serial: serial_core: Perform NULL checks for break_ctl ops

Doing fuzz test on sbsa uart device, causes a kernel crash
due to NULL pointer dereference:

------------[ cut here ]------------
Unable to handle kernel paging request at virtual address fffffffffffffffc
pgd = ffffffe331723000
[fffffffffffffffc] *pgd=0000002333595003, *pud=0000002333595003, *pmd=00000
Internal error: Oops: 96000005 [#1] PREEMPT SMP
Modules linked in: ping(O) jffs2 rtos_snapshot(O) pramdisk(O) hisi_sfc(O)
Drv_Nandc_K(O) Drv_SysCtl_K(O) Drv_SysClk_K(O) bsp_reg(O) hns3(O)
hns3_uio_enet(O) hclgevf(O) hclge(O) hnae3(O) mdio_factory(O)
mdio_registry(O) mdio_dev(O) mdio(O) hns3_info(O) rtos_kbox_panic(O)
uart_suspend(O) rsm(O) stp llc tunnel4 xt_tcpudp ipt_REJECT nf_reject_ipv4
iptable_filter ip_tables x_tables sd_mod xhci_plat_hcd xhci_pci xhci_hcd
usbmon usbhid usb_storage ohci_platform ohci_pci ohci_hcd hid_generic hid
ehci_platform ehci_pci ehci_hcd vfat fat usbcore usb_common scsi_mod
yaffs2multi(O) ext4 jbd2 ext2 mbcache ofpart i2c_dev i2c_core uio ubi nand
nand_ecc nand_ids cfi_cmdset_0002 cfi_cmdset_0001 cfi_probe gen_probe
cmdlinepart chipreg mtdblock mtd_blkdevs mtd nfsd auth_rpcgss oid_registry
nfsv3 nfs nfs_acl lockd sunrpc grace autofs4
CPU: 2 PID: 2385 Comm: tty_fuzz_test Tainted: G           O    4.4.193 #1
task: ffffffe32b23f110 task.stack: ffffffe32bda4000
PC is at uart_break_ctl+0x44/0x84
LR is at uart_break_ctl+0x34/0x84
pc : [<ffffff8393196098>] lr : [<ffffff8393196088>] pstate: 80000005
sp : ffffffe32bda7cc0
x29: ffffffe32bda7cc0 x28: ffffffe32b23f110
x27: ffffff8393402000 x26: 0000000000000000
x25: ffffffe32b233f40 x24: ffffffc07a8ec680
x23: 0000000000005425 x22: 00000000ffffffff
x21: ffffffe33ed73c98 x20: 0000000000000000
x19: ffffffe33ed94168 x18: 0000000000000004
x17: 0000007f92ae9d30 x16: ffffff8392fa6064
x15: 0000000000000010 x14: 0000000000000000
x13: 0000000000000000 x12: 0000000000000000
x11: 0000000000000020 x10: 0000007ffdac1708
x9 : 0000000000000078 x8 : 000000000000001d
x7 : 0000000052a64887 x6 : ffffffe32bda7e08
x5 : ffffffe32b23c000 x4 : 0000005fbc5b0000
x3 : ffffff83938d5018 x2 : 0000000000000080
x1 : ffffffe32b23c040 x0 : ffffff83934428f8
virtual start addr offset is 38ac00000
module base offset is 2cd4cf1000
linear region base offset is : 0
Process tty_fuzz_test (pid: 2385, stack limit = 0xffffffe32bda4000)
Stack: (0xffffffe32bda7cc0 to 0xffffffe32bda8000)
7cc0: ffffffe32bda7cf0 ffffff8393177718 ffffffc07a8ec680 ffffff8393196054
7ce0: 000000001739f2e0 0000007ffdac1978 ffffffe32bda7d20 ffffff8393179a1c
7d00: 0000000000000000 ffffff8393c0a000 ffffffc07a8ec680 cb88537fdc8ba600
7d20: ffffffe32bda7df0 ffffff8392fa5a40 ffffff8393c0a000 0000000000005425
7d40: 0000007ffdac1978 ffffffe32b233f40 ffffff8393178dcc 0000000000000003
7d60: 000000000000011d 000000000000001d ffffffe32b23f110 000000000000029e
7d80: ffffffe34fe8d5d0 0000000000000000 ffffffe32bda7e14 cb88537fdc8ba600
7da0: ffffffe32bda7e30 ffffff8393042cfc ffffff8393c41720 ffffff8393c46410
7dc0: ffffff839304fa68 ffffffe32b233f40 0000000000005425 0000007ffdac1978
7de0: 000000000000011d cb88537fdc8ba600 ffffffe32bda7e70 ffffff8392fa60cc
7e00: 0000000000000000 ffffffe32b233f40 ffffffe32b233f40 0000000000000003
7e20: 0000000000005425 0000007ffdac1978 ffffffe32bda7e70 ffffff8392fa60b0
7e40: 0000000000000280 ffffffe32b233f40 ffffffe32b233f40 0000000000000003
7e60: 0000000000005425 cb88537fdc8ba600 0000000000000000 ffffff8392e02e78
7e80: 0000000000000280 0000005fbc5b0000 ffffffffffffffff 0000007f92ae9d3c
7ea0: 0000000060000000 0000000000000015 0000000000000003 0000000000005425
7ec0: 0000007ffdac1978 0000000000000000 00000000a54c910e 0000007f92b95014
7ee0: 0000007f92b95090 0000000052a64887 000000000000001d 0000000000000078
7f00: 0000007ffdac1708 0000000000000020 0000000000000000 0000000000000000
7f20: 0000000000000000 0000000000000010 000000556acf0090 0000007f92ae9d30
7f40: 0000000000000004 000000556acdef10 0000000000000000 000000556acdebd0
7f60: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
7f80: 0000000000000000 0000000000000000 0000000000000000 0000007ffdac1840
7fa0: 000000556acdedcc 0000007ffdac1840 0000007f92ae9d3c 0000000060000000
7fc0: 0000000000000000 0000000000000000 0000000000000003 000000000000001d
7fe0: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
Call trace:
Exception stack(0xffffffe32bda7ab0 to 0xffffffe32bda7bf0)
7aa0:                                   0000000000001000 0000007fffffffff
7ac0: ffffffe32bda7cc0 ffffff8393196098 0000000080000005 0000000000000025
7ae0: ffffffe32b233f40 ffffff83930d777c ffffffe32bda7b30 ffffff83930d777c
7b00: ffffffe32bda7be0 ffffff83938d5000 ffffffe32bda7be0 ffffffe32bda7c20
7b20: ffffffe32bda7b60 ffffff83930d777c ffffffe32bda7c10 ffffff83938d5000
7b40: ffffffe32bda7c10 ffffffe32bda7c50 ffffff8393c0a000 ffffffe32b23f110
7b60: ffffffe32bda7b70 ffffff8392e09df4 ffffffe32bda7bb0 cb88537fdc8ba600
7b80: ffffff83934428f8 ffffffe32b23c040 0000000000000080 ffffff83938d5018
7ba0: 0000005fbc5b0000 ffffffe32b23c000 ffffffe32bda7e08 0000000052a64887
7bc0: 000000000000001d 0000000000000078 0000007ffdac1708 0000000000000020
7be0: 0000000000000000 0000000000000000
[<ffffff8393196098>] uart_break_ctl+0x44/0x84
[<ffffff8393177718>] send_break+0xa0/0x114
[<ffffff8393179a1c>] tty_ioctl+0xc50/0xe84
[<ffffff8392fa5a40>] do_vfs_ioctl+0xc4/0x6e8
[<ffffff8392fa60cc>] SyS_ioctl+0x68/0x9c
[<ffffff8392e02e78>] __sys_trace_return+0x0/0x4
Code: b9410ea0 34000160 f9408aa0 f9402814 (b85fc280)
---[ end trace 8606094f1960c5e0 ]---
Kernel panic - not syncing: Fatal exception

Fix this problem by adding NULL checks prior to calling break_ctl ops.
Signed-off-by: NJiangfeng Xiao <xiaojiangfeng@huawei.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/1574263133-28259-1-git-send-email-xiaojiangfeng@huawei.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1111,7 +1111,7 @@ static int uart_break_ctl(struct tty_struct *tty, int break_state)
 	if (!uport)
 		goto out;
 
-	if (uport->type != PORT_UNKNOWN)
+	if (uport->type != PORT_UNKNOWN && uport->ops->break_ctl)
 		uport->ops->break_ctl(uport, break_state);
 	ret = 0;
 out: