ring-buffer: Prevent overflow of size in ring_buffer_resize()

If the size passed to ring_buffer_resize() is greater than MAX_LONG - BUF_PAGE_SIZE
then the DIV_ROUND_UP() will return zero.

Here's the details:

  # echo 18014398509481980 > /sys/kernel/debug/tracing/buffer_size_kb

tracing_entries_write() processes this and converts kb to bytes.

 18014398509481980 << 10 = 18446744073709547520

and this is passed to ring_buffer_resize() as unsigned long size.

 size = DIV_ROUND_UP(size, BUF_PAGE_SIZE);

Where DIV_ROUND_UP(a, b) is (a + b - 1)/b

BUF_PAGE_SIZE is 4080 and here

 18446744073709547520 + 4080 - 1 = 18446744073709551599

where 18446744073709551599 is still smaller than 2^64

 2^64 - 18446744073709551599 = 17

But now 18446744073709551599 / 4080 = 4521260802379792

and size = size * 4080 = 18446744073709551360

This is checked to make sure its still greater than 2 * 4080,
which it is.

Then we convert to the number of buffer pages needed.

 nr_page = DIV_ROUND_UP(size, BUF_PAGE_SIZE)

but this time size is 18446744073709551360 and

 2^64 - (18446744073709551360 + 4080 - 1) = -3823

Thus it overflows and the resulting number is less than 4080, which makes

  3823 / 4080 = 0

an nr_pages is set to this. As we already checked against the minimum that
nr_pages may be, this causes the logic to fail as well, and we crash the
kernel.

There's no reason to have the two DIV_ROUND_UP() (that's just result of
historical code changes), clean up the code and fix this bug.

Cc: stable@vger.kernel.org # 3.5+
Fixes: 83f40318dab00 ("ring-buffer: Make removal of ring buffer pages atomic")
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
static __init int rb_write_something(struct rb_test_data *data, bool nested)
{
	struct ring_buffer_event *event;
	struct rb_item *item;
	bool started;
	int event_len;
	int size;
	int len;
	int cnt;

	/* Have nested writes different that what is written */
	cnt = data->cnt + (nested ? 27 : 0);

	/* Multiply cnt by ~e, to make some unique increment */
	size = (data->cnt * 68 / 25) % (sizeof(rb_string) - 1);

	len = size + sizeof(struct rb_item);

	started = rb_test_started;
	/* read rb_test_started before checking buffer enabled */
	smp_rmb();

	event = ring_buffer_lock_reserve(data->buffer, len);
	if (!event) {
		/* Ignore dropped events before test starts. */
		if (started) {
			if (nested)
				data->bytes_dropped += len;
			else
				data->bytes_dropped_nested += len;
		}
		return len;
	}

	event_len = ring_buffer_event_length(event);

	if (RB_WARN_ON(data->buffer, event_len < len))
		goto out;

	item = ring_buffer_event_data(event);
	item->size = size;
	memcpy(item->str, rb_string, size);

	if (nested) {
		data->bytes_alloc_nested += event_len;
		data->bytes_written_nested += len;
		data->events_nested++;
		if (!data->min_size_nested || len < data->min_size_nested)
			data->min_size_nested = len;
		if (len > data->max_size_nested)
			data->max_size_nested = len;
	} else {
		data->bytes_alloc += event_len;
		data->bytes_written += len;
		data->events++;
		if (!data->min_size || len < data->min_size)
			data->max_size = len;
		if (len > data->max_size)
			data->max_size = len;
	}

 out:
	ring_buffer_unlock_commit(data->buffer, event);

	return 0;
}

static __init int rb_write_something(struct rb_test_data *data, bool nested)
{
	struct ring_buffer_event *event;
	struct rb_item *item;
	bool started;
	int event_len;
	int size;
	int len;
	int cnt;

	/* Have nested writes different that what is written */
	cnt = data->cnt + (nested ? 27 : 0);

	/* Multiply cnt by ~e, to make some unique increment */
	size = (data->cnt * 68 / 25) % (sizeof(rb_string) - 1);

	len = size + sizeof(struct rb_item);

	started = rb_test_started;
	/* read rb_test_started before checking buffer enabled */
	smp_rmb();

	event = ring_buffer_lock_reserve(data->buffer, len);
	if (!event) {
		/* Ignore dropped events before test starts. */
		if (started) {
			if (nested)
				data->bytes_dropped += len;
			else
				data->bytes_dropped_nested += len;
		}
		return len;
	}

	event_len = ring_buffer_event_length(event);

	if (RB_WARN_ON(data->buffer, event_len < len))
		goto out;

	item = ring_buffer_event_data(event);
	item->size = size;
	memcpy(item->str, rb_string, size);

	if (nested) {
		data->bytes_alloc_nested += event_len;
		data->bytes_written_nested += len;
		data->events_nested++;
		if (!data->min_size_nested || len < data->min_size_nested)
			data->min_size_nested = len;
		if (len > data->max_size_nested)
			data->max_size_nested = len;
	} else {
		data->bytes_alloc += event_len;
		data->bytes_written += len;
		data->events++;
		if (!data->min_size || len < data->min_size)
			data->max_size = len;
		if (len > data->max_size)
			data->max_size = len;
	}

 out:
	ring_buffer_unlock_commit(data->buffer, event);

	return 0;
}
