net: sched: fix possible crash in tcf_action_destroy()

If the allocation done in tcf_exts_init() failed,
we end up with a NULL pointer in exts->actions.

kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 8198 Comm: syz-executor.3 Not tainted 5.3.0-rc8+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:tcf_action_destroy+0x71/0x160 net/sched/act_api.c:705
Code: c3 08 44 89 ee e8 4f cb bb fb 41 83 fd 20 0f 84 c9 00 00 00 e8 c0 c9 bb fb 48 89 d8 48 b9 00 00 00 00 00 fc ff df 48 c1 e8 03 <80> 3c 08 00 0f 85 c0 00 00 00 4c 8b 33 4d 85 f6 0f 84 9d 00 00 00
RSP: 0018:ffff888096e16ff0 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: dffffc0000000000
RDX: 0000000000040000 RSI: ffffffff85b6ab30 RDI: 0000000000000000
RBP: ffff888096e17020 R08: ffff8880993f6140 R09: fffffbfff11cae67
R10: fffffbfff11cae66 R11: ffffffff88e57333 R12: 0000000000000000
R13: 0000000000000000 R14: ffff888096e177a0 R15: 0000000000000001
FS:  00007f62bc84a700(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000758040 CR3: 0000000088b64000 CR4: 00000000001426e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 tcf_exts_destroy+0x38/0xb0 net/sched/cls_api.c:3030
 tcindex_set_parms+0xf7f/0x1e50 net/sched/cls_tcindex.c:488
 tcindex_change+0x230/0x318 net/sched/cls_tcindex.c:519
 tc_new_tfilter+0xa4b/0x1c70 net/sched/cls_api.c:2152
 rtnetlink_rcv_msg+0x838/0xb00 net/core/rtnetlink.c:5214
 netlink_rcv_skb+0x177/0x450 net/netlink/af_netlink.c:2477
 rtnetlink_rcv+0x1d/0x30 net/core/rtnetlink.c:5241
 netlink_unicast_kernel net/netlink/af_netlink.c:1302 [inline]
 netlink_unicast+0x531/0x710 net/netlink/af_netlink.c:1328
 netlink_sendmsg+0x8a5/0xd60 net/netlink/af_netlink.c:1917
 sock_sendmsg_nosec net/socket.c:637 [inline]
 sock_sendmsg+0xd7/0x130 net/socket.c:657
 ___sys_sendmsg+0x3e2/0x920 net/socket.c:2311
 __sys_sendmmsg+0x1bf/0x4d0 net/socket.c:2413
 __do_sys_sendmmsg net/socket.c:2442 [inline]

Fixes: 90b73b77d0 ("net: sched: change action API to use array of pointers to actions")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Vlad Buslov <vladbu@mellanox.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -2894,8 +2894,10 @@ out:
 void tcf_exts_destroy(struct tcf_exts *exts)
 {
 #ifdef CONFIG_NET_CLS_ACT
+	if (exts->actions) {
 		tcf_action_destroy(exts->actions, TCA_ACT_UNBIND);
 		kfree(exts->actions);
+	}
 	exts->nr_actions = 0;
 #endif
 }