net_sched: fix a null pointer dereference in tcindex_set_parms()

This patch fixes the following crash:

[   42.199159] BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
[   42.200027] IP: [<ffffffff817e3fc4>] tcindex_set_parms+0x45c/0x526
[   42.200027] PGD d2319067 PUD d4ffe067 PMD 0
[   42.200027] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
[   42.200027] CPU: 0 PID: 541 Comm: tc Not tainted 3.17.0-rc4+ #603
[   42.200027] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   42.200027] task: ffff8800d22d2670 ti: ffff8800ce790000 task.ti: ffff8800ce790000
[   42.200027] RIP: 0010:[<ffffffff817e3fc4>]  [<ffffffff817e3fc4>] tcindex_set_parms+0x45c/0x526
[   42.200027] RSP: 0018:ffff8800ce793898  EFLAGS: 00010202
[   42.200027] RAX: 0000000000000001 RBX: ffff8800d1786498 RCX: 0000000000000000
[   42.200027] RDX: ffffffff82114ec8 RSI: ffffffff82114ec8 RDI: ffffffff82114ec8
[   42.200027] RBP: ffff8800ce793958 R08: 00000000000080d0 R09: 0000000000000001
[   42.200027] R10: ffff8800ce7939a0 R11: 0000000000000246 R12: ffff8800d017d238
[   42.200027] R13: 0000000000000018 R14: ffff8800d017c6a0 R15: ffff8800d1786620
[   42.200027] FS:  00007f4e24539740(0000) GS:ffff88011a600000(0000) knlGS:0000000000000000
[   42.200027] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   42.200027] CR2: 0000000000000018 CR3: 00000000cff38000 CR4: 00000000000006f0
[   42.200027] Stack:
[   42.200027]  ffff8800ce0949f0 0000000000000000 0000000200000003 ffff880000000000
[   42.200027]  ffff8800ce7938b8 ffff8800ce7938b8 0000000600000007 0000000000000000
[   42.200027]  ffff8800ce7938d8 ffff8800ce7938d8 0000000600000007 ffff8800ce0949f0
[   42.200027] Call Trace:
[   42.200027]  [<ffffffff817e4169>] tcindex_change+0xdb/0xee
[   42.200027]  [<ffffffff817c16ca>] tc_ctl_tfilter+0x44d/0x63f
[   42.200027]  [<ffffffff8179d161>] rtnetlink_rcv_msg+0x181/0x194
[   42.200027]  [<ffffffff8179cf9d>] ? rtnl_lock+0x17/0x19
[   42.200027]  [<ffffffff8179cfe0>] ? __rtnl_unlock+0x17/0x17
[   42.200027]  [<ffffffff817ee296>] netlink_rcv_skb+0x49/0x8b
[   43.462494]  [<ffffffff8179cfc2>] rtnetlink_rcv+0x23/0x2a
[   43.462494]  [<ffffffff817ec8df>] netlink_unicast+0xc7/0x148
[   43.462494]  [<ffffffff817ed413>] netlink_sendmsg+0x5cb/0x63d
[   43.462494]  [<ffffffff810ad781>] ? mark_lock+0x2e/0x224
[   43.462494]  [<ffffffff817757b8>] __sock_sendmsg_nosec+0x25/0x27
[   43.462494]  [<ffffffff81778165>] sock_sendmsg+0x57/0x71
[   43.462494]  [<ffffffff81152bbd>] ? might_fault+0x57/0xa4
[   43.462494]  [<ffffffff81152c06>] ? might_fault+0xa0/0xa4
[   43.462494]  [<ffffffff81152bbd>] ? might_fault+0x57/0xa4
[   43.462494]  [<ffffffff817838fd>] ? verify_iovec+0x69/0xb7
[   43.462494]  [<ffffffff817784f8>] ___sys_sendmsg+0x21d/0x2bb
[   43.462494]  [<ffffffff81009db3>] ? native_sched_clock+0x35/0x37
[   43.462494]  [<ffffffff8109ab53>] ? sched_clock_local+0x12/0x72
[   43.462494]  [<ffffffff810ad781>] ? mark_lock+0x2e/0x224
[   43.462494]  [<ffffffff8109ada4>] ? sched_clock_cpu+0xa0/0xb9
[   43.462494]  [<ffffffff810aee37>] ? __lock_acquire+0x5fe/0xde4
[   43.462494]  [<ffffffff8119f570>] ? rcu_read_lock_held+0x36/0x38
[   43.462494]  [<ffffffff8119f75a>] ? __fcheck_files.isra.7+0x4b/0x57
[   43.462494]  [<ffffffff8119fbf2>] ? __fget_light+0x30/0x54
[   43.462494]  [<ffffffff81779012>] __sys_sendmsg+0x42/0x60
[   43.462494]  [<ffffffff81779042>] SyS_sendmsg+0x12/0x1c
[   43.462494]  [<ffffffff819d24d2>] system_call_fastpath+0x16/0x1b

'p->h' could be NULL while 'cp->h' is always update to date.

Fixes: commit 331b72922c5f58d48fd ("net: sched: RCU cls_tcindex")
Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-By: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_tcindex.c

@@ -381,7 +381,7 @@ tcindex_set_parms(struct net *net, struct tcf_proto *tp, unsigned long base,
 		f->result = new_filter_result;
 		f->next = NULL;
 
-		fp = p->h + (handle % p->hash);
+		fp = cp->h + (handle % cp->hash);
 		for (nfp = rtnl_dereference(*fp);
 		     nfp;
 		     fp = &nfp->next, nfp = rtnl_dereference(*fp))