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

    Call Trace:
    [<a000000100013940>] show_stack+0x40/0xa0
    [<a0000001000145a0>] show_regs+0x840/0x880
    [<a0000001000368e0>] die+0x1c0/0x2c0
    [<a000000100036a30>] die_if_kernel+0x50/0x80
    [<a000000100037c40>] ia64_fault+0x11e0/0x1300
    [<a00000010000bdc0>] ia64_leave_kernel+0x0/0x280
    [<a000000100482070>] check_modem_status+0xf0/0x360
    [<a000000100482300>] serial8250_get_mctrl+0x20/0xa0
    [<a000000100478170>] uart_read_proc+0x250/0x860
    [<a0000001001c16d0>] proc_file_read+0x1d0/0x4c0
    [<a0000001001394b0>] vfs_read+0x1b0/0x300
    [<a000000100139cd0>] sys_read+0x70/0xe0
    [<a00000010000bc20>] ia64_ret_from_syscall+0x0/0x20
    [<a000000000010620>] __kernel_syscall_via_break+0x0/0x20

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>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>