net/sched: fix NULL dereference on the error path of tcf_skbmod_init()

when the following command

 # tc action replace action skbmod swap mac index 100

is run for the first time, and tcf_skbmod_init() fails to allocate struct
tcf_skbmod_params, tcf_skbmod_cleanup() calls kfree_rcu(NULL), thus
causing the following error:

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
 IP: __call_rcu+0x23/0x2b0
 PGD 8000000034057067 P4D 8000000034057067 PUD 74937067 PMD 0
 Oops: 0002 [#1] SMP PTI
 Modules linked in: act_skbmod(E) psample ip6table_filter ip6_tables iptable_filter binfmt_misc ext4 snd_hda_codec_generic snd_hda_intel snd_hda_codec crct10dif_pclmul mbcache jbd2 crc32_pclmul snd_hda_core ghash_clmulni_intel snd_hwdep pcbc snd_seq snd_seq_device snd_pcm aesni_intel snd_timer crypto_simd glue_helper snd cryptd virtio_balloon joydev soundcore pcspkr i2c_piix4 nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip_tables xfs libcrc32c ata_generic pata_acpi qxl drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ttm drm virtio_console virtio_net virtio_blk ata_piix libata crc32c_intel virtio_pci serio_raw virtio_ring virtio i2c_core floppy dm_mirror dm_region_hash dm_log dm_mod [last unloaded: act_skbmod]
 CPU: 3 PID: 3144 Comm: tc Tainted: G            E    4.16.0-rc4.act_vlan.orig+ #403
 Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
 RIP: 0010:__call_rcu+0x23/0x2b0
 RSP: 0018:ffffbd2e403e7798 EFLAGS: 00010246
 RAX: ffffffffc0872080 RBX: ffff981d34bff780 RCX: 00000000ffffffff
 RDX: ffffffff922a5f00 RSI: 0000000000000000 RDI: 0000000000000000
 RBP: 0000000000000000 R08: 0000000000000001 R09: 000000000000021f
 R10: 000000003d003000 R11: 0000000000aaaaaa R12: 0000000000000000
 R13: ffffffff922a5f00 R14: 0000000000000001 R15: ffff981d3b698c2c
 FS:  00007f3678292740(0000) GS:ffff981d3fd80000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000008 CR3: 000000007c57a006 CR4: 00000000001606e0
 Call Trace:
  __tcf_idr_release+0x79/0xf0
  tcf_skbmod_init+0x1d1/0x210 [act_skbmod]
  tcf_action_init_1+0x2cc/0x430
  tcf_action_init+0xd3/0x1b0
  tc_ctl_action+0x18b/0x240
  rtnetlink_rcv_msg+0x29c/0x310
  ? _cond_resched+0x15/0x30
  ? __kmalloc_node_track_caller+0x1b9/0x270
  ? rtnl_calcit.isra.28+0x100/0x100
  netlink_rcv_skb+0xd2/0x110
  netlink_unicast+0x17c/0x230
  netlink_sendmsg+0x2cd/0x3c0
  sock_sendmsg+0x30/0x40
  ___sys_sendmsg+0x27a/0x290
  ? filemap_map_pages+0x34a/0x3a0
  ? __handle_mm_fault+0xbfd/0xe20
  __sys_sendmsg+0x51/0x90
  do_syscall_64+0x6e/0x1a0
  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
 RIP: 0033:0x7f36776a3ba0
 RSP: 002b:00007fff4703b618 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
 RAX: ffffffffffffffda RBX: 00007fff4703b740 RCX: 00007f36776a3ba0
 RDX: 0000000000000000 RSI: 00007fff4703b690 RDI: 0000000000000003
 RBP: 000000005aaaba36 R08: 0000000000000002 R09: 0000000000000000
 R10: 00007fff4703b0a0 R11: 0000000000000246 R12: 0000000000000000
 R13: 00007fff4703b754 R14: 0000000000000001 R15: 0000000000669f60
 Code: 5d e9 42 da ff ff 66 90 0f 1f 44 00 00 41 57 41 56 41 55 49 89 d5 41 54 55 48 89 fd 53 48 83 ec 08 40 f6 c7 07 0f 85 19 02 00 00 <48> 89 75 08 48 c7 45 00 00 00 00 00 9c 58 0f 1f 44 00 00 49 89
 RIP: __call_rcu+0x23/0x2b0 RSP: ffffbd2e403e7798
 CR2: 0000000000000008

Fix it in tcf_skbmod_cleanup(), ensuring that kfree_rcu(p, ...) is called
only when p is not NULL.

Fixes: 86da71b573 ("net_sched: Introduce skbmod action")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/act_skbmod.c

@@ -190,7 +190,8 @@ static void tcf_skbmod_cleanup(struct tc_action *a)
 	struct tcf_skbmod_params  *p;
 
 	p = rcu_dereference_protected(d->skbmod_p, 1);
-	kfree_rcu(p, rcu);
+	if (p)
+		kfree_rcu(p, rcu);
 }
 
 static int tcf_skbmod_dump(struct sk_buff *skb, struct tc_action *a,