cls_cgroup: avoid panic when receiving a packet before filter set

When a cgroup classifier is added, there is a small time interval in
which tp->root is NULL. If we receive a packet in this small time slice
a NULL pointer dereference will happen, leading to a kernel panic:

    # mkdir /sys/fs/cgroup/net_cls/0
    # echo 0x100001 >  /sys/fs/cgroup/net_cls/0/net_cls.classid
    # echo $$ >/sys/fs/cgroup/net_cls/0/tasks
    # ping -qfb 255.255.255.255 -I eth0 &>/dev/null &
    # tc qdisc add dev eth0 root handle 10: htb
    # while : ; do
    > tc filter add dev eth0 parent 10: protocol ip prio 10 handle 1: cgroup
    > tc filter delete dev eth0
    > done
    Unable to handle kernel NULL pointer dereference at virtual address 0000000000000028
    Mem abort info:
      ESR = 0x96000005
      Exception class = DABT (current EL), IL = 32 bits
      SET = 0, FnV = 0
      EA = 0, S1PTW = 0
    Data abort info:
      ISV = 0, ISS = 0x00000005
      CM = 0, WnR = 0
    user pgtable: 4k pages, 39-bit VAs, pgdp = 0000000098a7ff91
    [0000000000000028] pgd=0000000000000000, pud=0000000000000000
    Internal error: Oops: 96000005 [#1] SMP
    Modules linked in: sch_htb cls_cgroup algif_hash af_alg nls_iso8859_1 nls_cp437 vfat fat xhci_plat_hcd m25p80 spi_nor xhci_hcd mtd usbcore usb_common spi_orion sfp i2c_mv64xxx phy_generic mdio_i2c marvell10g i2c_core mvpp2 mvmdio phylink sbsa_gwdt ip_tables x_tables autofs4
    Process ping (pid: 5421, stack limit = 0x00000000b20b1505)
    CPU: 3 PID: 5421 Comm: ping Not tainted 5.1.0-rc6 #31
    Hardware name: Marvell 8040 MACCHIATOBin Double-shot (DT)
    pstate: 60000005 (nZCv daif -PAN -UAO)
    pc : cls_cgroup_classify+0x80/0xec [cls_cgroup]
    lr : cls_cgroup_classify+0x34/0xec [cls_cgroup]
    sp : ffffff8012e6b850
    x29: ffffff8012e6b850 x28: ffffffc423dd3c00
    x27: ffffff801093ebc0 x26: ffffffc425a85b00
    x25: 0000000020000000 x24: 0000000000000000
    x23: ffffff8012e6b910 x22: ffffffc428db4900
    x21: ffffff8012e6b910 x20: 0000000000100001
    x19: 0000000000000000 x18: 0000000000000000
    x17: 0000000000000000 x16: 0000000000000000
    x15: 0000000000000000 x14: 0000000000000000
    x13: 0000000000000000 x12: 000000000000001c
    x11: 0000000000000018 x10: ffffff8012e6b840
    x9 : 0000000000003580 x8 : 000000000000009d
    x7 : 0000000000000002 x6 : ffffff8012e6b860
    x5 : 000000007cd66ffe x4 : 000000009742a193
    x3 : ffffff800865b4d8 x2 : ffffff8012e6b910
    x1 : 0000000000000400 x0 : ffffffc42c38f300
    Call trace:
     cls_cgroup_classify+0x80/0xec [cls_cgroup]
     tcf_classify+0x78/0x138
     htb_enqueue+0x74/0x320 [sch_htb]
     __dev_queue_xmit+0x3e4/0x9d0
     dev_queue_xmit+0x24/0x30
     ip_finish_output2+0x2e4/0x4d0
     ip_finish_output+0x1d8/0x270
     ip_mc_output+0xa8/0x240
     ip_local_out+0x58/0x68
     ip_send_skb+0x2c/0x88
     ip_push_pending_frames+0x44/0x50
     raw_sendmsg+0x458/0x830
     inet_sendmsg+0x54/0xe8
     sock_sendmsg+0x34/0x50
     __sys_sendto+0xd0/0x120
     __arm64_sys_sendto+0x30/0x40
     el0_svc_common.constprop.0+0x88/0xf8
     el0_svc_handler+0x2c/0x38
     el0_svc+0x8/0xc
    Code: 39496001 360002a1 b9425c14 34000274 (79405260)

Fixes: ed76f5edcc ("net: sched: protect filter_chain list with filter_chain_lock mutex")
Suggested-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_cgroup.c

@@ -32,6 +32,8 @@ static int cls_cgroup_classify(struct sk_buff *skb, const struct tcf_proto *tp,
 	struct cls_cgroup_head *head = rcu_dereference_bh(tp->root);
 	u32 classid = task_get_classid(skb);
 
+	if (unlikely(!head))
+		return -1;
 	if (!classid)
 		return -1;
 	if (!tcf_em_tree_match(skb, &head->ematches, NULL))