printk: fix buffer overflow when calling log_prefix function from call_console_drivers

This patch corrects a buffer overflow in kernels from 3.0 to 3.4 when calling
log_prefix() function from call_console_drivers().

This bug existed in previous releases but has been revealed with commit
162a7e7500f9664636e649ba59defe541b7c2c60 (2.6.39 => 3.0) that made changes
about how to allocate memory for early printk buffer (use of memblock_alloc).
It disappears with commit 7ff9554bb578ba02166071d2d487b7fc7d860d62 (3.4 => 3.5)
that does a refactoring of printk buffer management.

In log_prefix(), the access to "p[0]", "p[1]", "p[2]" or
"simple_strtoul(&p[1], &endp, 10)" may cause a buffer overflow as this
function is called from call_console_drivers by passing "&LOG_BUF(cur_index)"
where the index must be masked to do not exceed the buffer's boundary.

The trick is to prepare in call_console_drivers() a buffer with the necessary
data (PRI field of syslog message) to be safely evaluated in log_prefix().

This patch can be applied to stable kernel branches 3.0.y, 3.2.y and 3.4.y.

Without this patch, one can freeze a server running this loop from shell :
  $ export DUMMY=`cat /dev/urandom | tr -dc '12345AZERTYUIOPQSDFGHJKLMWXCVBNazertyuiopqsdfghjklmwxcvbn' | head -c255`
  $ while true do ; echo $DUMMY > /dev/kmsg ; done

The "server freeze" depends on where memblock_alloc does allocate printk buffer :
if the buffer overflow is inside another kernel allocation the problem may not
be revealed, else the server may hangs up.

Signed-off-by: Alexandre SIMON <Alexandre.Simon@univ-lorraine.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
static void zap_locks(void)
{
	static unsigned long oops_timestamp;

	if (time_after_eq(jiffies, oops_timestamp) &&
			!time_after(jiffies, oops_timestamp + 30 * HZ))
		return;

	oops_timestamp = jiffies;

	debug_locks_off();
	/* If a crash is occurring, make sure we can't deadlock */
	raw_spin_lock_init(&logbuf_lock);
	/* And make sure that we print immediately */
	sema_init(&console_sem, 1);
}

static void zap_locks(void)
{
	static unsigned long oops_timestamp;

	if (time_after_eq(jiffies, oops_timestamp) &&
			!time_after(jiffies, oops_timestamp + 30 * HZ))
		return;

	oops_timestamp = jiffies;

	debug_locks_off();
	/* If a crash is occurring, make sure we can't deadlock */
	raw_spin_lock_init(&logbuf_lock);
	/* And make sure that we print immediately */
	sema_init(&console_sem, 1);
}
