net_sched: fix NULL pointer dereference when delete tcindex filter

Li Shuang reported the following crash:

[   71.267724] BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
[   71.276456] PGD 800000085d9bd067 P4D 800000085d9bd067 PUD 859a0b067 PMD 0
[   71.284127] Oops: 0000 [#1] SMP PTI
[   71.288015] CPU: 12 PID: 2386 Comm: tc Not tainted 4.18.0-rc8.latest+ #131
[   71.295686] Hardware name: Dell Inc. PowerEdge R730/0WCJNT, BIOS 2.1.5 04/11/2016
[   71.304037] RIP: 0010:tcindex_delete+0x72/0x280 [cls_tcindex]
[   71.310446] Code: 00 31 f6 48 87 75 20 48 85 f6 74 11 48 8b 47 18 48 8b 40 08 48 8b 40 50 e8 fb a6 f8 fc 48 85 db 0f 84 dc 00 00 00 48 8b 73 18 <8b> 56 04 48 8d 7e 04 85 d2 0f 84 7b 01 00
[   71.331517] RSP: 0018:ffffb45207b3f898 EFLAGS: 00010282
[   71.337345] RAX: ffff8ad3d72d6360 RBX: ffff8acc84393680 RCX: 000000000000002e
[   71.345306] RDX: ffff8ad3d72c8570 RSI: 0000000000000000 RDI: ffff8ad847a45800
[   71.353277] RBP: ffff8acc84393688 R08: ffff8ad3d72c8400 R09: 0000000000000000
[   71.361238] R10: ffff8ad3de786e00 R11: 0000000000000000 R12: ffffb45207b3f8c7
[   71.369199] R13: ffff8ad3d93bd2a0 R14: 000000000000002e R15: ffff8ad3d72c9600
[   71.377161] FS:  00007f9d3ec3e740(0000) GS:ffff8ad3df980000(0000) knlGS:0000000000000000
[   71.386188] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   71.392597] CR2: 0000000000000004 CR3: 0000000852f06003 CR4: 00000000001606e0
[   71.400558] Call Trace:
[   71.403299]  tcindex_destroy_element+0x25/0x40 [cls_tcindex]
[   71.409611]  tcindex_walk+0xbb/0x110 [cls_tcindex]
[   71.414953]  tcindex_destroy+0x44/0x90 [cls_tcindex]
[   71.420492]  ? tcindex_delete+0x280/0x280 [cls_tcindex]
[   71.426323]  tcf_proto_destroy+0x16/0x40
[   71.430696]  tcf_chain_flush+0x51/0x70
[   71.434876]  tcf_block_put_ext.part.30+0x8f/0x1b0
[   71.440122]  tcf_block_put+0x4d/0x70
[   71.444108]  cbq_destroy+0x4d/0xd0 [sch_cbq]
[   71.448869]  qdisc_destroy+0x62/0x130
[   71.452951]  dsmark_destroy+0x2a/0x70 [sch_dsmark]
[   71.458300]  qdisc_destroy+0x62/0x130
[   71.462373]  qdisc_graft+0x3ba/0x470
[   71.466359]  tc_get_qdisc+0x2a6/0x2c0
[   71.470443]  ? cred_has_capability+0x7d/0x130
[   71.475307]  rtnetlink_rcv_msg+0x263/0x2d0
[   71.479875]  ? rtnl_calcit.isra.30+0x110/0x110
[   71.484832]  netlink_rcv_skb+0x4d/0x130
[   71.489109]  netlink_unicast+0x1a3/0x250
[   71.493482]  netlink_sendmsg+0x2ae/0x3a0
[   71.497859]  sock_sendmsg+0x36/0x40
[   71.501748]  ___sys_sendmsg+0x26f/0x2d0
[   71.506029]  ? handle_pte_fault+0x586/0xdf0
[   71.510694]  ? __handle_mm_fault+0x389/0x500
[   71.515457]  ? __sys_sendmsg+0x5e/0xa0
[   71.519636]  __sys_sendmsg+0x5e/0xa0
[   71.523626]  do_syscall_64+0x5b/0x180
[   71.527711]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   71.533345] RIP: 0033:0x7f9d3e257f10
[   71.537331] Code: c3 48 8b 05 82 6f 2c 00 f7 db 64 89 18 48 83 cb ff eb dd 0f 1f 80 00 00 00 00 83 3d 8d d0 2c 00 00 75 10 b8 2e 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8
[   71.558401] RSP: 002b:00007fff6f893398 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
[   71.566848] RAX: ffffffffffffffda RBX: 000000005b71274d RCX: 00007f9d3e257f10
[   71.574810] RDX: 0000000000000000 RSI: 00007fff6f8933e0 RDI: 0000000000000003
[   71.582770] RBP: 00007fff6f8933e0 R08: 000000000000ffff R09: 0000000000000003
[   71.590729] R10: 00007fff6f892e20 R11: 0000000000000246 R12: 0000000000000000
[   71.598689] R13: 0000000000662ee0 R14: 0000000000000000 R15: 0000000000000000
[   71.606651] Modules linked in: sch_cbq cls_tcindex sch_dsmark xt_CHECKSUM iptable_mangle ipt_MASQUERADE iptable_nat nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack nf_coni
[   71.685425]  libahci i2c_algo_bit i2c_core i40e libata dca mdio megaraid_sas dm_mirror dm_region_hash dm_log dm_mod
[   71.697075] CR2: 0000000000000004
[   71.700792] ---[ end trace f604eb1acacd978b ]---

Reproducer:
tc qdisc add dev lo handle 1:0 root dsmark indices 64 set_tc_index
tc filter add dev lo parent 1:0 protocol ip prio 1 tcindex mask 0xfc shift 2
tc qdisc add dev lo parent 1:0 handle 2:0 cbq bandwidth 10Mbit cell 8 avpkt 1000 mpu 64
tc class add dev lo parent 2:0 classid 2:1 cbq bandwidth 10Mbit rate 1500Kbit avpkt 1000 prio 1 bounded isolated allot 1514 weight 1 maxburst 10
tc filter add dev lo parent 2:0 protocol ip prio 1 handle 0x2e tcindex classid 2:1 pass_on
tc qdisc add dev lo parent 2:1 pfifo limit 5
tc qdisc del dev lo root

This is because in tcindex_set_parms, when there is no old_r, we set new
exts to cr.exts. And we didn't set it to filter when r == &new_filter_result.

Then in tcindex_delete() -> tcf_exts_get_net(), we will get NULL pointer
dereference as we didn't init exts.

Fix it by moving tcf_exts_change() after "if (old_r && old_r != r)" check.
Then we don't need "cr" as there is no errout after that.

Fixes: bf63ac73 ("net_sched: fix an oops in tcindex filter")
Reported-by: Li Shuang <shuali@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_tcindex.c

@@ -447,11 +447,6 @@ tcindex_set_parms(struct net *net, struct tcf_proto *tp, unsigned long base,
 		tcf_bind_filter(tp, &cr.res, base);
 	}
 
-	if (old_r)
-		tcf_exts_change(&r->exts, &e);
-	else
-		tcf_exts_change(&cr.exts, &e);
-
 	if (old_r && old_r != r) {
 		err = tcindex_filter_result_init(old_r);
 		if (err < 0) {
@@ -462,6 +457,8 @@ tcindex_set_parms(struct net *net, struct tcf_proto *tp, unsigned long base,
 
 	oldp = p;
 	r->res = cr.res;
+	tcf_exts_change(&r->exts, &e);
+
 	rcu_assign_pointer(tp->root, cp);
 
 	if (r == &new_filter_result) {