net/sched: act_sample: fix NULL dereference in the data path

Matteo reported the following splat, testing the datapath of TC 'sample':

 BUG: KASAN: null-ptr-deref in tcf_sample_act+0xc4/0x310
 Read of size 8 at addr 0000000000000000 by task nc/433

 CPU: 0 PID: 433 Comm: nc Not tainted 4.19.0-rc3-kvm #17
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS ?-20180531_142017-buildhw-08.phx2.fedoraproject.org-1.fc28 04/01/2014
 Call Trace:
  kasan_report.cold.6+0x6c/0x2fa
  tcf_sample_act+0xc4/0x310
  ? dev_hard_start_xmit+0x117/0x180
  tcf_action_exec+0xa3/0x160
  tcf_classify+0xdd/0x1d0
  htb_enqueue+0x18e/0x6b0
  ? deref_stack_reg+0x7a/0xb0
  ? htb_delete+0x4b0/0x4b0
  ? unwind_next_frame+0x819/0x8f0
  ? entry_SYSCALL_64_after_hwframe+0x44/0xa9
  __dev_queue_xmit+0x722/0xca0
  ? unwind_get_return_address_ptr+0x50/0x50
  ? netdev_pick_tx+0xe0/0xe0
  ? save_stack+0x8c/0xb0
  ? kasan_kmalloc+0xbe/0xd0
  ? __kmalloc_track_caller+0xe4/0x1c0
  ? __kmalloc_reserve.isra.45+0x24/0x70
  ? __alloc_skb+0xdd/0x2e0
  ? sk_stream_alloc_skb+0x91/0x3b0
  ? tcp_sendmsg_locked+0x71b/0x15a0
  ? tcp_sendmsg+0x22/0x40
  ? __sys_sendto+0x1b0/0x250
  ? __x64_sys_sendto+0x6f/0x80
  ? do_syscall_64+0x5d/0x150
  ? entry_SYSCALL_64_after_hwframe+0x44/0xa9
  ? __sys_sendto+0x1b0/0x250
  ? __x64_sys_sendto+0x6f/0x80
  ? do_syscall_64+0x5d/0x150
  ? entry_SYSCALL_64_after_hwframe+0x44/0xa9
  ip_finish_output2+0x495/0x590
  ? ip_copy_metadata+0x2e0/0x2e0
  ? skb_gso_validate_network_len+0x6f/0x110
  ? ip_finish_output+0x174/0x280
  __tcp_transmit_skb+0xb17/0x12b0
  ? __tcp_select_window+0x380/0x380
  tcp_write_xmit+0x913/0x1de0
  ? __sk_mem_schedule+0x50/0x80
  tcp_sendmsg_locked+0x49d/0x15a0
  ? tcp_rcv_established+0x8da/0xa30
  ? tcp_set_state+0x220/0x220
  ? clear_user+0x1f/0x50
  ? iov_iter_zero+0x1ae/0x590
  ? __fget_light+0xa0/0xe0
  tcp_sendmsg+0x22/0x40
  __sys_sendto+0x1b0/0x250
  ? __ia32_sys_getpeername+0x40/0x40
  ? _copy_to_user+0x58/0x70
  ? poll_select_copy_remaining+0x176/0x200
  ? __pollwait+0x1c0/0x1c0
  ? ktime_get_ts64+0x11f/0x140
  ? kern_select+0x108/0x150
  ? core_sys_select+0x360/0x360
  ? vfs_read+0x127/0x150
  ? kernel_write+0x90/0x90
  __x64_sys_sendto+0x6f/0x80
  do_syscall_64+0x5d/0x150
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
 RIP: 0033:0x7fefef2b129d
 Code: ff ff ff ff eb b6 0f 1f 80 00 00 00 00 48 8d 05 51 37 0c 00 41 89 ca 8b 00 85 c0 75 20 45 31 c9 45 31 c0 b8 2c 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 6b f3 c3 66 0f 1f 84 00 00 00 00 00 41 56 41
 RSP: 002b:00007fff2f5350c8 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
 RAX: ffffffffffffffda RBX: 000056118d60c120 RCX: 00007fefef2b129d
 RDX: 0000000000002000 RSI: 000056118d629320 RDI: 0000000000000003
 RBP: 000056118d530370 R08: 0000000000000000 R09: 0000000000000000
 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000002000
 R13: 000056118d5c2a10 R14: 000056118d5c2a10 R15: 000056118d5303b8

tcf_sample_act() tried to update its per-cpu stats, but tcf_sample_init()
forgot to allocate them, because tcf_idr_create() was called with a wrong
value of 'cpustats'. Setting it to true proved to fix the reported crash.

Reported-by: Matteo Croce <mcroce@redhat.com>
Fixes: 65a206c ("net/sched: Change act_api and act_xxx modules to use IDR")
Fixes: 5c5670f ("net/sched: Introduce sample tc action")
Tested-by: Matteo Croce <mcroce@redhat.com>
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_sample.c

@@ -69,7 +69,7 @@ static int tcf_sample_init(struct net *net, struct nlattr *nla,
 
 	if (!exists) {
 		ret = tcf_idr_create(tn, parm->index, est, a,
-				     &act_sample_ops, bind, false);
+				     &act_sample_ops, bind, true);
 		if (ret) {
 			tcf_idr_cleanup(tn, parm->index);
 			return ret;