net: dsa: Check existence of .port_mdb_add callback before calling it

The dsa framework has optional .port_mdb_{prepare,add,del} callback fields
for drivers to handle multicast database entries. When adding an entry, the
framework goes through a prepare phase, then a commit phase. Drivers not
providing these callbacks should be detected in the prepare phase.

DSA core may still bypass the bridge layer and call the dsa_port_mdb_add
function directly with no prepare phase or no switchdev trans object,
and the framework ends up calling an undefined .port_mdb_add callback.
This results in a NULL pointer dereference, as shown in the log below.

The other functions seem to be properly guarded. Do the same for
.port_mdb_add in dsa_switch_mdb_add_bitmap() as well.

    8<--- cut here ---
    Unable to handle kernel NULL pointer dereference at virtual address 00000000
    pgd = (ptrval)
    [00000000] *pgd=00000000
    Internal error: Oops: 80000005 [#1] SMP ARM
    Modules linked in: rtl8xxxu rtl8192cu rtl_usb rtl8192c_common rtlwifi mac80211 cfg80211
    CPU: 1 PID: 134 Comm: kworker/1:2 Not tainted 5.3.0-rc1-00247-gd3519030752a #1
    Hardware name: Allwinner sun7i (A20) Family
    Workqueue: events switchdev_deferred_process_work
    PC is at 0x0
    LR is at dsa_switch_event+0x570/0x620
    pc : [<00000000>]    lr : [<c08533ec>]    psr: 80070013
    sp : ee871db8  ip : 00000000  fp : ee98d0a4
    r10: 0000000c  r9 : 00000008  r8 : ee89f710
    r7 : ee98d040  r6 : ee98d088  r5 : c0f04c48  r4 : ee98d04c
    r3 : 00000000  r2 : ee89f710  r1 : 00000008  r0 : ee98d040
    Flags: Nzcv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
    Control: 10c5387d  Table: 6deb406a  DAC: 00000051
    Process kworker/1:2 (pid: 134, stack limit = 0x(ptrval))
    Stack: (0xee871db8 to 0xee872000)
    1da0:                                                       ee871e14 103ace2d
    1dc0: 00000000 ffffffff 00000000 ee871e14 00000005 00000000 c08524a0 00000000
    1de0: ffffe000 c014bdfc c0f04c48 ee871e98 c0f04c48 ee9e5000 c0851120 c014bef0
    1e00: 00000000 b643aea2 ee9b4068 c08509a8 ee2bf940 ee89f710 ee871ecb 00000000
    1e20: 00000008 103ace2d 00000000 c087e248 ee29c868 103ace2d 00000001 ffffffff
    1e40: 00000000 ee871e98 00000006 00000000 c0fb2a50 c087e2d0 ffffffff c08523c4
    1e60: ffffffff c014bdfc 00000006 c0fad2d0 ee871e98 ee89f710 00000000 c014c500
    1e80: 00000000 ee89f3c0 c0f04c48 00000000 ee9e5000 c087dfb4 ee9e5000 00000000
    1ea0: ee89f710 ee871ecb 00000001 103ace2d 00000000 c0f04c48 00000000 c087e0a8
    1ec0: 00000000 efd9a3e0 0089f3c0 103ace2d ee89f700 ee89f710 ee9e5000 00000122
    1ee0: 00000100 c087e130 ee89f700 c0fad2c8 c1003ef0 c087de4c 2e928000 c0fad2ec
    1f00: c0fad2ec ee839580 ef7a62c0 ef7a9400 00000000 c087def8 c0fad2ec c01447dc
    1f20: ef315640 ef7a62c0 00000008 ee839580 ee839594 ef7a62c0 00000008 c0f03d00
    1f40: ef7a62d8 ef7a62c0 ffffe000 c0145b84 ffffe000 c0fb2420 c0bfaa8c 00000000
    1f60: ffffe000 ee84b600 ee84b5c0 00000000 ee870000 ee839580 c0145b40 ef0e5ea4
    1f80: ee84b61c c014a6f8 00000001 ee84b5c0 c014a5b0 00000000 00000000 00000000
    1fa0: 00000000 00000000 00000000 c01010e8 00000000 00000000 00000000 00000000
    1fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
    1fe0: 00000000 00000000 00000000 00000000 00000013 00000000 00000000 00000000
    [<c08533ec>] (dsa_switch_event) from [<c014bdfc>] (notifier_call_chain+0x48/0x84)
    [<c014bdfc>] (notifier_call_chain) from [<c014bef0>] (raw_notifier_call_chain+0x18/0x20)
    [<c014bef0>] (raw_notifier_call_chain) from [<c08509a8>] (dsa_port_mdb_add+0x48/0x74)
    [<c08509a8>] (dsa_port_mdb_add) from [<c087e248>] (__switchdev_handle_port_obj_add+0x54/0xd4)
    [<c087e248>] (__switchdev_handle_port_obj_add) from [<c087e2d0>] (switchdev_handle_port_obj_add+0x8/0x14)
    [<c087e2d0>] (switchdev_handle_port_obj_add) from [<c08523c4>] (dsa_slave_switchdev_blocking_event+0x94/0xa4)
    [<c08523c4>] (dsa_slave_switchdev_blocking_event) from [<c014bdfc>] (notifier_call_chain+0x48/0x84)
    [<c014bdfc>] (notifier_call_chain) from [<c014c500>] (blocking_notifier_call_chain+0x50/0x68)
    [<c014c500>] (blocking_notifier_call_chain) from [<c087dfb4>] (switchdev_port_obj_notify+0x44/0xa8)
    [<c087dfb4>] (switchdev_port_obj_notify) from [<c087e0a8>] (switchdev_port_obj_add_now+0x90/0x104)
    [<c087e0a8>] (switchdev_port_obj_add_now) from [<c087e130>] (switchdev_port_obj_add_deferred+0x14/0x5c)
    [<c087e130>] (switchdev_port_obj_add_deferred) from [<c087de4c>] (switchdev_deferred_process+0x64/0x104)
    [<c087de4c>] (switchdev_deferred_process) from [<c087def8>] (switchdev_deferred_process_work+0xc/0x14)
    [<c087def8>] (switchdev_deferred_process_work) from [<c01447dc>] (process_one_work+0x218/0x50c)
    [<c01447dc>] (process_one_work) from [<c0145b84>] (worker_thread+0x44/0x5bc)
    [<c0145b84>] (worker_thread) from [<c014a6f8>] (kthread+0x148/0x150)
    [<c014a6f8>] (kthread) from [<c01010e8>] (ret_from_fork+0x14/0x2c)
    Exception stack(0xee871fb0 to 0xee871ff8)
    1fa0:                                     00000000 00000000 00000000 00000000
    1fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
    1fe0: 00000000 00000000 00000000 00000000 00000013 00000000
    Code: bad PC value
    ---[ end trace 1292c61abd17b130 ]---

    [<c08533ec>] (dsa_switch_event) from [<c014bdfc>] (notifier_call_chain+0x48/0x84)
    corresponds to

	$ arm-linux-gnueabihf-addr2line -C -i -e vmlinux c08533ec

	linux/net/dsa/switch.c:156
	linux/net/dsa/switch.c:178
	linux/net/dsa/switch.c:328

Fixes: e6db98db8a ("net: dsa: add switch mdb bitmap functions")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/switch.c

@@ -153,6 +153,9 @@ static void dsa_switch_mdb_add_bitmap(struct dsa_switch *ds,
 {
 	int port;
 
+	if (!ds->ops->port_mdb_add)
+		return;
+
 	for_each_set_bit(port, bitmap, ds->num_ports)
 		ds->ops->port_mdb_add(ds, port, mdb);
 }