Fix possible NULL pointer access in 8250 serial driver

Fix possible NULL pointer access in 8250 serial driver

I encountered the following kernel panic.  The cause of this problem was
NULL pointer access in check_modem_status() in 8250.c.  I confirmed this
problem is fixed by the attached patch, but I don't know this is the
correct fix.

sadc[4378]: NaT consumption 2216203124768 [1]
Modules linked in: binfmt_misc dm_mirror dm_mod thermal processor fan
container button sg e100 eepro100 mii ehci_hcd ohci_hcd

Pid: 4378, CPU 0, comm: sadc
psr : 00001210085a2010 ifs : 8000000000000289 ip : [<a000000100482071>]
Not tainted
ip is at check_modem_status+0xf1/0x360
unat: 0000000000000000 pfs : 0000000000000289 rsc : 0000000000000003
rnat: 800000000000cc18 bsps: 0000000000000000 pr : 0000000000aa6a99
ldrs: 0000000000000000 ccv : 0000000000000000 fpsr: 0009804c8a70033f
csd : 0000000000000000 ssd : 0000000000000000
b0 : a000000100481fb0 b6 : a0000001004822e0 b7 : a000000100477f20
f6 : 1003e2222222222222222 f7 : 0ffdba200000000000000
f8 : 100018000000000000000 f9 : 10002a000000000000000
f10 : 0fffdccccccccc8c00000 f11 : 1003e0000000000000000
r1 : a000000100b9af40 r2 : 0000000000000008 r3 : a000000100ad4e21
r8 : 00000000000000bb r9 : 0000000000000001 r10 : 0000000000000000
r11 : a000000100ad4d58 r12 : e0000000037b7df0 r13 : e0000000037b0000
r14 : 0000000000000001 r15 : 0000000000000018 r16 : a000000100ad4d6c
r17 : 0000000000000000 r18 : 0000000000000000 r19 : 0000000000000000
r20 : a00000010099bc88 r21 : 00000000000000bb r22 : 00000000000000bb
r23 : c003fffffc0ff3fe r24 : c003fffffc000000 r25 : 00000000000ff3fe
r26 : a0000001009b7ad0 r27 : 0000000000000001 r28 : a0000001009b7ad8
r29 : 0000000000000000 r30 : a0000001009b7ad0 r31 : a0000001009b7ad0

Call Trace:
[<a000000100013940>] show_stack+0x40/0xa0
sp=e0000000037b7810 bsp=e0000000037b1118
[<a0000001000145a0>] show_regs+0x840/0x880
sp=e0000000037b79e0 bsp=e0000000037b10c0
[<a0000001000368e0>] die+0x1c0/0x2c0
sp=e0000000037b79e0 bsp=e0000000037b1078
[<a000000100036a30>] die_if_kernel+0x50/0x80
sp=e0000000037b7a00 bsp=e0000000037b1048
[<a000000100037c40>] ia64_fault+0x11e0/0x1300
sp=e0000000037b7a00 bsp=e0000000037b0fe8
[<a00000010000bdc0>] ia64_leave_kernel+0x0/0x280
sp=e0000000037b7c20 bsp=e0000000037b0fe8
[<a000000100482070>] check_modem_status+0xf0/0x360
sp=e0000000037b7df0 bsp=e0000000037b0fa0
[<a000000100482300>] serial8250_get_mctrl+0x20/0xa0
sp=e0000000037b7df0 bsp=e0000000037b0f80
[<a000000100478170>] uart_read_proc+0x250/0x860
sp=e0000000037b7df0 bsp=e0000000037b0ee0
[<a0000001001c16d0>] proc_file_read+0x1d0/0x4c0
sp=e0000000037b7e10 bsp=e0000000037b0e80
[<a0000001001394b0>] vfs_read+0x1b0/0x300
sp=e0000000037b7e20 bsp=e0000000037b0e30
[<a000000100139cd0>] sys_read+0x70/0xe0
sp=e0000000037b7e20 bsp=e0000000037b0db0
[<a00000010000bc20>] ia64_ret_from_syscall+0x0/0x20
sp=e0000000037b7e30 bsp=e0000000037b0db0
[<a000000000010620>] __kernel_syscall_via_break+0x0/0x20
sp=e0000000037b8000 bsp=e0000000037b0db0

Fix the possible NULL pointer access in check_modem_status() in 8250.c.  The
check_modem_status() would access 'info' member of uart_port structure, but it
is not initialized before uart_open() is called.  The check_modem_status() can
be called through /proc/tty/driver/serial before uart_open() is called.

Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Taku Izumi <izumi2005@soft.fujitsu.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>