv4 tun bridge stp llc ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter intel_rapl x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel snd_hda_codec_hdmi snd_hda_codec_realtek kvm snd_hda_codec_generic hp_wmi iTCO_wdt sparse_keymap rfkill mei_wdt iTCO_vendor_support wmi_bmof gpio_ich irqbypass crct10dif_pclmul crc32_pclmul ghash_clmulni_intel pcbc aesni_intel snd_hda_intel crypto_simd cryptd snd_hda_codec glue_helper snd_hda_core snd_hwdep snd_seq snd_seq_device snd_pcm pcspkr i2c_i801 snd_timer snd sg lpc_ich soundcore wmi mei_me
  mei ie31200_edac nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip_tables xfs libcrc32c sr_mod cdrom sd_mod ahci libahci crc32c_intel i915 ixgbe serio_raw libata video dca i2c_algo_bit sfc drm_kms_helper syscopyarea mtd sysfillrect mdio sysimgblt fb_sys_fops drm e1000e i2c_core
 CR2: 0000000000000000
 ---[ end trace 3c9e9d1a77df4026 ]---
 RIP: 0010:tcf_action_exec+0xb8/0x100
 Code: 00 00 00 20 74 1d 83 f8 03 75 09 49 83 c4 08 4d 39 ec 75 bc 48 83 c4 10 5b 5d 41 5c 41 5d 41 5e 41 5f c3 49 8b 97 a8 00 00 00 <48> 8b 12 48 89 55 00 48 83 c4 10 5b 5d 41 5c 41 5d 41 5e 41 5f c3
 RSP: 0018:ffffa020dea03c40 EFLAGS: 00010246
 RAX: 0000000020000001 RBX: ffffa020d7ccef00 RCX: 0000000000000054
 RDX: 0000000000000000 RSI: ffffa020ca5ae000 RDI: ffffa020d7ccef00
 RBP: ffffa020dea03e60 R08: 0000000000000000 R09: ffffa020dea03c9c
 R10: ffffa020dea03c78 R11: 0000000000000008 R12: ffffa020d3fe4f00
 R13: ffffa020d3fe4f08 R14: 0000000000000001 R15: ffffa020d53ca300
 FS:  00007f5a46942740(0000) GS:ffffa020dea00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 0000000104218002 CR4: 00000000001606f0
 Kernel panic - not syncing: Fatal exception in interrupt
 Kernel Offset: 0x26400000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
 ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---

Fixes: 9c5f69bbd7 ("net/sched: act_csum: don't use spinlock in the fast path")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tc_act/tc_csum.h

@@ -7,7 +7,6 @@
 #include <linux/tc_act/tc_csum.h>
 
 struct tcf_csum_params {
-	int action;
 	u32 update_flags;
 	struct rcu_head rcu;
 };

net/sched/act_csum.c

@@ -91,7 +91,7 @@ static int tcf_csum_init(struct net *net, struct nlattr *nla,
 	}
 	params_old = rtnl_dereference(p->params);
 
-	params_new->action = parm->action;
+	p->tcf_action = parm->action;
 	params_new->update_flags = parm->update_flags;
 	rcu_assign_pointer(p->params, params_new);
 	if (params_old)
@@ -561,7 +561,7 @@ static int tcf_csum(struct sk_buff *skb, const struct tc_action *a,
 	tcf_lastuse_update(&p->tcf_tm);
 	bstats_cpu_update(this_cpu_ptr(p->common.cpu_bstats), skb);
 
-	action = params->action;
+	action = READ_ONCE(p->tcf_action);
 	if (unlikely(action == TC_ACT_SHOT))
 		goto drop_stats;
 
@@ -599,11 +599,11 @@ static int tcf_csum_dump(struct sk_buff *skb, struct tc_action *a, int bind,
 		.index   = p->tcf_index,
 		.refcnt  = p->tcf_refcnt - ref,
 		.bindcnt = p->tcf_bindcnt - bind,
+		.action  = p->tcf_action,
 	};
 	struct tcf_t t;
 
 	params = rtnl_dereference(p->params);
-	opt.action = params->action;
 	opt.update_flags = params->update_flags;
 
 	if (nla_put(skb, TCA_CSUM_PARMS, sizeof(opt), &opt))